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Caterpillar defence methods affect lifespan and offspring numbers

25 January 2011

Caterpillars regularly using a defence mechanism to deter predators not only produce fewer eggs but also grow more slowly and often die earlier, according to research published today.

Scientists from the Universities of Liverpool and Glasgow studied the caterpillars of large 'cabbage white' butterflies (Pieris brassicae). They found that the caterpillars regurgitate semi-digested cabbage to make them smell and taste unpleasant to predators, but if they have to defend themselves from frequent attacks this has an adverse effect.

Pieris brassicae caterpillar. Credit - University of Liverpool

The study suggests that natural predators, such as farmland birds, do not have to eat large numbers of these caterpillars to have a significant effect on the population size. Despite surviving predator attacks, around 40% of caterpillars frequently using the regurgitation defence method die before transforming into a butterfly. Those that do survive grow more slowly, produce around half the number of eggs produced by a larger caterpillar, and apparently choose to metamorphose into smaller butterflies.

Dr Mike Speed, from the University of Liverpool, explains, "Research has shown that large insects produce more eggs than smaller ones. This is commonly assumed to always be the case, but we have found that those that regurgitate food as a defence against predators, have fewer eggs, similar to the numbers of offspring smaller insects have. We also found that these insects grow at a slower rate and even those that successfully change into a butterfly, are smaller than normal."

Dr Andrew Higginson, from the University of Glasgow, said, "Interestingly, the caterpillars that grew at a slower rate were not forced, as a result of the attack, to metamorphose prematurely. They could have fed for longer, grown larger and produced more offspring, despite the daily use of their defences, but they appear to 'choose' to change into a smaller butterfly. More study is required to understand why they do this, but it could be that the threat of a fatal attack is too large for them to remain at the larval stage for too long and prompts them to transform into a butterfly early."

Dr Speed added, "This work demonstrates that it is important to maintain the diversity of predators such as wild birds, particularly in areas where large numbers of insects can destroy food crop. We now need to look at the defence mechanisms of a variety of insects to understand if other species react in similar ways."

The study, funded by the Natural Environment Research Council, is published in the Journal of Animal Ecology.

Further information

Samantha Martin
Senior Press Officer
University of Liverpool
Tel: 0151 794 2248
Mob: 07970 247836

NERC Press Office
Natural Environment Research Council
Polaris House, North Star Avenue
Swindon, SN2 1EU
Tel: 01793 411561
Mob: 07917 557215

Notes

1. Dr Andrew Higginson is now at the School of Biological Sciences, University of Bristol.

Press release: 04/11

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