Flyfocals: Vision and Vectors Help Hunting Robber Flies

Image credit: Thomas Shahan

Robber flies (Asilidae family) are not your typical house flies. They are small, predatory insects that feed on a vast array of other arthropods. While they are small in size (10 times smaller than a dragonfly), these guys are serious hunters. For example, Mallophora omboides is known as the “Florida bee killer” for its taste for honey bees. Other robber flies hunt down wasps, dragonflies, spiders, or grasshoppers, just to name a few. Perhaps almost as impressive as the types of prey is how they are subdued. Typically, robber flies will perch out in an open sunny place and wait, seizing their prey in flight and injecting it with neurotoxic or proteolytic enzymes that both immobilizes it and liquefies its insides.

A recent study in Current Biology took a closer look at the robber fly’s “aerial attack strategy.” The authors focused on the genus Holcocephala, a group native to the Americas. Let’s start by going over something you know about but probably never realized had an actual term: constant bearing angle (CBA) strategy. Initially, I tried to describe this just using text, but it is really best visualized with the help of a supplemental graphic from the paper.

Figure S1 from Wardill et al. (2017). Diagram showing how the constant bearing angle strategy (CBA) and proportional navigation can be used to intercept targets. It looks like an eye, but you are actually looking down on the "Human" and seeing the top of the head (black) and shoulders (white). 

Visualize this: You are walking along and ahead of you a ball is rolling along the ground from your left. But you decide that you want to get to the ball before it would intercept your path. If you want to catch the ball you can’t run straight for where you see it or it will have rolled past that spot before you get there. If you want to intercept that ball while it is still on your left, you will technically have to turn to backward, changing your “bearing angle.” You must anticipate where it will be and run in a straight line to that spot. This line is a “parallel range vector.” There are several of these vectors, depending on when you choose to change course.

The study considered whether the flies were using this CBA strategy to catch their prey. To do this, the researchers went out to a field and hung up a big white sheet as a backdrop for their high-speed video cameras. Next, they set up their “fly teaser,” a custom made plastic frame that housed a stepper motor and several pulleys to move taut fishing line. This allowed for precise, computer controlled movements of the beads they attached to the fishing line. When a robber fly perched on a blade of grass in their study area, they “teased” it with a bead (a.k.a. dummy prey item for the fly). They included several variations including bead size and direction. They recorded the fly with two synchronized cameras running at 1000 fps to get a 3D view of the attack. For each attack video, they analyzed the frame at which the flies started to take off and until it began a terminal deceleration on final approach of the target. Then: measure, measure, measure, math, math, math.

They found that the flies were fairly consistent with the CBA model. If they decelerated or reversed the bead during the attack, the robber flies compensated, actively keeping the range vectors parallel. One unexpected finding occurred in cases where the bead moved in front of the fly and it took off with a head-on collision course. They found that the fly still intercepted the bead while flying at a backward angle, meaning that the latter part of its trajectory was distinctly curved. When they took a closer look, the found the results to reflect a “lock-on” process “during which the fly has a new heading and the speed is fixed to a value slightly higher than that of the prey.” This lock-on strategy has not been described in any other flying animal. The flies were able to compensate for unexpected changes in the target’s velocity and uncertainties in the location, size, and speed of the target.

Adapted from paper's graphical abstract

This type of hunting relies very heavily on vision. So each robber fly was captured for later, high detailed analysis of the head and eyes. It is important to remember that insects have compound eyes. Repeating units (the ommatidia, which have hexagonal faces called facets) that make up the eyes function as separate receptors that, when put together, assemble view of the environment. The researchers measured several parts and angles within the eyes, and once again math, math, math. This revealed the ommatidia in the front, center portion of each eye (colored red in the picture) to be nearly double the size of those in other areas, have extended focal lengths and smaller receptors. This means that the flies can reduce diffraction, focus incident light, and optimize resolution in this area. This results in a frontal fovea, or area within the eye that provides greater visual acuity than the rest of the eye. Sort of like the embedded lens of bifocal glasses; while that is an incredibly simplified way to look at it, it tells you a lot about how the flies might strategize prey capture. Also, they could be judging distance using stereopsis. This is when they use both eyes in combination to depth and 3D structure. The authors sum things up nicely, so I'll leave it in their words: "[It is kind of amazing the] accurate performance that a miniature brain can achieve in highly demanding sensorimotor tasks."

Interested in more details? Here’s a video summary put together by the researchers:

Wardill, T., Fabian, S., Pettigrew, A., Stavenga, D., Nordström, K., & Gonzalez-Bellido, P. (2017). A Novel Interception Strategy in a Miniature Robber Fly with Extreme Visual Acuity Current Biology, 27 (6), 854-859 DOI: 10.1016/j.cub.2017.01.050


Read more about robber flies at University of Florida's Featured Creatures page.

Mutualism a.k.a Caterpillars Drugging Ants To Do Their Bidding

From the study - Figure 1. Attendant Workers of
Pristomyrmex punctatus standing on or around
Narathura japonica caterpillars

The manuscript is done! Submitted! Summer interns are finished. Boot up Normal Life Mode, please. Recommence blogging. So many good papers have come out during my hiatus. Where to start…where to start…

If you have read this blog for any amount of time then you will come across my fascination with ant manipulation, particularly zombification. This is why my cursor stopped over a new paper in Current Biology about caterpillars manipulating ants to do their bidding.

Let’s start with mutualism. This is a topic that I have visited in the past, and in ants for that matter. It’s a nice little relationship between species that involves an exchange of goods and/or services. In the natural world, this often means food and protection.

In this study, the researchers chose the Japanese oakblue butterfly (Narathura japonica), a lycaenid belonging to the Theclinae subfamily of butterflies. Many in this group are myrmecophilic, meaning they associate (often mutualistically) with ants in some way. The Japanese oakblue caterpillar has a specialized exocrine gland, the “dorsal nectary organ (DNO),” that is located on the seventh abdominal segment and is flanked by tentacle organs (TO). The DNO secretes sugar- and amino acid-rich honeydew while the TO secretes scents to “talk” to the ants. A “Come on down!” or “Danger, Will Robinson!” type thing. The ants tend to the caterpillars and keep them safe for a nice, sugary food reward. But is that all to the story? Obviously not or this post would end here.

To do this experiment, butterfly eggs and their associated ants (Pristomyrmex punctatus) were collected and reared separately. Then three test situations were set up with 50 ants per treatment:

1. “Experienced” ants – had free access to the caterpillars and their DNO secretions
2. “Inexperienced” ants – no caterpillar access, just some sugar soaked cotton balls
3. “Unrewarded” ants – had access only to caterpillars that had their DNO’s blocked (a little bit of clear nail polish goes a long way)

After 3 days in their test situation, 10 ants from each treatment were moved to Petri dishes that were set on pieces of white paper with a line on it to divide the dishes into 2 halves. After the ants acclimated to their new little plastic arenas, they were observed to see how many times they crossed the center line (“locomotory activity”). Also at the 3 day time point, ants and caterpillars were frozen in liquid nitrogen until their brains could be dissected out, specifically removing the optic lobes. Now, I’ve done some pretty small dissections, but those come nowhere close to ant brain removal! Wow, just wow. Once those itty bitty brains were out, they were processed for liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) for serotonin, dopamine, octopamine, and tyramine. Very simply, that means making an ant-brain-aerosol that is then separated and identified by component.

