Tag Archives: caterpillar

Nicotine, a Natural Insecticide

Nicotine, the addictive agent in cigarettes, comes from the leaves of tobacco plants (Nicotiana). Plants, of course, do not manufacture nicotine as a favor to smokers, but for their own benefit: nicotine is one of the most powerful insecticides in the world, highly effective at stopping hungry, leaf-munching pests in their six-footed tracks.

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Tobacco flowers and leaves. Photo by Joachim Mullerchen, licensed under CC BY 2.5.

Nicotine is a neurotoxin, and to understand how it works, you’ll need to understand a few things about the nervous system. Nerves are simply long, thin cells that run through your body, carrying signals as electrical impulses. Here’s the problem: at the junction between two nerve cells, or between a nerve and a muscle cell, there’s a space across which electrical impulses cannot travel. This space is called the synapse.

To keep the message going, the signal-sending nerve cell sends out an army of molecules called neurotransmitters, who boldly drift across the synapse like astronauts from space-ship to satellite. When a neurotransmitter arrives at the receiving nerve or muscle cell, it enters through a tube-shaped receptor molecule.

There are many kinds of neurotransmitters, each with its own unique receptor. Take acetylcholine, which carries signals from nerve cells to muscle cells. When you recoil from a hot stove, it’s acetylcholine that tells your muscles to get moving.

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A synapse between two nerve cells. Figure in public domain.

If a poison kept all your acetylcholine receptors closed, you would be paralyzed: your muscles wouldn’t get any signals from the nervous system. If, on the other hand, the toxin kept receptors constantly open, your muscles would be constantly trying to move. You would go into convulsions, unable to control your body. Eventually you would exhaust yourself and die.

That’s how nicotine works (Zevin et al. 1998). In small doses, like in a cigarette, it works as a mild stimulant, keeping a few more receptors open than usual. In massive doses, like when a caterpillar eats a tobacco leaf, it works like a doorstop, keeping all acetylcholine receptors wide open. The unfortunate insect convulses, contracting all its muscles simultaneously until it runs out of energy and expires.

For many years, farmers used nicotine as a pesticide, spraying it over their crops to poison any hungry insects. However, nicotine is quite toxic to mammals, including humans. It isn’t sold as a pesticide anymore in the U.S. or Europe, replaced by neonicotinoids. The new pesticides are similar to nicotine, highly effective, and work in the same way, but are safer for people (not insects, of course).

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The tobacco hornworm. Photo by Tom Murray, used with permission.

There are insects that eat tobacco leaves, and those insects have acquired an immunity to nicotine. The best-known example is the tobacco hornworm (Manduca sexta), a big, fat, bright green caterpillar that ultimately transforms into a hawkmoth. In fact, these caterpillars have evolved the ability to store nicotine in their own bodies, making themselves toxic to caterpillar-eating predators. Experiments have shown that wolf spiders normally avoid tobacco hornworms, but if the caterpillars are fed a diet lacking in nicotine, the spiders attack without hesitation (Kumar et al. 2013).

Cited:

Kumar P., S.S. Pandit, A. Steppuhn, and I.T. Baldwin. 2013. Natural history-driven, plant-mediated RNAi-based study reveals CYP6B46’s role in a nicotine-mediated antipredator herbivore defense. Proceedings of the National Academy of Sciences U.S.A. 111(4): 1245-1252.

Zevin S., S.G. Gourlay, and N.L. Benowitz. 1998. Clinical pharmacology of nicotine. Clinics in Dermatology 16(5): 557-564.

Silver and Green

by Joseph DeSisto

Sunlight in the western U.S. can be intense, and all the more so when it catches the shimmering green carapace of a jewel scarab:

A Beyer's jewel scarab from Arizona. Photo by Joseph DeSisto.

A Beyer’s jewel scarab from Arizona. Photo by Joseph DeSisto.

The regal purple legs of this beetle identify it as Beyer’s jewel scarab (Chrysina beyeri). Beyer’s jewel scarabs spend most of their lives as white grubs in rotting logs, where they slowly eat their way through the wood. When they finally emerge as adult beetles, they eat oak leaves.

Although nearly a hundred jewel scarab species are found in Mexico and Central America, only four are known from the U.S.A., all in the southeastern portion of the country. Probably the most visually stunning is the so-called glorious jewel scarab (Chrysina gloriosa), with a lime-green exoskeleton sporting thick stripes of metallic silver.

The glorious jewel scarab, also from Arizona. Photo by Joseph DeSisto.

The glorious jewel scarab, also from Arizona. Photo by Joseph DeSisto.

There is something truly awe-inspiring about a beetle in which you can see your own reflection. We know that, of course, the glorious jewel scarab is not beautiful for our own gratification, but what adaptive purpose could metallic stripes possibly fulfill?

There are likely two. First, the glorious jewel scarab feeds on the leaves of juniper trees, which are common in the beetle’s high elevation habitats in Arizona and Texas. Juniper trees have tiny leaves along narrow branches, and thin strips of sun can create a shimmering effect at the right time of day. The beetle’s shining armor may actually serve as camouflage, by mimicking the narrow rays of sunlight that flicker through the trees.

