Caterpillars under threat metamorphasise early

Caterpillars that protect themselves from predators by regurgitating semi-digested cabbage pay for it by producing fewer eggs and developing into smaller butterflies, if they manage to survive at all.

Caterpillars of the large cabbage white butterfly regurgitate food to deter predators Credit Wikipedia/Sannse

The caterpillars of the large cabbage white butterfly – Pieris brassicae – regurgitate partially digested cabbage to make themselves smell and taste unpleasant to predators, such as farmland birds. If the caterpillar has to defend themselves from frequent attacks, this can have adverse effects, including death before metamorphosis.

Around 40% of caterpillars frequently using the regurgitation defence method will die before they transform into a butterfly – those that survive tend to grow more slowly, produce fewer eggs and metamorphose into smaller butterflies.

“Interestingly, the caterpillars that grew at a slower rate were not forced, as a result of the attack, to metamorphose prematurely,” said Dr Andrew Higginson from the University of Glasgow. “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.”

Higginson said more study is required to understand why they do this, but he believes the threat of a fatal attack may be too large for them to remain at the larval stage for too long, so they transform into a butterfly early.

“This work demonstrates that it is important to maintain the diversity of predators such as wild birds, particularly where large numbers of insects can destroy food crop,” said Dr Mike Speed from the University of Liverpool. “We now need to look at the defence mechanism of a variety of insects to understand if other species react in similar ways.”