They found that experienced ants had significantly less locomotory activity than the other two groups. So what does an ant walking, or in this case not walking, across a line even mean? Well, the fact that the ants are staying put signals that they are “standing guard” for the caterpillars. Okay, let’s say that standing means guarding, how do we know that this is really caterpillar-related and not just standing there? Well, first of all, it was only the experienced ants that did this. Second, the researchers observed that the caterpillars often “everted their TOs,” meaning that they turned them outward. This is typically a response the caterpillar makes when it is attacked by a predator – “Raise shields!” Experienced ants responded differently than the other two when they saw this caterpillar behavior in that they responded aggressively. This aggression is a response to the caterpillars’ alarm, one that has the ants defending against the predator. The fact that only experienced ants had these responses suggests that something in the DNO secretions is eliciting these defense behaviors.

So what is it about these secretions? That’s where the LC-MS/MS comes in. Biogenic amines are known function as neurotransmitters, neuromodulators, and/or neurohormones. This means that they can modify behavior in insects. DNO secretions contain biogenic amines. This analysis showed that experienced ant brains had low dopamine levels. Now, that’s important because dopamine has been shown to be involved in both locomotory activity and aggression in well studied organisms like fruit flies. Starting to see some links here, yes? To confirm the linkage, ants from each treatment were given reserpine, a small-molecule inhibitor that depletes dopamine but not serotonin in the brain. This test resulted the same behaviors, but the LC-MS/MS showed increased dopamine and serotonin in the ant brains. So same but different.

There is another aspect to consider: Who loses if the mutualism goes away? The honeydew is not the sole source of nourishment for the ants. They can leave and be still be fine. The caterpillar has much more to lose than the ant (its life via predation). So the caterpillars must be doing something besides sugar-loading their ants.  This is where the caterpillar gets sneaky - finding a way to make their ants to both stick around and defend against predators. As the authors put it, they they insert “manipulative drugs [into the honeydew] that could function to enforce cooperative behavior…from attendant ants.” Put that way, I’m okay calling it “ant mind control.”


Hojo, M., Pierce, N., & Tsuji, K. (2015). Lycaenid Caterpillar Secretions Manipulate Attendant Ant Behavior Current Biology DOI: 10.1016/j.cub.2015.07.016

p.s. The supplementary materials have a nice little video of ants in lined Petri dishes.

(image is Figure 1 from the above paper)

Vampire Plants: Sucking Life into the Community

I would like to introduce you to the yellow rattle (Rhinanthus minor). This little plant has pretty yellow flowers that belie a dark secret: it is a vampire plant. Okay, technically, it is a hemiparasite. The yellow rattle has green, photosynthetic leaves to make its own food, but its roots latch onto those of nearby plants to steal their water and nutrients. See? Vampire. I guess you could say that it hasn’t gone full parasite. So what’s wrong with a little leaching of material from your neighbor? Must be like stealing cable, right? Well, not exactly. Hemiparastitic plants can have impacts that are highly disproportionate to their small size. By stealing water and nutrients, they reduce the host plants’ photosynthetic rate and biomass, making them less competitive in their habitat. When you are less competitive in a highly-competitive environment then you don’t last all that long. But this is looking at it on an individual, plant-by-plant, level. Let’s scale it up a bit.

We’ll start by following the logic trail. Hemiparastic plants are widespread in ecosystems such as meadows where they attach to dominant grasses (in this case, dominant meaning numbers). As individual grass plants are weakened or killed, overall grass biomass decreases. When the dominants are gone there is more room for other types of plants to move in, increasing species richness. More types of plants attract more primary consumers (like insects) to feed on them. More insects attract more secondary consumers, and on and on. All in all that one little plant has caused indirect community level changes. A new paper by Hartley et al. in Ecology looks at how this one little plant can affect multiple trophic levels (or levels in the food chain).

The researchers laid out 13 blocks, each containing four 1 by 1 meter plots, in a field site in Sussex, UK. Three out of those four plots were allowed to be naturally colonized by yellow rattle. These “infected plots” were then randomly assigned to one of three treatments: yellow rattle removed, present, or enhanced. Then, during maximum vegetation and invertebrate abundance, plots were censused. Counts, ground cover, vegetation height, and species richness were measured for plants. Invertebrates were sampled with a Vortis suction sampler (a.k.a. bug vacuum) and hand counts. They were identified and classified into their trophic levels: herbivore, predator and detritivore.

They found that the density of yellow rattle had pronounced effects on the plant community. Grass cover was lower, plant diversity higher (although not richness), and plant height lower in plots where the hemiparasite was present or enhanced. Going up a trophic level, sap-feeding insects (Hemiptera) increased by 130%, caterpillars (Lepidoptera) by 217% and weevils (Curculionidae) by 188%. Whoa! Going up another level to the predators, spiders and harvestmen (Araneae and Opiliones) increased by 142% and bees and wasps (Hymenoptera) by 180%. Continuing with the whoa! Going down to the detritivore groups, wood lice (Isopoda) went up by 116% and springtails (Arthropleona) by 58%. Of course, not all animals fit into a single trophic group, but as you probably guessed by the trend you’ve been seeing, those groups went up too. Slugs and snails (Gastropoda) by 103% and mites (Acari) by 57%. That’s quite a list, but not everything increased. There were some non-significant groups including flies (Diptera), rove beetles (Staphylinidae), and springtails (Neelipleona and Symphypleona). It is also important to note that these numbers are from enhanced compared to removed plots. The present plots had increases in many of the same categories, though not as dramatic.

In scientist-speak these results showed that “the manipulation of a single sub-dominant plant species causes substantial changes in the abundance and diversity of organisms across four trophic levels in individual plots in a complex grassland community.” Basically, they demonstrated that grasses went down and just about everything else went up just by the addition of one, small and less numerous hemiparasite. This one little plant species caused striking (their term), rapid changes in the community. The less obvious effects of yellow rattle were how the effect was actually occurring. Was it on plant community composition or plant quality or stand height? The authors cite a greenhouse study that showed insects to prefer parasitized to unparasitized grasses when given the choice. Add to that that it is known that parasitic plants impact nutrient cycling, especially enhancing the nitrogen content of vegetation overall. This suggests that it may be plant quality that is the driving mechanism. So the observed, large increase of sap-sucking invertebrates, particularly the Hemiptera, with altered plant quality makes sense as does the increase in detritivores with newly nutrient-rich litter. At this point, perhaps you have considered the impact of these plant-attacking invertebrates as well. Good on you. They are probably themselves doing some damage on and affecting change on the plant community, but this work doesn’t go into that. Time for another study I think!