A glorious jewel scarab on its host, juniper. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

A glorious jewel scarab on its host, juniper. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

Another Arizona insect uses the same trick. The royal moth’s caterpillar (Sphingicampa) is large and green, with a red stripe on each side and metallic spines jutting out of its back. Yet on their host plants, these caterpillars are well camouflaged. How? Because the plants they live on (locust, acacia, and others) all have small leaves with tiny spaces between them. The spines on royal caterpillars mimic the spaces between the leaves, and the light that flows through them.

A Sphingicampa caterpillar, showing off its metallic spines. Photo by Joseph DeSisto.

A Sphingicampa caterpillar, showing off its metallic spines. Photo by Joseph DeSisto.

The other reason for metallic strips is less obvious. It turns out that jewel and a few other scarab beetles can be so shiny because their exoskeletons reflect a unique kind of light, called circularly polarized light. To understand that, we need to understand a bit of physics.

What we call light is actually the product of tiny packets of energy called photons, travelling through space at the speed of … light. Photons travel in waves, undulating up and down or side to side. Normally, each photon has a wave pointed in its own direction, but sometimes, all the waves are oriented the same way. In other words, all the waves are on the same plane:

When light is polarized, all photons have wavelengths with the same orientation. Figure in public domain.

When light is polarized, all photons have wavelengths with the same orientation. The red wave shows the path of photons in an example. Figure in public domain.

If all the waves are on the same plane, the light is said to be polarized. Polarized light is fairly common in nature — although humans cannot identify polarized light, many insects such as bees use it to orient themselves with respect to the sun. This allows them to navigate between flowers, and to and from their hives.

Circularly polarized light, on the other hand, is extremely rare. In this situation, waves are still on the same plane as each other, but the plane rotates. If you could see the photons as they traveled directly towards you, it would look like a single photon is moving in a spiral, but in fact many photons are moving together, each at slightly an angle to its neighbor:

In circularly polarized light, the plane on which the waves are travelling rotates. Figure in public domain.

In circularly polarized light, the plane on which the waves are travelling rotates. Figure in public domain.

I won’t belabor this — physics is not my strong suit and I would rather omit details than risk getting things wrong. The point is, it takes a very special and rare kind of surface to reflect light in this way — the surface of a jewel scarab.

That’s pretty cool by itself, but it also happens that the same beetles are one of the few animals that can see circularly polarized light. There are lenses that allow us to see the same light, and if we look at a glorious jewel scarab through one of those lenses, it will appear not green and silver, but black (Sharma et al. 2009).

This can help otherwise well-camouflaged jewel scarabs find mates. Experiments with glorious jewel scarabs have shown that they are attracted to and will fly towards circular polarized light (Brady and Cummings 2010). Closely related, but less flashy beetles show no such preference. So while a predator looks at a beetle-filled juniper tree and sees nothing but leaves and shimmering sunlight, beetles can easily spot each other as big, black, radiant spots in a green world.

The Beyer's jewel scarab. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

The Beyer’s jewel scarab. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

Cited:

Brady P. and M. Cummings. 2010. Differential response to circularly polarized light by the jewel scarab beetle Chrysina gloriosa. The American Naturalist 175(5): 614-620.

Vivek S., M. Crne, J.O. Park, and M. Srinivasarao. 2009. Structural origin of circularly polarized iridescence in jeweled beetles. Science 325: 449-451.

Living Illusions

by Joseph DeSisto

Good camouflage requires more than just color. Millions of years of natural selection have favored birds that can easily identify a brown moth on a brown background, but some species are a little more sophisticated. Lappet moths hide on tree bark – their odd shape, combined with mottled color, helps break up their outline, so visual predators such as birds have a hard time recognizing them as moths.

A lappet moth (Phyllodesma americana) from Arkansas. Photo by Marvin Smith, licensed under CC BY-ND-NC 1.0.

A lappet moth (Phyllodesma americana) from Arkansas. Photo by Marvin Smith, licensed under CC BY-ND-NC 1.0.

These moths lay their eggs on trees including birch, oak, poplar, willow, and many others. The caterpillars munch on leaves by night, hiding on twigs and bark by day. They are also well-hidden, but because they have to be able to live on a variety of different trees, each of which has a differently-colored bark, lappet caterpillars don’t have a color that matches a particular background. Instead they, like their parent moths, have bodies with distorted outlines, specifically a lateral fringe of long hairs.

On bark, this helps a caterpillar “merge” with the bark on which it rests. On a twig, maybe not so much:

A resting lappet caterpillar on one of its favorite hosts, scrub oak. Photo by Joseph DeSisto.

A resting lappet caterpillar on one of its favorite hosts, scrub oak. Photo by Joseph DeSisto.

Animals that depend on camouflage have to stay very still to avoid detection, but if they are spotted, staying still quickly becomes futile. Many animals use color to startle predators as a backup plan, the best-known example being the red-eyed tree frog. At rest, the frogs appear a solid leafy-green, but if disturbed, they quickly open their eyes. The sudden appearance of two giant, bright red eyes can be enough to startle a predator, which might give the frog time enough to make a hasty escape.

