Tag Archives: pest

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.

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Worms Have Mothers Too: Meet the Heteroderids

by Joseph DeSisto

Today’s story comes from another group of agricultural pests — not insects this time, but cyst-forming nematodes in the family Heteroderidae. What are nematodes, you ask? Nematodes are one of many animal phyla referred to as “worms.” These worms in particular are unsegmented, surrounded by a hard cuticle or exoskeleton, and often microscopic.

There are around 25,000 known species of nematodes, but as many as a million are likely undiscovered. Some are free-living and eat bacteria or other tiny animals, but most are parasites either on animals (including humans) or plants. The heteroderids belong to the last category — here are some:

A whole bunch of nematode cysts, among potato roots. Photo by Xiaohong Wang, in public domain.

A whole bunch of potato cyst nematodes, Globodera pallida. Photo by Xiaohong Wang, in public domain.

The wormy-looking things are not worms at all, but the roots of a potato plant. Instead, the worms are packed away in cysts, the tiny yellow spheres nestled among the roots.

Within each of these cysts is a cluster of eggs which, when conditions are right, will emerge as worms. If conditions do not support hatching, the eggs can survive in these cysts for up to 20 years! Below is a recently hatched cyst nematode, beside an unhatched egg. This image was taken with an electron microscope — the colors do not reflect the colors of the actual animals.

The soybean cyst nematode (Heterodera glycines). Photo from Agricultural Research Service, in public domain.

The soybean cyst nematode (Heterodera glycines). Photo from Agricultural Research Service, in public domain.

Although called “cyst nematodes,” the heteroderids are not unique in forming cysts. Lots of nematodes do it — the reason it’s so important to cook pork thoroughly is that parasitic nematode cysts can survive in meat long after the pig has been slaughtered. What’s unusual about many heteroderids is how and when they form their cysts, but to appreciate that we need to start at the beginning.

For now, let’s consider the soybean cyst nematode, the species in the image above (see Davis and Mitchum 2005). When the nematode first hatches, it is microscopic and barely small enough to burrow into the root. Once inside, it secretes chemicals that trick the plant into forming special tissues where the worm can feed.

After reaching a certain size, a male soybean cyst nematode leaves the inside of the root and migrates over the surface, looking for mates. The female is lazier — not leaving her special feeding site, she eats and grows until her body expands so much that her rear end bursts out of the root, exposed to the soil. At this point she has eaten so much that if you were to dig up the soybean plant, you could see her exposed body without a microscope.

The soybean (Glycine max) is one of the most economically important crop plants in the world -- and the most important food plant for the soybean cyst nematode. Photo by Harry Rose, licensed under CC BY 2.0.

Originally from East Asia, the soybean (Glycine max) is now one of the most economically important crop plants in the world — and the most important food plant for the soybean cyst nematode. Photo by Harry Rose, licensed under CC BY 2.0.

She’s not done yet, though. With her rear end now outside the root and able to swell freely, the nematode “lays” her eggs into a jelly-filled cavity, still inside her body. This hardens to form an egg sac until finally, the nematode herself dies and her dried cuticle becomes the cyst’s protective skin. For the female cyst nematode, laziness becomes the biggest sacrifice a mother can make. Her eggs, numbering up to 400, can now survive for years until it’s time to hatch.

Soybean cyst nematodes can be pretty damaging to crops, but pale in comparison to the potato cyst nematode (Globodera pallida — see what I did there?). Nicol et al. (2011) estimate that just in the United Kingdom, yearly losses to pallida total roughly £50 million. Fields infested by the potato cyst nematode have reported crop losses of up to 50%. That’s a lot of potatoes. Thanks to strict quarantine protocols, the worm has not successfully invaded the United States, but pallida is so scary that a small outbreak in Idaho caused Japan to boycott U.S. potatoes for decades until imports were resumed in 2006.

Controlling nematodes is not easy, and this is especially true for cyst-formers. Not only are the cysts extremely resilient, the adult worms are hard to monitor and control as they spend their lives sheltered inside the roots of their host plants. Conventional pesticides, even those designed to kill nematodes, are seldom effective. Including all species, nematode damage to crops worldwide is estimated to cost $80 billion every year (Nicol et al. 2011).

A newly hatched cyst nematode, Meloidogyne incognita, entering the root of a tomato plant. Photo by William Wergin and Richard Sayre, licensed under CC BY 2.0.

A newly hatched cyst nematode, Meloidogyne incognita, entering the root of a tomato plant. The colors aren’t real — this was taken with an electron microscope. Photo by William Wergin and Richard Sayre, licensed under CC BY 2.0.

Modern science, however, may provide solutions. Last year the complete genome of the potato cyst nematode was sequenced, giving scientists a first look at the role genes play in the worm’s complex life cycle (Cotton et al. 2014). Cotton et al. were able to isolate the genes responsible for several key proteins, each vital to the ability of cyst nematodes to invade, manipulate, and feed on their hosts. Hopefully these discoveries will pave the way for newer, more effective methods of controlling pallida and its relatives.

For a more technical description of the heteroderid life cycle, I recommend Davis and Mitchum’s 2005 paper in Plant Physiology. If you enjoy reading about life histories and especially chemical ecology, you will find I have barely scratched the surface of what there is to know about these strange and amazing creatures.

Cited:

Cotton J.A., C.J. Lilley, L.M. Jones, T. Kikuchi, A.J. Reid, P. Thorpe, I.J. Tsai, H. Beasley, V. Blok, P.J.A. Cock, S.E. den Akker, N. Holroyd, M. Hunt, S. Mantelin, H. Naghra, A. Pain, J.E. Palomares-Ruis, M. Zarowiecki, M. Berriman, J.T. Jones, and P.E. Urwin. 2014. The genome and life-stage specific transcriptomes of Globodera pallida elucidate key aspects of plant parasitism by a cyst nematode. Genome Biology 15(3):

Davis E.L. and M.G. Mitchum. 2005. Nematodes: Sophisticated parasites of legumes. Plant Physiology 137(4): 1182-1188.

Nicol J.M., S.J. Turner, D.L. Coyne, L. den Nijs, S. Hockland, and Z. Tahna Maafi. 2011. Current nematode threats to world agriculture. In: J. Jones, G. Gheysen, and C. Fenoll (Eds.), Genomics and Molecular Genetics of Plant-Nematode Interactions (21-43). Netherlands: Springer.

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.