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Manipulating insects

Herbivory insects manage to influence the stomata opening of the plant they feed on. Most do this to keep the stomata closed so they minimise the release of volatile organic compounds. These scent molecules are after all responsible for attracting the enemies; predatory insects. But the tomato leafminer (Phthorimaea absoluta) does the opposite. It forces the stomata open and uses the released benzaldehyde as a cue that it is best to lay its eggs on an uninfested plant a new study found.

It all started out with a question of why this tomato pests became so successful. It not only manages to escape predators that might keep it in check but also expands its range where it can be found. To find out how this insect manged this feat, the researchers of this latest study started with studying it. Finding that its larvae do quite well on already infested plants. However, despite larvae success on infested plants, egg laying females preferred to find an uninfested plant to lay their eggs.

This behaviour of the tomato leafminer appears to be an exception. The researchers found that egg laying females of other common tomato pests preferred already infested plants to lay their eggs on.

Scent signs

Subsequent analysis of the volatiles the infested plants released showed that those released after a tomato leafminer infestation were different from those released by plants infested with other herbivory insects. Suggesting that the key to the leafminer’s behaviour might be in the scent bouquet that is released.

Comparing this scent bouquet from infested and uninfested plants identified multiple potential signalling compounds. But analysing their effect on egg laying leafminer females reduced them to two compounds: Benzaldehyde and (Z)-3-hexenol. Of those, only benzaldehyde turned out to affect the egg laying female leafminers in the more nature realistic setting of a wind tunnel.

Having identified the volatile compound that influences the insect behaviour, next up was to identify what caused the plant to release benzaldehyde in the first place. Surprisingly this was not due to making more of it. Looking in more detail to the genes whose activity is affected by the leafminer infestation the researchers found that the activity of seven stomata related genes were different compared to the uninfested controls. One of these, IQM1, is encoding a protein known to influence stomata opening.

Open stomata

To confirm if stomata opening is indeed specifically affected by the tomato leafminer infestation, the researchers checked the stomata opening of plants infested with this and other herbivory insects. However, in contrast to the tomato leafminer, those other analysed herbivory insects did not cause stomata to open.

To study the effect of this forced stomata opening, the researchers created tomato plants without a functioning IQM1 gene. Those plants grew normal. But during a tomato leafminer infestation they did not open their stomata like tomato plants with a functioning IQM1 gene. With as result; the egg laying adult tomato leafminers did not go in search for an uninfested plant. They were laying their eggs on the infested one. This caused the food supply of the larvae to exhaust quicker and the death of the larvae. Resulting in a decrease in population growth.

So, the tomato leafminer learned to recognise the release of benzaldehyde by its host plant as a sign to move to an uninfested one. Preventing larvae dead by exhausted plants. It appears that it even manipulates its unwilling host to release this sign. Which molecules in the insect saliva are responsible for this manipulation however still needs to be found out.

Literature

L. Chen, X. Li, Y. Wang, et al. Herbivory-Induced Stomatal Opening Triggers the Dispersal of an Invasive Insect Herbivore. Advanced Science (2026): e77207. https://doi.org/10.1002/advs.77207

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