Hartley, S., Green, J., Massey, F., Press, M., Stewart, A., & John, E. (2015). Hemiparasitic plant impacts animal and plant communities across four trophic levels Ecology DOI: 10.1890/14-1244.1



(image via Urban Butterfly Garden UK)

Not So Simple: Social Evolution in Silk-Weaving Ants

Silk weaving ants. That in and of itself is really neat. Then you see this picture of Polyrhachis shattuck...I mean, look at her! How many cool points can one animal rack up? A new study in Behavioral Ecology and Sociobiology takes a look at these arboreal nesting and silk-weaving ants.

Let's begin with sociality. It is one of those subjects in biology that is considered its own discipline. When you think of social animals you probably think of herds of mammals or maybe schools of fish. Sociality reaches its peak in eusociality, a surprisingly complex and truly social organization. These animals live in groups, cooperatively care for juveniles, divide labor, and overlap in generations. Studies of these social systems has shed light onto broader concepts of collective decision making, even leading to advances in our own technology (traffic flow, communications networks, internet searches, etc.). However, as much as we know about the social mechanics, we know very little about the evolution of such systems.

Most eusocial animals are found in the phylum Arthropoda, with the order Hymenoptera being the largest and most well-known, and all ants classified as eusocial species. And ants are the focus of today's chosen study. Ant larvae spin individual protective cocoons of silk and, depending on the species, that silk is either allocated to the colony or sequestered for the larvae’s individual needs. Donated silk is used by the worker caste to weave together leaves into nests. The “lowest grade” of arboreal (a.k.a. tree-dwelling) ants, Dendromyrmex, have larvae that produce silk without any interaction or provocation from the worker ants. In “intermediate grade” ants, Camponotus senex and Polyrhachis ?doddi (re-described as Polyrhachis robsoni (Kohout 2006)), the workers hold larvae at the work site and, with simple ritualized behaviors, the workers collect the silk. In the “highest grade” genus, Oecophylla, the larvae donate their silk supply to the colony. A worker ant will use highly ritualized behaviors - bring the larvae to the work site, straddle a leaf seam, use antennae to tap the head of the larva (telling it to extrudes silk from its salivary glands), use silk to glue together the seam, repeat.

A comparison of ant genera in this way, simple to complex, is thought to represent possible evolutionary steps in nest-weaving behavior. However, molecular sequence data suggests that nest-weaving has evolved independently in each of the four genera in which it occurs. This new study focuses on Polyrhachis ants. This genus offers good within-taxa comparison of multiple life strategies as different species vary in their nesting locations, from intertidal to subterranean to arboreal, the presence of nest construction, and even silk sources.

A total of 37 specimens of ants from all 13 currently recognized (*grr*, an often frustrating term in insect taxonomy) subspecies and five outgroup taxa were used for this study. The researchers isolated total genomic DNA and amplified and sequenced DNA from six fragments using specific primers for each gene region. After they collected the sequences, they analyzed and aligned them using computer programs. In their complicated analysis (they used Bayesian…that always makes my eyes cross) they input the gene level data along with variables of nesting preference (ground = soil, logs, stones vs. arboreal = twigs or leaves in trees) and nest construction (silk weaving vs. no silk vs. other silk). All of this allowed them to construct phylogenies (like an evolutionary family tree) and infer relationships among the species and ancestral states for behavior.

Their results showed robust phylogeny with strong support for the monophyly of the genus Polyrhachis, further supported by the inclusions of nesting preference and nest construction.This is good because it provides a nice, solid ancestral reconstruction for the evolution of the different species and their relationships to each other. It also allows for the comparison of the different nesting strategies within the framework of evolution. The investigation turned up some very interesting results. Simply, their results do not support the stepwise evolution of simple to complex. They found that the production of arboreal silk nests is the ancestral state with at least two transitions to subterranean nesting and the loss of silk weaving as species become more derived. There is also some flexibility and reversal in the behavior. Basically, the ants evolve, abandon and then re-evolve the nest weaving practices. The loss of silk nest weaving seems to occur with the transition from arboreal to terrestrial nesting followed by the re-evolution of silk nest weaving. This suggests a strong but flexible link between nesting preference and nest construction.

To illustrate this evolve-abandon-re-evolve point, the researchers present the example of Hedomyrma, a subgenus within a larger clade of subterranean nesters. This larger clade has already lost both arborality and nest weaving. But there are 2 species of Hedomyrma (Polyrhachis argentosa and Polyrhachis fervens) that have reverted to arborality. The re-evolution of this nesting preference has come with the modification of building nests within the hollow internodes of bamboo sans silk. Another reversal pattern is seen in a third species of Hedomyrma (Polyrhachis turneri), which has larvae that retain all of their silk for their own cocoon-constructing needs. Rather, worker ants steal silk from spiders to build nests on the sides of rocks. So the nest construction characteristic is what has re-evolved, just with a different mechanism. Larval cocoons have been lost in 2 of the arboreal nest-weaving species studied, and the allocation of larval silk to colony rather than individual need is considered a more derived but decoupled characteristic of nest construction.

I think that both the flexibility and the rapid evolution (or re-evolution) of this system is what attracted me to this paper. We know that evolution is a complex concept that we often boil down to from-simple-to-complex, and in many cases it is exactly that. This study almost reads like a sequel, a what-happens-next sort of thing.


Robson, S., Kohout, R., Beckenbach, A., & Moreau, C. (2015). Evolutionary transitions of complex labile traits: Silk weaving and arboreal nesting in Polyrhachis ants Behavioral Ecology and Sociobiology DOI: 10.1007/s00265-014-1857-x


(image of Polyrhachis shattuck, Maliau Basin, Sabah via AntWiki via California Academy of Science Ant Course)

The Charge of the Crazy Ant: Chemical Warfare Between Invading Species

LeBrun, Jones, and Gilbert (2014) Figure 1A

I’ll be the first to admit that I've been a little blog-negligent lately. Even when all of the ice and snow we've gotten here on the East Coast forced me to stay inside I just binge watched shows on Netflix instead. I’m not sure what brought me out of my procrastination funk and compelled me to do a little reading and writing. If you've been following the Facebook page then you've been getting a lot of yummy sciency tidbits, but it’s time for me to get back on the hard science wagon. I think I’ll start off with a great couple of papers about ant chemical warfare.

