Call for help from plants before imminent pest attacks

Large White butterfly
© Giuseppe Andrea Mosca (CC BY-NC)

The deposition of eggs by insect pests has been shown to trigger the release of chemical signals from plants, attracting species to assist the plant in resisting attack. This is a valuable tool in the plant’s defences as it is initiated at the first sign of the pest, even before feeding has begun. The research was carried out on Black Mustard (Brassica nigra), a relative of cabbage, and the Large White, also known as the Cabbage Butterfly (Pieris brassicae). The eggs of this butterfly trigger specific volatile compounds, called oviposition-induced plant volatiles (OIPVs), to be released from Black Mustard.
These volatiles were found to attract two parasitic wasps: Trichogramma brassicae and Cotesia glomerata. 

Black Mustard plants call upon  T. brassicae and C. glomerata to parasitise the butterfly eggs, reducing the number that hatch and begin feeding on the host plant. Of the caterpillars that do manage to hatch from their eggs, it is likely that some will fall victim to other parasitoids, which are also attracted by the plant’s chemical signals. The research, headed by Nina Fatouros of Wageningen University, also found that OIPVs produced by Black Mustard repelled gravid butterflies, reducing the number of additional eggs that are laid on the plant. This is beneficial to the butterfly as well, as it means she can detect the plants which are free of Large White butterfly eggs, meaning her offspring will have less competition for necessary resources.

Eggs of the Large White butterfly © Dean Morley (CC BY-ND)

Analysis of the OIPVs found that they are specific to the eggs that triggered their release. Eggs are deposited by the butterflies along with an adhesive to help the eggs stay firmly on the plant. It is this substance which causes characteristic changes to the chemistry and structure of the leaf, and triggers the release of specific OIPVs.

In contrast to this the generalist cabbage moth (Mamestra brassicae), also studied as part of this research, produces a thinner and less effective adhesive. This causes fewer changes to the leaf morphology and caused different OIPVs to be released. Unlike with the OIPVs triggered by the Large White, these signals did not attract the parasitic wasps T. brassicae and C. glomerata. It is thought that the plant has less incentive to produce a response to the cabbage moth as this species doesn’t survive well on Black Mustard, so would move on soon after it hatched anyway.

This research, published in PLoS ONE last month, highlights the response of plants before feeding by pests even begins, and the affect that this has on species from different trophic levels.

References

Fatouros NE, Lucas-Barbosa D, Weldegergis BT, Pashalidou FG, van Loon JJA, et al. (2012) Plant Volatiles Induced by Herbivore Egg Deposition Affect Insects of Different Trophic Levels. PLoS ONE 7 (8). doi:10.1371/journal.pone.0043607

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