Locusts use regurgitation as a chemical defense against predators

How does regurgitation protect locusts? Experts have long observed that locusts regurgitate when attacked by predators such as ants, mantises, or birds. This behavior increases survival rates by repelling attackers.

Using the migratory locust Locusta migratoria and fire ant Solenopsis invicta as a model prey-predator system, the researchers conducted behavioral bioassays, chemical analyses, and electrophysiological experiments. Colony and individual foraging behaviors of ants showed that locust regurgitation and mealworms smeared with it significantly repelled ant attacks.

Chemical analysis identified 907 compounds in the regurgitation, but only small-molecule compounds not proteins or volatiles exerted the defensive effect. Among them, a mixture of diethyl glutarate (DG) from the salivary glands, and 3-methylindole (3MI) and 2-(formylamino)benzoic acid (FBA) from the crop, deterred ant predation.

We have known for years that regurgitation repels some predators, but what we did not know until now was the exact chemical mechanism, explains Prof. Le Kang, corresponding author of the study. These three compounds, when mixed, mediate defense by stimulating ant gustation at concentrations lower than those found individually in regurgitation.

Electrophysiological trials confirmed that the mixture evokes gustatory responses in ants chaetic sensilla on the antennae, leading to aversion. Single compounds at natural concentrations did not elicit the same effect, highlighting the synergistic role of the mixture.

The team discovered that the defensive effect relies on gustation, not olfaction. Electroantennogram EAG recordings showed no olfactory response, while single sensillum recordings SSR revealed enhanced spikes in response to the mixture.

The mixture elicits responses at lower concentrations than individual compounds, suggesting synergy DG and 3MI as primary agonists, FBA as an allosteric modulator enhancing efficacy.

This knowledge could inform broader understanding of chemical defenses in insects. Research groups are exploring similar mechanisms in other species. Our findings about the pivotal compound mixture could help model prey-predator dynamics, Kang says. For the first time, they provide evidence for regurgitation defensive role.

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