The iconic red-eyed tree frog (Agalychnis callidryas). Photo by Carey James Balboa, in public domain.

The iconic red-eyed tree frog (Agalychnis callidryas). Photo by Carey James Balboa, in public domain.

Lappet caterpillars have a similar but more elaborate trick. If a potential predator (or human finger) brushes against a caterpillar, it first flexes its body so the hairs on its back part. This reveals two striking, blood-red bands, a warning this caterpillar might be toxic.

Photo by Joseph DeSisto.

The lappet caterpillar has only to flex its body to reveal these striking bands. Photo by Joseph DeSisto.

Let’s say this doesn’t work, and the bird isn’t intimidated. If a bird’s beak (or my tweezers) pinches the caterpillar, it bends its head back over its body, revealing legs surrounded by pitch-black spots:

Photo by Joseph DeSisto.

This caterpillar doesn’t like me very much. Photo by Joseph DeSisto.

Still not startled? The lappet caterpillar has one last show. In desperation, it falls from its perch in the trees and, on hitting the ground, flops upside-down, revealing a bright yellow-and-black belly:

Photo by Joseph DeSisto.

Yellow and black are universal warning signs — many toxic animals share these colors. Photo by Joseph DeSisto.

If this doesn’t work, the caterpillar is pretty much toast. For all its show, the lappet caterpillar isn’t poisonous – at worst, some of its hairs are mildly irritating to the skin. Like the red-eyed tree frog, it relies entirely on visual illusions to ward off predators. It sounds risky, but it’s worked at least some of the time for millions of years.

Deadly Caterpillars

by Joseph DeSisto

Today’s article is about the hemileucines, caterpillars with venom-injecting spines. While most have harmless, if painful, stings, a few have venom powerful enough to kill an adult human. But before things get too dark, let’s take some time to appreciate some harmless little beasties:

First-instar caterpillars of the io moth (Automeris io), munching on some oak leaves. Photo by Shawn Hanrahan, licensed under CC BY-SA 2.5.

First-instar caterpillars of the io moth (Automeris io), munching on some oak leaves. Photo by Shawn Hanrahan, licensed under CC BY-SA 2.5.

These are the newly-hatched caterpillars of the io moth, found in eastern and central North America. As caterpillars, they like to move and feed in groups, which provides some protection against predators. The moth, with a wingspan approaching four inches, looks like this:

An adult io moth, ready to take flight and find a mate. Photo by Patrick Coin, licensed under CC BY-SA 2.5.

An adult io moth, ready to take flight and find a mate. Photo by Patrick Coin, licensed under CC BY-SA 2.5.

Not bad, right? Sadly, for all its showy appearance, io moths don’t have mouthparts and cannot feed as adults. A lucky moth dies when its energy reserves run out after about a week, if it can manage to avoid being snapped up by bats, spiders, and other hunters.

This has its benefits — a moth with no appetite can spend its time and energy mating and laying eggs as much as possible. It also has consequences — the caterpillars, to prepare for the most important week of their lives, have to eat a lot. Within a few weeks of devouring as much greenery as physically possible, an io caterpillar can go from being a half-inch-long worm to a nearly three-inch-long monstrosity, brilliant green with red and white racing stripes:

An io moth caterpillar, almost ready to spin a coccoon. Photo by Tim Lethbridge, licensed under CC BY-ND-NC 1.0.

An io moth caterpillar, almost ready to spin a coccoon. Photo by Tim Lethbridge, licensed under CC BY-ND-NC 1.0.

See the spiky, poisonous-looking tufts growing out of the caterpillar’s back and sides? Io caterpillars are indeed capable, and more than willing, to deliver a painful sting. If you brush up against these spines, the tips will break off and start to inject venom. It’s not nice, but not much worse than an encounter with stinging nettle plants which, incidentally, uses many of the same toxins. The strategy is shared by other hemileucine caterpillars, among which are some of the largest and most conspicuous moths in North America.

Southern Brazil has its own hemileucines, including members of the genus LonomiaLonomia moths are large and attractive, if perhaps a bit less flashy than their northern brethren:

An adult Lonomia moth from Brazil. Photo by Benjamint, licensed under GFDL 1.2.

An adult Lonomia moth from Brazil. Photo by Benjamint, licensed under GFDL 1.2.

Their caterpillars, too, are less conspicuous, which is why they sometimes wind up stinging people. A single sting contains only a miniscule amount of venom — not nearly enough to do any real harm. The problem comes from the fact that hemileucine caterpillars tend to feed in groups. If a person accidentally brushes up against 20 or more Lonomia at the same time, the result can be severe.

Unlike the io caterpillar, Lonomia has venom designed to do more than just irritate the skin. As soon as it enters the bloodstream, anticoagulants work to prevent the blood from clotting, while other proteins punch holes in the victim’s blood vessels. The result is violent internal bleeding. In the worst cases, death is usually the result of internal bleeding in the brain (Pinto et al. 2010).

Each of these spines contains a tiny amount of venom. Photo from Centro de Informações Toxicológicas de Santa Catarina, in public domain.