These papers focus on invasive ants, a big problem in many regions. To really grasp one of the underlying aspects of their warfare strategies, you must first understand the basics of an invasive species. Start by recognizing the difference between a native species and an exotic species. Put simply, a native species occurs naturally (or natively) to a habitat and an exotic species does not. Exotics can come in any biological form, but they are not necessarily a problem to their new habitat (think: earthworms). It’s when an exotic species becomes an invasive species that there is a problem because invasives cause environmental, economic, and/or human health harms. The reason for this is that they did not evolve together with the ecosystem in which they find themselves. There are no checks and balances in place to curb their population growth, things like predators, parasites, and competitors. Their unnaturally large population numbers then become harmful to the native species that suddenly have to deal with and compete against them, dramatically altering the community and habitat.

It is often the case that multiple species invade a region. Throughout the rest of this post I’ll be discussing new papers by Michael Kaspari and Michael Weiser and by LeBrun, Jones, and Gilbert (specifically at the latter) that take a look at just such a case in ants. The red imported fire ant (Solenopsis invicta) first came to the United States from South America around 1930. This species is far more aggressive than your typical American ant, not only in how they like the bite the hell out you (that’s a lot of personal experience talking) but also in their predatory abilities and landscape re-engineering. Now enter the tawny crazy ant (Nylanderia fulva). This new exotic invasive species was transported to the southeastern U.S. in the early 1980s and has begun to spread.. These two species have common source assemblages, their native ranges overlapping in northern Argentina, Paraguay, and southern Brazil. Until the introduction of crazy ants, the fire ant has enjoyed an uninterrupted domination of the native grassland ant assemblages. But now that the crazy ant has arrived on the scene they are displacing the fire ants. Why is this?

Since the fire and crazy ants have overlapping native habitats, they have evolved to compete directly for resources. The tawny crazy ant easily expels the fire ant from any food items it controls, up to 93 percent of the time. Also, tawny crazy ants have often been found living inside fire ant mounts, having usurped the mound and evicted the owners. Fire ants are strong and resilient and so the crazy ants must have a strong competitive advantage.

Now, finally, we get to the meat of the post: chemical warfare. If you've been stung by a fire ant (or ants, plural, as is usually the case) then you know that they pack a wallop! They have an alkaloid venom called Solenopsin that to humans causes a painful, fiery sting, and to other ants acts as a topical insecticide. The crazy ants do not have stingers but instead possess an acidopore (a specialized exocrine gland) on the end of the abdomen that sprays their venom into a mist of formic acid. They will charge into masses of fire ants misting as they go. But the fire ants don’t just stand by idly to be sprayed with venom and die, they fight back. The fire ants “gaster flag,” extruding venom from their stingers and dabbing it onto a nearby attacking ant. Normally this would result in the death of said ant. However, LeBrun and his colleagues have observed what they are calling a “detoxifying behavior” in the attacking tawny crazy ants. In this behavior, an afflicted ant stands on its hind legs, run its front legs through its mandibles, and grooms itself vigorously, periodically reapplying its acidopore to its mandibles (check out the video!).

To test this behavior the researchers conducted a series of experiments to see if there is really a detoxifying component, to see where it is coming from, and to evaluate the species-level specificity of the behavior. For the first they staged antagonistic interactions between the two species, sealing a portion of the crazy ant acidopores, and then observing afflicted individuals for behavior and survivorship. They found that those tawny crazy ants that had had their acidopores sealed had a low survival rate (only 48 percent). However, those with working acidophores had a 98 percent survival rate, supporting the detoxifying hypothesis. The Dufour’s and venom glands (exocrine glands used for communication and defense) both duct to the acidopore in this species. To see where the detoxifying agent was coming from they applied solutions of fire ant venom and tawny crazy ant glandular products to Argentine ants (Linepithema humile), which are morphologically similar to crazy ants but do not have the detoxifying capability. These tests showed the venom gland of the crazy ant to contain the detoxifying agent. When the crazy ant’s formic acid was tested it was found to be the compound responsible for detoxifying fire ant venom.

The production and application of this antidote is a potentially costly endeavor for the crazy ants. Yes, it is the difference between life and death, but when to apply it must be considered. Why use a costly resource if you don’t have to? The authors conducted a series of ant interaction tests where they had crazy ants interact independently with eight Texas ant species including fire ants, observing when the crazy ants chose to apply their detoxifier. They found that after chemical conflict with fire ants, crazy ants detoxified themselves with almost 7 times more frequently than the average response to other ant species. This suggests that this detoxifying behavior is specifically adapted to competition with fire ants, and it is probably a key factor in the displacement of invasive fire ants now underway in the southern United States.


LeBrun, E., Jones, N., & Gilbert, L. (2014). Chemical Warfare Among Invaders: A Detoxification Interaction Facilitates an Ant Invasion Science, 343 (6174), 1014-1017 DOI: 10.1126/science.1245833


Kaspari, M., & Weiser, M. (2014). Meet the New Boss, Same as the Old Boss Science, 343 (6174), 974-975 DOI: 10.1126/science.1251272


U.S. Fish and Wildlife Service's page on Invasive Species
The University of Texas at Austin Fire Ant Project
Texas A&M AgriLife Research Extension page on Tawny Crazy Ants

Pinpointing the Pollen: Honeybees and a Host Jumping Virus

Lately I've been revisiting some of my past topics and continuing the story with new research. Such is the case today. A relatively popular post of mine from 2010 called The Buzz on the Bees described a study from that year by Jerry Bromenshenk et al. investigating Colony Collapse Disorder (CCD). CCD describes the mysterious, sudden and serious die-off seen honeybee (Apis mellifera) colonies across the U.S. It is characterized by sudden colony death with a lack of adult bees in front of the die-outs. Honey stores and recent brood rearing are often evidenced, and sometimes the queen and a small number of survivor bees remain. The 2010 study found CCD colonies to contain an iridescent virus (IIV) (Iridoviridae; a DNA virus) that tracks with the microsporidia, Nosema apis and N. ceranae (specifically the latter), when compared to healthy colonies. This and previous scientific studies, using sensitive genome-based and proteomic methods, have also found small RNA bee viruses. These RNA viruses, alone or in conjunction with other pathogens, have frequently been implicated in CCD.

A new study published a couple of days ago in mBio by Ji Lian Li et al. correspondingly takes a look at the role of viruses in CCD. Evidence from previous studies shows that viruses that cause common infections in honeybees also infect other hymenopteran pollinators. A study published by Singh et al. (2010) even showed these viruses to be present and infective in pollen pellets.

I’ll conjecture that if you ask most people you’ll find that they don’t really think of plants as capable of getting viruses. That is until some disease comes and kills all of the fruit trees in their back yards. But plant viruses are like other viruses, obligate intracellular parasites, and they require a way to transmit from one plant to another. As you may have noticed, plants don’t generally get up and move around. This means that their viruses need a vector. Generally, these vectors are herbivorous insects. These insects are carriers, usually by carrying around infected pollen spreading the virus from one plant to another without themselves getting sick. To date, only a few plant viruses are known to also affect their insect vectors.

