Adult size and egg number are reduced when Large Cabbage White caterpillars use their defence against predators

Background

Many species of small herbivores, including some that are major agricultural pests, use chemicals from their host plants to defend themselves against their predators. By lowering predation rates such defences could limit the impact of natural enemies, such as farmland birds, on populations of insect herbivores. However, there may be costs of such defences that affect herbivore growth and reproduction. One such method of defence, used by the caterpillars of the large white butterfly Pieris brassicae among others, is to regurgitate semi-digested cabbage leaves so that they smell and taste unpleasant.

Findings

In a laboratory experiment, we simulated attacks on caterpillars to make them regurgitate and then measured their survival, growth, and number of eggs in female butterflies. The results showed that the act of regurgitation reduces survival and growth, so that individuals that were made to regurgitate frequently were less likely to metamorphose and turned in to smaller adults. Importantly, frequent regurgitation caused females to have fewer eggs, and this was even true for individuals that managed to reach a large size. Researchers have frequently shown in the lab that large female insects have more eggs and this is commonly assumed to be always true. Our results show that in the real world that contains predators this might not be the case.

Implications

Our result also suggests that natural enemies need not actually kill insects to have strong impacts on their population growth rates, because due to defending themselves as caterpillars the adults have fewer offspring. Hence, predators such as farmland birds might have a much more beneficial impact on agricultural pests than can be appreciated by just measuring how many insects they consume. This study suggests that the loss of diversity of natural predators in semi-natural ecosystems might be even more economically costly than is currently appreciated.

Subject

Behavioural ecology


Subject Group

Zoology and Ecology


Keywords

Predation

Antipredator defence

Gregarious caterpillars

Regurgitation


Posted by

Andrew Higginson

on Fri Sep 01 2017


Article ID

FKZ4HCHMY


Details of original research article:

Higginson AD, Delf J, Ruxton GD, Speed MP. Growth and reproductive costs of larval defence in the aposematic lepidopteran Pieris brassicae. Journal of Animal Ecology. 2011;80:384-392.

Preceded by:

Changes in the timing and size of animals when they metamorphose can depend on the defences they can use against predators

Posted by: Andrew Higginson Posted Fri Sep 01 2017


Followed by:

Animals reduce their use of costly defence against their predators when living in groups

Posted by: Andrew Higginson Posted Fri Sep 01 2017


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