Each of these spines contains a tiny amount of venom. Photo from O Centro de Informações Toxicológicas de Santa Catarina, in public domain.

Over the last few decades, cases in southern Brazil have been on the rise, and scientists at the Butantan Institute in São Paulo have been studying the molecular aspects of Lonomia venom. The most important result is that Brazilian hospitals now have access to an antivenom that quickly halts the venom’s action.

To North Americans a venomous caterpillar, even a deadly one, might seem like an esoteric problem. Yet in parts of southern Brazil, the caterpillar causes more medical emergencies than any snake, spider, or scorpion (Pinto et al. 2010). The Brazilian Ministry of Health estimated that in 2008, for every 100,000 people in the caterpillar’s range, there were eight stings. Meanwhile, incidents have become more common as rain forest has been replaced by fruit tree plantations, which happen to provide ideal habitat for Lonomia, and the stage for deadly encounters.

A cluster of Lonomia obliqua caterpillars. Photo from Photo from O Centro de Informações Toxicológicas de Santa Catarina, in public domain.

A cluster of Lonomia obliqua caterpillars. Photo from O Centro de Informações Toxicológicas de Santa Catarina, in public domain.

Cited:

Pinto A.F.M., M. Bergerm J. Reck, R.M.S. Terra, and J.A. Guimaraes. 2010. Lonomia obliqua venom: In vivo effects and molecular aspects associated with the hemorrhagic syndrome. Toxicon 56(7): 1103-1112.

Caterpillar Medicine

by Joseph DeSisto

Meet the ornate tiger moth (Grammia ornata):

The ornate tiger moth, showing of its warning colors. Photo by Jim Moore, licensed under CC BY-ND-NC 1.0.

The ornate tiger moth, showing of its warning colors. Photo by Jim Moore, licensed under CC BY-ND-NC 1.0.

Tiger moths are some of the most beautiful in North America, with flashy warning colors to startle predators. But the real action, and the story we’re going to talk about today, takes place before the moth ever hatches from its coccoon. Here is a tiger moth’s caterpillar (different species, this time Grammia doris):

The caterpillar of the Doris tiger moth, Grammia doris. Photo by Tom Murray, used with permission.

The caterpillar of the Doris tiger moth (Grammia doris). Photo by Tom Murray, used with permission.

Tiger moths all belong to the subfamily Arctiinae, and their caterpillars are sometimes called woolly bears, a reference to their often dense coating of hair. Don’t be fooled by their fuzzy, warm appearance — those hairs are barbed and meant to protect the caterpillar against predators. Birds and other attackers find it hard to get a hold on such a hairy insect. Even if they do, hairs easily detach and get stuck in a predator’s skin, where they can be extremely irritating.

Yet irritating hairs are no defense against predators that eat from the inside out. Tachinid flies, for example, lay their eggs on the caterpillar while it is still alive. The fly maggots, upon hatching, begin to eat away at the insides of the caterpillar, growing at their host’s expense. Finally, when the maggots are ready to emerge as adult flies, they burst out of the caterpillar’s body, killing it in the process.

A typical adult tachinid fly. Photo by Tom Murray, used with permission.

A typical tachinid fly. Photo by Tom Murray, used with permission.

Once the fly’s eggs are attached, a caterpillar doesn’t have many options — unless it’s a woolly bear. At least one of these caterpillars, Grammia incorrupta, has an unusual little trick. Many caterpillars are specialists, eating only one or a few types of plant — monarchs, for example, eat only milkweed. Woolly bears are just the opposite, and will eat almost anything leafy and green, from dandelions to clover to lettuce.

Despite their catholic appetites, woolly bears understandable avoid poisonous food. When a caterpillar ingests toxins, its body has to break them down and minimize any damage. That can take a lot of energy, energy better invested in growing as quickly as possible.

All of that changes when a tachinid maggot arrives. If given the opportunity, a maggot-laden caterpillar will actually seek out toxic plants, especially those that are full of pyrrolidine alkaloids. Pyrroladine alkaloids are chemicals that many plants make — tobbacco and carrot leaves have them, for example. Like many plant-manufactured alkaloids, they evolved for the exact purpose of killing or deterring insects, but the woolly bear can use this to its advantage.

The hairs on tobacco stems and leaves are coated with droplets of pyrrolidine alkaloids, natural pesticides. Photo by Stan Shebs, licensed under CC BY-SA 3.0.

The hairs on tobacco stems and leaves are coated with droplets of pyrrolidine alkaloids, natural pesticides. Photo by Stan Shebs, licensed under CC BY-SA 3.0.

By stuffing itself with toxins, a woolly bear can kill the maggot inside its body. Experiments have shown that even though caterpillars without maggots can become sick if they eat too much toxic food, caterpillars with maggots have a much higher survival rate if they are allowed to eat food with pyrroladine alkaloids (Singer et al. 2009). As with any medicine, there is a fine balance — overdose can be fatal. But the risk is worth the reward, since otherwise, the odds of survival for a maggot-carrying caterpillar are essentially zero.