Li et al.’s new study takes a closer look at the role of pollen in virus transmission in honeybees. Initially, they carried out a study to screen bees and pollen loads of bee colonies for the presence of frequent and rare viruses. This resulted in the chance detection of a plant virus, tobacco ringspot virus (TRSV). This virus is a type species of the genus Nepovirus within the family Secoviridae, and it is known to infect a wide range of herbaceous crops and woody plants. Like other members of this genus, TRSV has a bipartite genome of positive-sense, single-stranded polyadenylated RNA molecules, RNA-1 and RNA-2, encapsidated in separate virions of similar size. For you non-biologists, this basically means that the virus has two genome segments/virus particles and can be directly translated into the desired viral proteins by the host cell. RNA viruses typically mutate very fast and are really good at working around host defenses (HIV and hepatitis are good examples of RNA viruses).

It is known that honeybees transmit TRSV from infected plants to healthy ones, but its presence in the researchers' screens got them to wondering if this plant virus could cause systemic infection in the exposed honeybees. To answer this, they collected adult worker bees, samples of the pollen being processed by a colony, and the ectoparasitic mite Varroa destructor (great name!) within the hive. They assessed 10 colonies for 1 year, classifying them as strong or weak based on the size of adult populations, amount of sealed brood, and presence of food stores. From their samples they purified virus particles from the adult bees and used them for cDNA library construction, virus-specific primer design, total RNA extraction, conventional RT-PCR, in situ hybridization, cDNA sequencing, and a phylogenetic analysis.

The observations of the colonies revealed an increase in bee deaths starting in the autumn and peaking in the winter. The researchers found both TRSV and IAPV (Israeli acute paralysis virus, common in honeybees) to be absent in colonies classified as strong, but both were found in weak colonies. Weak colonies too were found to have more multiple virus infections. These weak colonies were the ones less likely to survive through the cold winter months. Additionally, TRSV of the same strain was detected in the mites of infected colonies suggesting they obtain it from their bee hosts.

These results are the first evidence that honeybees exposed to virus-contaminated pollen can also be infected and that the infection can be systemic and spread throughout their entire body. Any host jumping is not without its challenges. In order for a virus to jump to a new host it must have the opportunity to come into contact with a perspective host, undergo genetic changes so that it may enter a new type of host cell, and gain the ability to spread horizontally between individuals within the new host populations. It seems that TRSV has been successful in overcoming all of these challenges. Its presence in the mites suggests that they could be a vector for the horizontal transmission between colonies. However, food-borne transmission (via pollen) is the most important route for transmission. Their results suggest that TRSV is neurotropic (affecting the nerves) in the honeybees, potentially causing severe functional impairment of nerves and muscles.

Do these results definitively conclude that TRSV is the cause of CCD? Well, no. But this study does add to a growing body of evidence that implicate parasites and pathogens as the key culprits.


Ji Lian Lia, et al. (2014). Systemic Spread and Propagation of a Plant-Pathogenic Virus in European Honeybees, Apis mellifera mBio, 5 (1) DOI: 10.1128/mBio.00898-13


NY Times article: "Bee Deaths May Stem From Virus, Study Says"

Also, check out these links for more information on Colony Collapse Disorder:
Mid-Atlantic Apiculture Research and Extension Consortium (MAAREC)
United States Department of Agriculture: Agricultural Resource Service
United States Department of Agriculture: National Agricultural Library
The Ohio State University's Agriculture Network Information Center's Bees and Pollination Page


(image via Wikipedia)

Dung Beetles and Ball-Rolling: Star Light, Star Bright

Lately, it seems that poo is a popular topic in science news sections, and the dung beetle seems to be up front and center. I suppose that, if you are a dung beetle, you've solved all sorts of poo-related problems. If you recall the dung beetles and ball-cooling post from November, you will remember that these insects use their dung balls to help cool off their feet on the blazing hot African sands. But what if you are a beetle that works at night? You can chuck out the hot feet problem and worry about a whole new one: navigation. A new paper published in Current Biology suggests how dung beetles may solve this navigation dilemma.

For African ball-rolling dung beetles (Scarabaeus satyrus), the best ball rolling strategy is the straight line. The straighter the path the beetle uses to roll the ball away from the dung pile the less likely it is their ball will be stolen by rival beetles. They spent all that time to pinch and roll the poo together, it is a waste of time and energy if it is stolen. Competition is fierce near the dung heap, so a quick and straight exit strategy is best. Getting that ball to roll straight isn't simply a matter of putting one tarsus in front of another, it usually involves exploiting such celestial features as the sun and the moon to orientate. However, it has been observed that many beetles will still manage to orientate along straight paths on clear moonless nights. So what are they using to help them navigate?

To answer this question, a group of researchers set up some beetles in arenas. I know, it already sounds good. On a starlit night, they placed dung beetles with their dung balls in a flattened, leveled, and enclosed circular arena.They first wanted to know how accurately the dung beetles could orientate along straight paths when they were prevented from seeing any celestial cues at all. So they made little hats for them. No kidding. They made little caps from small pieces of cardboard and attached them to the beetles' heads so that their dorsal field of view was obscured but their ventral eyes were unimpeded. Then they let them roll, filming them from above so that the rolling paths could be reconstructed and measured. The sight impeded beetles had path lengths almost 4 times longer than beetles that could see the moonless night sky. Okay, so maybe the beetles are using landmarks, like trees, to help them. To test this, the researchers made another arena that removed all visual cues (including the observer), enclosing it with a circular black cloth wall. Because they removed all observer cues, like the camera, they had to design the arena such that it could tell them when the beetles were at the edge without the researchers filming or looking. So they made the arena wall with a slightly larger diameter than the floor so that there was a gap large enough to allow the beetles reaching the edge to fall from the floor into a trough below, resulting in an audible thump sound. Since ball rolling speed is relative to path straightness (the straighter your path the faster you get to the edge), they just had to time the thumps. Clever. Under a full moon, starry night the beetles took 21.4 seconds to exit the arena and on a moonless, starry night they took a reduced, but not significantly so, 40.1 seconds. The story changes when you put the little beetle hats back on. With the caps they take a significantly longer 124.5 seconds (note: this is not significantly different than an overcast night at 117.4 seconds).

