Plants Can Turn Caterpillars Into Cannibals Using A Chemical Curse

The Beet armyworm is a cannibal in waiting. batjaket/Shutterstock

We might often think of plants as peace-loving things that passively sit around waiting to get eaten by a caterpillar, but some plants have developed the most ingenious and devilish way to deal with their herbivorous enemies.

New research has demonstrated how some plants avoid getting eaten by pesky herbivores – they force them into cannibalism. The study, published in the journal Nature: Ecology and Evolution, shows how tomato plants (Lycopersicon lycopersicum) produce defensive chemicals that prompt a species of caterpillar, the beet armyworm, to eat each other.

When the caterpillars begin to feast on the plant's leaves, it oozes out an organic compound called methyl jasmonate. This chemical makes the plant taste really bad to the pest, forcing them to resort to cannibalism for their food intake.

"Many insects are known to become cannibalistic when the going gets tough," study author Professor John Orrock said in a statement"From the plant's perspective, this is a pretty sweet outcome, turning herbivores on each other. Cannibals not only benefit the plant by eating herbivores, but cannibals also don't have as much appetite for plant material, presumably because they're already full from eating other caterpillars."

The methyl jasmonate also has another effect. When it’s secreted by the plant, it acts like an emergency siren to its other neighboring plants, warning them that the enemies are at the gates. This prompts the other plants to invest in their own defenses and start producing their own stocks of methyl jasmonate.

It might seem strange that the caterpillars are so quick to turn on each other and become cannibals, but as the researchers explain, cannibalism is a double-edged sword that can be surprisingly helpful when the going gets tough.

“A cannibal that eats a victim has essentially found ‘the perfect meal,’” Associate Professor Bret Elderd said in a statement. “If I’m a cannibal, my prey has all the protein ratios and micronutrients that I need, because it’s essentially me. The disadvantage, however, is that my victim is also the perfect host for any suite of pathogens or parasites that would also like to feast on me. The thought here is that I shouldn’t be cannibalistic because I have a high probability of contracting a particular disease if I feed on an organism of my own species.”

In this light, the researchers now want to find out whether this cannibalism among insects actually helps or hinders the spread of pathogens. If it does, the plant’s defense mechanism could prove to be even more effective than we ever thought.

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