The tale of the woolly bear is one of the best-known examples of self-medication by an invertebrate. We now know, however, that many other insects seek out chemicals to help rid themselves of parasites.

Wood ants are known to gather bits of pine resin, hardened sap, to embed within the walls of their nests. The resin is toxic to many of the ants’ diseases, including harmful bacteria and fungi (Chapuisat et al. 2007). Unlike pyrrolidines, resin isn’t an insecticide, so the ants can use it as a preventative medicine, hoarding it in their nests even when diseases aren’t around (Castella et al. 2008).

A wood ant, Formica lugubris. Photo by Richard Bartz, licensed under CC BY-SA 2.5.

A wood ant, Formica lugubris. Photo by Richard Bartz, licensed under CC BY-SA 2.5..

One of my favorite insect medicine stories comes from the humble fruit fly. Alcohols are usually quite toxic to animals, even humans. Fruit flies, though, are the lead-bellies of the animal kingdom — they need to have a high tolerance for alcohol, since they eat the yeast that grows on fermenting fruit. So when they find themselves full of parasites, they drink a little extra (Milan et al. 2012).

No symbolism there. None at all.

Cited:

Castella G., M. Chapuisat, and P. Christe. 2008. Prophylaxis with resin in wood ants. Animal Behavior 75(4): 1591-1596.

Chapuisat M., A. Oppliger, P. Magliano, and P. Christe. 2007. Wood ants use resin to protect themselves against pathogens. Proceedings of the Royal Society of London B 274: 2013-2017.

Milan N.F., B.Z. Kacsoh, and T.A. Schlenke. 2012. Alcohol consumption as self-medication against blood-borne parasites in the fruit fly. Current Biology 22(6): 488-493.

Singer M.S., K.C. Mace, and E.A. Bernays. Self-medications as adaptive plasticity: Increased ingestion of plant toxins by parasitized caterpillars. PLoS One 4(3): e4796. doi: 10.1371/journal.pone.0004796

Changes

by Joseph DeSisto

The idea that an animal that looks like this:

An early-stage caterpillar of the promethea moth (Callosamia promethea). Photo by Joseph DeSisto.

An early-stage caterpillar of the promethea moth (Callosamia promethea). Photo by Joseph DeSisto.

can transform into something like this:

An adult promethea moth. Photo by Tom Murray, used with permission.

An adult promethea moth. Photo by Tom Murray, used with permission.

has captivated us for centuries. The caterpillar and moth above belong to the species Callosamia promethea, commonly called the promethea moth. Prometheas can have wingspans approaching 4 inches across, and throughout their eastern North American range they are some of the biggest moths around. The moth is truly a spectacular beast. What often goes unappreciated, however, are the changes that go on before the caterpillar even forms its cocoon.

All caterpillars have to molt several times before they are large enough to go through metamorphosis. The stages between molts are called instars, and sometimes successive instars can look very different from one another.

The first photo was of a caterpillar in its second instar, meaning it has molted once since it hatched out of an egg. At this point the caterpillar has grown from nearly microscopic to a respectable centimeter or so — now it’s ready to molt again. It does so by splitting the front of its exoskeleton and, slowly and patiently, pulling itself out:

From second to third instar: a molting promethea caterpillar. Photo by Joseph DeSisto.

From second to third instar: a molting promethea caterpillar. Photo by Joseph DeSisto.

The old, empty skin is left behind:

The left-over exoskeletons of just-molted third instar caterpillars. Photo by Joseph DeSisto.

The left-over exoskeletons of just-molted third instar caterpillars. Photo by Joseph DeSisto.

When we started rearing these caterpillars in the lab, I wasn’t familiar with their life history. You can imagine my surprise when a container full of black-and-yellow-striped caterpillars, overnight, became a container full of these charming little creatures:

A third instar promethea caterpillar. Photo by Joseph DeSisto.

A third instar promethea caterpillar. Photo by Joseph DeSisto.

Promethea caterpillars are generalists and eat leaves off a variety of woody plants — these ones are munching on black cherry (Prunus serotina).

Third (left) and second (right) instar promethea caterpillars. Photo by Joseph DeSisto.

Third (left) and second (right) instar promethea caterpillars. Photo by Joseph DeSisto.

Why do the caterpillars change so radically and suddenly? That question remains very much unanswered. Perhaps the two instars simply have slightly different lifestyles, and different lifestyles require different adaptations. Or maybe having two different-looking life stages keeps predators from developing an accurate “search image.” In other words, by the time a bird learns to recognize the second instar as prey, it changes into a new, unfamiliar caterpillar.

Tomorrow I leave to spend a week in Arizona, New Mexico, and Texas. I’m supposed to spend that time looking for caterpillars and moths although I hope to see many other interesting animals — scorpions, rattlesnakes, and giant centipedes are at the top of my list. No writing while I’m gone, sadly, but plenty of picture-taking, so brace yourselves.

Potato Wars: Meet the Gelechiids

by Joseph DeSisto

Today’s story comes from the tiny but remarkable twirler moths, the Gelechiidae. The moths themselves are small and typically brown, attracting little attention, while the caterpillars go by names like splitworm, bollworm, pinworm, tuberworm, and so on, usually referring to the plant tissues they consume.