Figure 2 from Dacke et al. (2013) showing the effect of stars on dung beetle orientation

Now we know that stars are important in getting a dung beetle to roll its ball straight. Good. But we also know that most stars are too dim for tiny beetle eyes to discriminate. It is probably unlikely that the beetles are picking out constellations for their navigating needs. So what orientation information are they extracting from a starry sky? To answer this question, the researchers grabbed their beetle arena and took it to the Johannesburg planetarium, where they could manipulate the sky the beetles were seeing. Again, clever. They performed the experiments under five different conditions: (1) complete starry sky, with more than 4,000 stars and the Milky Way, (2) Milky Way only, (3) dim stars, with the brightest 18 stars excluded, (4) 18 brightest stars only, or (5) total darkness. They found that the beetles took the same amount of time to exit the arena, irrespective of whether they could see the full projection of the starry sky or only the Milky Way. This means that the dung beetles are using the bright band of light produced by the Milky Way. The Milky Way is a bright band because it is made up of stars, and when the Milky Way part of the projection (a diffuse streak of light) is removed you still see a sky where a higher density of stars defines the galaxy's axis. In this case, the beetles were still able to use the star density but it took them somewhat longer to reach the edge of the arena. It is high density of light forming into the streak across the sky that is visible, and therefore usable, to the beetles.

There is all sorts of navigating going on in the animal kingdom. Now, it appears that we've found another one. One that may be more widespread than we yet know. We just need to go looking.

Dacke, M., Baird, E., Byrne, M., Scholtz, C., & Warrant, E. (2013). Dung Beetles Use the Milky Way for Orientation Current Biology DOI: 10.1016/j.cub.2012.12.034

Some press stories on this paper:
National Geographic: "Dung Beetles Navigate Via the Milky Way, First Known in Animal Kingdom"
ScienceNOW: "Dung Beetles Navigate by the Milky Way"
The Naked Scientists: "Dung Beetles Navigate by the Light of the Milky Way"
The New Yorker: "Dung Beetles, Dancing to the Milky Way"
Wired: "Lowly Dung Beetles Are Insect Astronomers"

(dung beetle hat photo credit to Eric Warrant via the NatGeo link above)

Dung Beetles and Ball-Cooling: The Secret of the Poo

You’re a dung beetle. That isn’t an insult, it’s a visualization aid. You are a dung beetle, you live in South Africa, you roll up feces into balls, you push those balls to a storage location, and you use the balls as food or for brooding. Now, as a human visualizing yourself as a dung beetle, consider the environment you are rolling your dung ball across: the sands of the South African desert. Are your feet hot? How do you cool them down?

The authors of a new paper in Current Biology asked just these questions. The hot desert sands of South Africa can exceed temperatures of 60°C (140°F). Even for the resident dung beetle (Scarabaeus lamarcki) that’s hot. It is known that many species will seek refuges to cool down in these hot climes. For example, desert ants will spend up to 75 percent of their foraging time cooling down on elevated thermal refuges (like stalks of grass). It would make sense that dung beetles, which work under similar hot conditions, would seek refuges as they roll their poo-balls across the sand. Returning to your imagined-dung-beetle-state, what do you do to cool off?

The researchers used infrared thermography and behavioral experiments to see how dung beetles use their dung ball as a mobile thermal refuge onto which they climb to cool down. Jochen Smolka and his colleagues set up two sandy, circular, 3 meter diameter arenas in a natural South African habitat. One of the arenas was shaded in the morning to keep the ground temperature cooler. The other arena was exposed to full sunlight. They found that at the cooler ground temperatures, below 50°C, the beetles roll their dung balls straight across the arena without stopping. On the hotter ground, the beetles were observed to occasionally stop, climb up onto their ball and preen their front legs with their mouth-parts. It is likely that this preening covers the legs in regurgitated liquid, cooling them down by evaporative cooling. After the preening, the beetles perform an orientation dance and continue to roll their balls across the arena.

Fig 1. The dung ball as a mobile thermal refuge. (A) With rising soil temperature, beetles climb onto their dung balls more frequently while rolling (B) Temperature of the right front leg (red) and thorax (blue) of a beetle during its first three ball climbs (periods of rolling are grey) (C) Front leg temperature profile averaged over 84 ball climbs from 7 beetles (D) With silicone ‘boots’ on their legs, beetles perform fewer ball climbs. Similarly, beetles climb onto cool balls less often than hot balls

Ground temperature also significantly affected the frequency of this ball climbing behavior. At progressively high temperatures, the beetles climbed up on their balls more often, spending almost 70 percent of their time on top of their balls when the ground temperature went above 60°C.

So why climb balls? Answer: Ball-cooling. Infrared thermography shows that when the beetles roll their dung balls, the surface temperature of the beetles’ front legs increases by as much as 10°C, but when they climb up on their balls that temperature decreases again. That’s quite a bit, but is it really their hot feet that causes the beetles to ball climb? To this, they applied dental silicone to the beetles’ front legs. Pause: Beetle-booties, fun to say and I’m sure fun to see, and reminds me of the awesomeness that is ants on stilts. They found that these beetle-boots doubled the beetles’ ball rolling time, decreasing their ball climbing by 35 percent. This suggests that the ball climbing behavior is related to ground temperature and the heating up of beetle feet. As it turns out, the poo-balls are acting as thermoregulators in three ways:

1. They are portable, elevated platforms that can be used to escape the hot sand.

2. They are heat sinks. The moist dung ball undergoes evaporative cooling, keeping it the much cooler temperature of 31.8°C. This is substantially cooler than the beetle and the sand.

3. They are sand-coolers. Essentially, they are performing another heat sinking duty, sort of a heat vacuum, if you will. The dung ball draws the heat from the sand so it is cooler for the beetles to walk on.

If the poo-balls are actually acting as heat sinks, both during rolling and while the beetle is on it, then warmer balls should be less efficient heat sinks and the beetles should climb on them more often. The researchers tested this by giving beetles cold balls and hot balls. They found that the beetles climbed the hot balls 73 percent more often than the cold balls, supporting the heat sink hypothesis. “Because beetles roll their ball rather than drag it, the ball, preceding the beetle, cools down the sand the beetle is about to step on” by 1.5°C.

Put together, these mechanisms allow dung beetles to operate during a time of day when most arthropods, and other animals for that matter, seek a cool shelter. I guess there are a lot more good things about poo than I ever realized. And it appears that dung beetles have uncovered the secret of the poo.

Smolka, J., Baird, E., Byrne, M., el Jundi, B., Warrant, E., & Dacke, M. (2012). Dung beetles use their dung ball as a mobile thermal refuge Current Biology, 22 (20) DOI: 10.1016/j.cub.2012.08.057


Here are a few news outlets that have picked up the story:
From Wired UK "Study: Dung beetles cool their heels atop balls of poo"
Discovery News' story "Why Dung Beetles Like to Chill on Poop Balls"
LiveScience's "That's Hot! Beetles Dance on Poop Balls to Keep Cool"
The Naked Scientists' report "Beetles use dung balls to keep cool"

The Rocky Mountain Parnassius Problem

Lately, I've been thinking about butterflies. I won't subject you to the interesting, if slightly convoluted, train of thought that led me to today's paper (this post is long enough as it is), but suffice it to say that we are back on the topic of butterflies and climate change. If you remember, back in March I wrote about a paper that explored how a single climate parameter can determine population dynamics in a butterfly species, the Mormon Fritillary (Speyeria mormonia) - An Early Spring Isn't Always a Good Thing. In that case, it was how snow melt time in the first year would affect butterfly fecundity through flower abundance.