Human interest in twirler moths, then, is largely focused on their relationships with the plants that both humans and twirlers relish — potatoes, tomatoes, grains, almonds, and many others.

The gelechiid caterpillar Chionodes pseudofondella, freshly coaxed from its silken shelter. Photo by M.J. Hatfield, licensed under CC BY-ND-NC 1.0.

The gelechiid caterpillar Chionodes pseudofondella, freshly coaxed from its silken shelter. Photo by M.J. Hatfield, licensed under CC BY-ND-NC 1.0.

Twirler caterpillars live sheltered lives — they typically either live inside twigs, fruit, leaves, or other plant tissues, or else use silk to roll leaves into shelters. This way the caterpillars can dine in relative peace, with fewer predators able to find and attack them.

The opening to a shelter in mountain mint, made by the caterpillar above.

The opening to a shelter in mountain mint, made by the caterpillar above. Photo by M.J. Hatfield, licensed under CC BY-ND-NC 1.0.

Shelters can be effective against spiders and wasps, but they haven’t stopped humans from using all kinds of strategies, from pesticides to natural predator, to keep twirler caterpillars out of their crops. With good reason — many of these caterpillars cost farmers a huge portion of their annual yield.

Today we are going to focus on one of these moths, the Guatemalan potato moth, Tecia solanivoria (GPM). GPM caterpillars feed underground by boring into a potato and hollowing out a set of tunnels, in which they live until ready to emerge as adult moths. By eating out the inside of the potato, the caterpillars not only damage the crop themselves, they also allow fungi to enter the potato and begin the decomposition process, resulting in a rotten vegetable.

The Guatemalan potato moth's caterpillar, in all its glory. Photo from Hayden et al. (2013).

The Guatemalan potato moth’s caterpillar, in all its glory. Photo from Hayden et al. (2013).

The caterpillars can feed on potatoes both before and long after harvest. As a result, they are easily spread when infested potatoes are shipped across long distances. Although GPM is native to Guatemala, it was first discovered in Costa Rica where it had been introduced in imported potatoes. Entomologists estimate that the introduction occurred in 1970, but the species was not formally described until three years later (Povolny 1973).

Why so long between introduction and description? Here I must editorialize — if Central America had had an active expert in gelechiid taxonomy, the species that was causing so much damage would surely have been discovered much earlier, perhaps even before the species ever left Guatemala! Instead the species was described by a Czech taxonomist, by which time Central American farmers were reporting 20-40% crop losses to the unnamed pest (Povolny 1973). The “outsourcing” of tropical biodiversity research to scientists in first-world countries can have dire consequences.

For reference, here's a map showing Central America and northwestern South America. GPM is native to Guatemala in the northwest, then quickly spread through Central America, Venezuela, Colombia, and Ecuador. Map data: Google, Landsat.

For reference, here’s a map showing Central America and northwestern South America. GPM originated in Guatemala (upper left) and has quickly spread southeast as far as Ecuador and Colombia. Click to enlarge. Map data: Google, Landsat.

Before 1970, the most significant potato-eating caterpillar in Latin America was the potato tuberworm (Phthorimaea operculella — try saying that out loud). Unlike GPM, the tuberworm is a leaf-miner, munching its way through the 2-dimensional world inside leaves. After its introduction to Costa Rica, however, GPM saw a meteoric rise, and quickly stole the show as the most economically injurious caterpillar for Latin American potato growers (Carrillo and Torrado-Leon 2013). In 1983 GPM was accidentally imported (via infested potatoes) into Venezuela from Costa Rica, and South America became the new front in the moth’s conquest. By 1996 the caterpillars had reached Ecuador, and in 1999 a population was even introduced to the Canary Islands off the coast of Northwest Africa.

Managing GPM is an economic necessity. Unfortunately, this can be difficult since once the caterpillar hatches and starts making its tunnels inside a potato, you’ve pretty much lost the potato. Pesticides are generally targeted toward the more vulnerable eggs and adult moths, but these stages are short-lived. Timing is crucial, and only pesticides applied at the right stage will reduce crop losses. Despite this, potato growers frequently err on the side of caution, applying pesticides intensively throughout the growing season (Carrillo and Torrado-Leon 2013). This is not only a waste of money, it can also lead to the moths becoming resistant to the pesticides, not to mention the environmental and human safety consequences of pesticide overuse.

A pinned specimen of the Guatemalan potato moth. Photo from Hayden et al. (2013).

A pinned specimen of the Guatemalan potato moth. Photo from Hayden et al. (2013).

The most effective methods of reducing GPM infestation are not chemical but cultural (Gallegos et al. 2002). Growers can help protect their plants by tilling the soil to break up clumps (havens for moths and eggs), planting their potatoes deeper in the soil, and removing “leftover” potatoes from the soil.

Here’s the twist: under certain conditions, it can actually benefit the farmer to let the caterpillars feed.

In 2010, Poveda and colleagues experimented with GPM abundance in potato fields in Colombia. Surprisingly, fields in which caterpillars were allowed to feed in moderation actually showed higher total yield than when caterpillars were completely excluded. It turns out that when caterpillars feed, the chemicals in their saliva are strewn about the potato, and the plant is able to recognize and react to these chemicals.