Along these lines, a preprint in the journal Ecology takes a look at how regional climate, particularly winter and winter extremes, affects annual rates of population change. We know that climate change is causing range shifts in many species. Good examples of this can be seen in high elevation, typically mountainous regions. The idea here is that a warmer climate facilitates growth in areas where a colder climate had previously prevented growth. However, this warming trend is not the only prediction attached to climate change. Variability in climate and weather and the extremes of seasons and events are also expected to have a large impact on ecological processes. This means that not only do species have to respond to general climate warming but also to general and local extremes. Long-lived vertebrate species with overlapping generations may be buffered to this because such these extreme changes act primarily on a single age class or cohort. Short-lived, univoltine (one brood or generation per year) ectothermic species have little to no buffering, meaning the entire population is affected by these extreme events.

The authors of this study use long-term (15 year) estimates of population size for 21 subpopulations of the Rocky Mountain Apollo butterfly (Parnassius smintheus Doubleday) in Alberta, Canada. This species is common in the alpine meadows of the Rocky Mountains of North America. They are known to overwinter as pharate larvae inside the egg, hatching in May, feeding on their obligate host plant (Sedum lanceolatum), pupating in late June, emerging as adults in late July, and the females ovipositing on their host plant through August. Although they are common, they tend to occur in relatively small subpopulations, having limited dispersal, which makes them good for metapopulation studies and studies of local changes. The researchers estimated population size in each subpopulation using mark-recapture data. The climate variable they chose was the Pacific Decadal Oscillation (PDO) index, an index shown to have strong correlations with their chosen study site. This index “contrasts the spatial distribution of sea temperatures between the northeastern and northwestern Pacific Ocean after correction for mean global temperature…providing a single integrative measure of climate across western North America through its strong temporal correlation with both temperature and precipitation.” A positive PDO means that warm water is along the coast and are associated with warm, dry years inland. A negative PDO means that cooler water lies along the coast and are associated with cool, wet years. They used both annual PDO as well as seasonal PDO values corresponding to stages of the life-cycle that were of particular interest. Then they ran some models that I won’t go into (I’ve used up a lot of space and I haven’t even gotten to the results yet!).

These models showed that “more frequent climate extremes pose important consequences or animal population growth affected by climate.” They found that winter values of the PDO were a strong predictor of annual population growth. The effects of climate in these butterflies was found to be curvilinear wherein both extremes (too warm and too cold) result in population decline. This suggests that the variability and extremes predicted by climate change models will greatly affect the population dynamics of species such as this and that there may be less opportunity for them to adapt to general climate warming as the occurrences of these extremes increases. Additionally, the curvilinear nature of these results suggests some complications in the mechanisms involving range shifts. Their data support range shifts (either poleward or elevational) in that climate warming may sustain a positive population growth, although low latitude and low-elevation range margins might be affected more causing negative growth.

Are these results applicable to all species? No. P. smintheus is an alpine species that is naturally subjected to a cold, unpredictable environment, and, as such, they exhibit several behavioral, morphological and physiological adaptations. This means that curvilinear results of the model suggest multiple climate-related factors that need to be teased out (temperature, precipitation, snow cover, snow distribution, etc.) and that the PDO index itself may have a range with extreme values on its edges. Because these are extreme factors rather than just gradual shifts in climate, conservation planning could be more difficult over the long term. The extremes themselves decrease populations and the variability shrinks geographic ranges (depending on event and climate interactions) also causing decreases. Perhaps helping to curtail the effects of the short-term weather extremes may help in the long-term. As yet it is unknown, and, as with most science, needs more investigation.

I encourage you to read the entire paper. There are additional ideas and fleshing out of these conclusions that are particularly interesting.

Roland, J., & Matter, S. (2012). Variability in winter climate, and winter extremes, reduce population growth of an alpine butterfly Ecology DOI: 10.1890/12-0611.1


There are also a couple of articles that have nice interviews with the authors:
From EurekAlert! and the University of Alberta: "Climate change isolates Rocky Mountain butterflies"
ScienceDaily's article: "Climate Change Isolates Rocky Mountain Butterflies" 


(image via GeoLocation)

Castrating the Zombie Ant

Last year I posted Attack of the Zombie Ant! A post that turned out to be quite popular. To sum it up, there are fungal parasites (genus Ophiocordyceps) that infect ants and take control of their bodies. The fungus then compels the ant to crawl up into the forest canopy and clamp down on a leaf while the fungus grows inside the body, eventually producing a hyphae and stroma (fruiting body) that grows out of the head and produces and releases spores. And repeat.

A recent paper in PLoS ONE takes a closer look the coevolution between ant colonies and these rare, specialized fungi. Broadly, the term coevolution is used to describe how two or more species reciprocally affect each other's evolution. In the case of the zombie ants, it is host-parasite coevolution. The parasite evolves to infect the host, and the host evolves to resistant to the parasite. An arms race, if you will. The virulence and defense traits of specialized parasites, such as the zombie-ant fungi, are shaped by these arms races. This is even more true in species like Ophiocordyceps that rely on host behavior for their reproductive success. On the other side of the coevolutionary coin, ant colonies (in this case Formica and Camponotus ants) are long-lived and live in high density, continuously interacting colonies. This behavior has been shown to be a type of social immunity where there is a strong selection for efficient prophylactic defenses and where ant parasites pose a limited threat to infecting an entire colony. Meaning that individual ants my die from the fungal disease but that the mortality of the colony is low.

This paper specifically looks at the trade-offs experienced by Ophicordyceps manipulating ants into dying in nearby graveyards. When the fungus compels an ant to leave their nest and die close to their host colony, many infected individuals will end up in one area, forming high-density ant graveyards that may persist for years. Considering the life span of your average ant, that is a long time. The authors of this paper used data from previous studies of O. unilateralis in Thailand and collected a new data set from O. camponoti-rufipedis from Brazil to construct a developmental-stage-structured model describing this ant-fungus interaction. In this new collection, they identified ants infected with O. camponoti-rufipedis and marked areas covering entire graveyards, tagging all dead infected ants. Each cadaver ant was then characterized in terms of parasite development: (1) freshly killed ant, (2) dead ant with parasite stroma, (3) dead ant with mature fruiting body, (4) dead ant at stage 2 or 3, but hyperparasitized by other fungi, or (5) dead ant whose status could not be identified. To estimate the infectivity of the fungi's fruiting bodies, they collected a sample of dead ants and brought them to the lab for study. From these data, they were able to formalize the "life-cycle" of parasitized ants and calculate a growth rates and fungal developmental stage distributions of the graveyards.

The researchers found that only 6.5 percent of the O. camponoti-rufipedis fruiting bodies were effectively producing spores. Most of the dead ants that they found were sterile because they were either immature, damaged or hyperparasitized (secondarily parasite develops within a previously existing parasite). They also found that only 42 percent of the fruiting bodies were shooting spores at a particular time interval. When the apparently "healthy" cadaver ants were dissected, they found them to have been invaded by the larvae of small unidentified arthropods which may have reduced the likelihood of the fungi reaching maturity. Add to this that out of all of the ant colony members, only the foragers face the risk of encountering spores, then you end up seeing a rather low infection and transmission rate. So only if graveyards are stable or growing will infection levels be stable.

Interestingly, the authors found that the zombie-ant fungi are themselves vulnerable to attack by other parasites. Their model suggests that the stroma life stages or immature fruiting body stages are highly vulnerable to biotic attack. So much so that hyperparasitism is nearly negligible in the mature life stage. Whether that is because the mature life stage has a much more efficient immune defense or some other cause is unclear. What is clear is that hyperparasitic fungi prevent the infected zombie-ant fungus from spreading spores which, in turn, means that fewer of the ants will become zombies. This means that the rate of infections is much less than the size of some graveyards might suggest. It is known that O. unilateralis has a range of asexual stages (synanamorphs) with spores adapted for persistence or aerial dispersal. However, O. camponoti-rufipedis is known to produce a single anamorph. So the horizonatal transmission of this species may depend on the movement of the infected ants themselves. And that means all kinds of other intriguing studies may be in the works. Can't wait!


Sandra B. Andersen, Matthew Ferrari, Harry C. Evans, Simon L. Elliot, Jacobus J. Boomsma, & David P. Hughes (2012). Disease Dynamics in a Specialized Parasite of Ant Societies PLoS ONE, 7 (5) DOI: 10.1371/journal.pone.0036352


About the Authors:
Sandra Andersen's bio
David Hughes' Lab (includes zombie-ant videos on his Projects page)

Articles:
Penn State Science "The Zombie-Ant Fungus Is Under Attack, Research Reveals"
Discover Magazine "Zombie Ant Parasite Has Its Own Parasite - a Fungus that Attacks Fungi"
The Guardian "Zombie-ant parasitic fungus kept in check by hyperparasitic fungus"


(image from the Hughes Lab website)

Ants as Art

Andrey Pavlov is a photographer from Moscow, Russia and he takes wonderful pictures of ants. He spends hours setting up props and sets for the ants to explore, taking their pictures in wonderful detail. He doesn't use any digital effects on his photos, he simply uses his macro lens to capture their curiosity. Here are a few of his wonderful pictures:

View 80 more of Mr. Pavlov's ant photos here: Artist's Gallery

Story Links:
NY Daily News "Ants as art: Check out this f-ant-asy world"
Mirror "Weird ant-ics: Bugs pictured weightlifting and sewing a button"
The Telegraph "The fantasy world of ants: photographs by Andrey Pavlov"
Daily Mail "Antsy fantasy: Russian photographer creates a fairytale world with obliging insects"
Huffington Post "Andrey Pavlov, Russian Photographer, Takes Fairytale-Like Pictures of Ants"
Incrediblethings "Amazing Ant Photography"

An Early Spring Isn't Always a Good Thing

It is pretty well agreed upon within the scientific community that climate change is happening. It has become increasingly urgent that we nail down all of the individual facets of this very large change and the operation of environmental drivers. One of these is the operation of weather as a driver for population dynamics.

It is important at this stage to define weather versus climate. Weather is the state of the atmosphere at a particular time and place such as heat, cloudiness, dryness, sunshine, wind, rain, etc. Climate is the weather conditions prevailing in a place over a long period of time. So when we talk about weather, it is as a single climate driver that may have detectable effects on populations of organisms. These effects can be direct density-independent, have indirect effects on populations (act of food sources, predators, etc.), or act of population dynamics (age structure, life stages, etc.).

A new study, published in Ecology Letters, takes a look at how a single climate parameter can determine population dynamics in a butterfly species. The focal butterfly species they chose to work with was the Mormon Fritillary (Speyeria mormonia) because it is a species that is distributed throughout the North American Rocky Mountains and has non-overlapping generations. The adults of this species feed on nectar (the preferred floral nectar host is Erigeron speciosus) and young males feed from mud, dung and carrion. The females will mate once and lay their eggs singly in some leaf litter located near a host plant.The larvae will over-winter as unfed first instars and will develop into adults in about 6 weeks. Previous studies have shown that the fecundity of adult females declines linearly with their food intake with their eggs deriving up to 80% of their carbon composition from the sugars taken up by the adults. The host plants are useful in that they can be counted for flour availability which can be a broad indicator of nectar (food) availability for these females. Then add in the weather component. Frosts occurring early in the growing season will kill developing flower buds, reducing the food availability in the system. Keeping this in mind, the researchers hypothesized that snow melt time in the first year would affect butterfly fecundity through flower abundance (a delayed density-dependent indirect effect). In the year following this they predicted that snow melt time would directly affect the developing larvae. Remember, the larvae overwinter and mortality could occur due to exposure (a density-independent direct effect).

To test this, the researchers set up study sites in the Rocky Mountains of Colorado. They determined the floral hosts preferences and distribution of the hosts plants of the butterflies. They counted the blooms every other day in teach of their plots every year from 1975 to 2009 (except 1990). They also caught butterflies, took their demographics (size, sex, etc.), numbered their wings, and observed their feeding behaviors. Finally they recorded the snow melt timing within their sites.

After all kinds of population growth analysis that I'm not going to go into (you're welcome), they found that the timing of the snow melt affects the population dynamics of these butterflies both directly and through the density-dependent indirect effects on flower availability. The early snow melt reduced the flower (and therefore the food) supply, adversely affecting the butterfly population growth rate. In the second, consecutive year, the combined effects explained more than four-fifths of the variation in the population growth rate. Just a single weather parameter (in this case snow melt) can have multiple effects on population growth. This study is among the first to demonstrate these indirect effects as well as documenting the multiple effects that a single weather parameter can have on population dynamics. These researchers were able to take long-term data and apply it to data models to understand both the effects on a single species as well as understanding species interactions in the context of climate change. These types of experiments and models can help to predict changes in populations in the future and even across populations and species.

Here's the paper:


Boggs, Carol L. and David W. Inouye. (2012) A single climate driver has direct and indirect effects on insect population dynamics. Ecology Letters: published online March 14, 2012 (DOI: 10.1111/j.1461-0248.2012.01766.x)

Science Daily article about this paper:  Early Spring Drives Butterfly Population Declines: 'Ahead-of-Time' Snowmelt Triggers Chains of Events in the Mormon Fritillary Butterfly

(image from http://www.wildutah.us/html/butterflies_moths/nymphalidae/h_btfly_fritillary_mormon.html)