Flowers from a potato plant. Photo by Keith Weller, in public domain.

Flowers from a potato plant. Photo by Keith Weller, in public domain.

Each potato plant contains many tubers, the things we call potatoes. If less than 10% of these are infested with caterpillars, the plant compensates by diverting resources to the remaining tubers. The result: a greater number of extra-large potatoes, and an increase in overall yield. Plants with limited caterpillar infestations yielded 2.5 times as much useable potato mass as plants in which there were no caterpillars at all.

The lesson? Plant-insect relationships are complicated, and understanding the subtle details can matter. So can the taxonomy of small, “boring” moths. There are a lot of important things we wouldn’t know about GPM if it weren’t for the hard work and dedication of entomologists from Colombia to the Czech Republic. With invasive pests an increasing problem, we need to make sure to invest in both applied and basic research, and study even the insects that most people would rather ignore.

This post marks the start of a kind of experiment for me. Starting here, I will be posting a new article every Tuesday and Thursday. Here’s the catch: each post will focus on a different family of invertebrates, and I can’t cover the same family twice! The goal is to try and write about as many families as possible, starting with Gelechiidae.

Cited:

Carrillo D. and E. Torrado-Leon. 2013. Tecia solanivora Povolny (Lepidoptera: Gelechiidae), an Invasive Pest of Potatoes Solanum tuberosum L. in the Northern Andes. In: J.E. Pena (Ed.), Potential Invasive Pests of Agricultural Crops (126-136). Boston, Massachusetts: CABI.

Gallegos P., J. Suquillo, F. Chamorro, P. Oyarzun, H. Andrade, F. Lopez, C. Sevillano, et al. 2002. Determinar la eficiencia del control quimico para la polilla de la papa Tecia solanivora, en condiciones del campo. In: Memorias del II Taller Internacional de Pollila Guatemalteca Tecia solanivora, Avances en Investigacion y Manejo Integrado de la Plaga, 4-5 June 2002, Quito, Ecuador pp. 7.

Hayden, J.E., S. Lee, S.C. Passoa, J. Young, J.F. Landry, V. Nazari, R. Mally, L.A. Somma, and K.M. Ahlmark. 2013. Digital Identification of Microlepidoptera on Solanaceae. USDA-APHIS-PPQ Identification Technology Program (ITP). Fort Collins, CO. 7 July 2015 <http://idtools.org/id/leps/micro/&gt;

Povolny D. 1973. Scrobipalpopsis solanivora sp. n. — a new pest of potato (Solanum tuberosum) from Central America. Acta Universitatis Agriculturas, Facultas Agronomica 21(1): 133-146.

The Fork-tailed Dragons

by Joseph DeSisto

Meet Furcula borealis, the caterpillar of the white furcula moth:

Furcula cinerea, the caterpillar of a medium-sized, gray moth. Photo by Joseph DeSisto.

Furcula borealis, the white furcula caterpillar. Photo by Joseph DeSisto.

It’s a weird-looking caterpillar already, but even weirder when viewed head-on:

The fork-tailed dragon caterpillar, Furcula borealis. Photo by Joseph DeSisto.

The fork-tailed dragon caterpillar, Furcula borealis. Photo by Joseph DeSisto.

Like all insects, caterpillars have only six legs, and these are located near the front of the body, just behind the head. The sticky, climbing appendages along the rest of the trunk are not true legs but “prolegs,” which are lost during metamorphosis. In Furcula caterpillars (and the related Cerura, shown below), the last pair of prolegs are modified into long, rigid “tails.”

Cerura scitiscripta, showing six true legs near the head (upper left), four typical prolegs on the trunk, and the special modified pair of prolegs at the rear of the body -- the forked

Cerura scitiscripta, showing six true legs near the head (upper left), four typical prolegs on the trunk, and the special modified pair of prolegs at the rear of the body — the forked “tail.” Photo by Joseph DeSisto.

The furcula moths belong to the strange and beautiful family Notodontidae. These are sometimes called prominent moths, despite mostly being brown, gray, or some combination thereof. Recall, however, that in my last post about caterpillars I referred to notodontid caterpillars as the dragon caterpillars. Furcula, then, are unofficially dubbed the fork-tailed dragon caterpillars.

In case you’re wondering what F. borealis looks like as a moth, here’s an example specimen:

The white furcula moth, Furcula borealis. Photo by Tom Murray, used with permission.

The white furcula moth, Furcula borealis. Photo by Tom Murray, used with permission.

So what is the forked tail for? To find out, we simply tap our little dragon on the head. This is what happens:

Furcula borealis. Photo by Joseph DeSisto.

Furcula borealis, beginning its display. Photo by Joseph DeSisto.

Bright red tentacles begin to emerge from the  modified prolegs. In less than a second they are fully everted:

Don't mess with the fork-tailed dragon! Photo by Joseph DeSisto.

Furcula borealis, tentacles emerging from modified prolegs. Photo by Joseph DeSisto.

The fork-tailed dragon then waves these tentacles about, and even attempts to rub them onto the offending party (i.e., my finger). The “strike” reminds me of a scorpion trying to sting, if scorpions dressed up as clowns and went to birthday parties. After tapping me, the tentacles disappear as quickly as they emerged, while the caterpillar tucks its head and braces itself for another attack.

Furcula borealis. Photo by Joseph DeSisto.

Furcula borealis. Photo by Joseph DeSisto.

In a less dramatic fashion, many insects use eversible organs to rub toxins on their predators. Rove beetles are an example — this explains the way many of them run, with abdomens held high in the air like scorpions. If you grab one, it will use its flexible abdomen to rub a cocktail of nasty chemicals on you, some of which can cause blistering. But in Furcula and Cerura caterpillars, the tentacle-rubbing is actually a harmless show designed to scare off predators. I must admit, were I a foraging bird, I would think twice about attacking a caterpillar with such a bizarre and intimidating display.

And yet, in an ironic twist, these caterpillars are not harmless. If sufficiently disturbed, they can fire off a burst of formic acid — the stuff fire ants sting you with — from glands behind the head. In F. borealis, these are stored in spiky, poisonous-looking projections:

If you're mean enough, this caterpillar might just spray formic acid at you. Photo by Joseph DeSisto.

If you’re mean enough, this caterpillar might just spray formic acid at you. Photo by Joseph DeSisto.

Don’t mess with the fork-tailed dragons!

Pictures of Cats

by Joseph DeSisto

By cats I mean caterpillars, of course.

This summer, in addition to working on centipedes, I have a job working in the lab of Dr. David Wagner, an entomologist here at UConn. He studies caterpillars, which means I get to watch hundreds of different species go through metamorphosis. It’s pretty great. Today I’ll share some of my favorites — the caterpillars I give a few extra leaves every day, just because I love them.

But first, a quick lesson on caterpillar anatomy. Here’s Catocala amica, in profile:

Catocala amica, the caterpillar of a large and beautiful underwing moth. Photo by Joseph DeSisto.

Catocala amica, the caterpillar of a large and beautiful underwing moth. Photo by Joseph DeSisto.

Although caterpillars look like they have lots of legs, they actually only have six tiny ones at the front of the body, like all insects. The larger, pad-shaped structures are prolegs, and they will disappear during metamorphosis. Most caterpillars, including this Catocala, have five pairs of prolegs, but others such as inchworms (family Geometridae) have fewer.

Now let’s look at the head:

Another view of Catocala amica. Photo by Joseph DeSisto.

Another view of Catocala amica. Photo by Joseph DeSisto.

The bulbous portions of the head are not eyes — the simple eyes, or stemmata, are clustered in the lower corners of the caterpillar’s face. The head is so large because it holds all the muscles needed for what the caterpillar spends virtually all its time doing: eating. The antennae are small and project down, toward the leaf.

In case you’re curious, Catocala amica turns into a large moth, affectionately known as the friendly underwing:

The friendly underwing, same species as the caterpillar above. Photo by Ilona L., licensed under CC BY-ND-NC 1.0.

The friendly underwing, same species as the caterpillar above. Photo by Ilona L., licensed under CC BY-ND-NC 1.0.

Because a lot of caterpillars turn into butterflies, it might be easy to assume that caterpillars are the “ugly ducklings” of the insect world, a kind of lowly purgatory for insects before they transform into something beautiful. But in many cases, such as in the family Notodontidae, the caterpillar is by far the more visually stunning. Here, for example, is Notodonta torva the moth, pinned and spread as a museum specimen:

An adult Notodonta torva, a European moth. Photo by M. Virtala, in public domain.

An adult Notodonta torva, a European moth. Photo by M. Virtala, in public domain.

But in the Wagner lab, this is what I get to feed and clean up after, every day until it pupates:

Notodonta torva, as a caterpillar. Photo by Joseph DeSisto.

Notodonta torva, as a caterpillar. Photo by Joseph DeSisto.

Notodontids have some of the wildest-looking caterpillars, with strange outgrowths and protuberances giving them a dragon-like appearance. Here’s another notodontid we’re rearing, in the genus Schizura (I don’t know the species):

Schizura, species unknown (to me). Photo by Joseph DeSisto.

Schizura, species unknown (to me). Photo by Joseph DeSisto.

Schizura caterpillars are fairly opportunistic — they’ll eat almost any woody plant, unlike many caterpillars which have very specific diets (think of the monarch, which eats only milkweed).

Another Schizura, munching away. Photo by Joseph DeSisto.

Another Schizura, munching away. Photo by Joseph DeSisto.

Notodontids are awesome. In the future I think I’ll refer to them as dragon caterpillars. That isn’t their real common name — I just made it up. But of course, all common names are just names people make up. If they make their subjects sound cool, it’s nobody’s loss.

That’s it for caterpillars today, but you can bet there will be more posts about caterpillars in the future.

Although I took the caterpillar pictures in this post, thanks are owed to Dr. David Wagner, who is the proprietor of both the camera and the caterpillars. You can learn more about his work on his lab website, here.