Deadly colours of caterpillars

February 18, 2000

A team of Stirling University scientists hopes to solve the mystery of why an African species of caterpillar changes colour in crowded conditions.

The African army worm is a major pest in East Africa, where it eats staple crops such as wheat, sorghum and maize as well as pasture grasses used for cattle grazing. Finding out more about the caterpillars may lead to a means of controlling them.

The Natural Environment Research Council is funding a one-year study to explore why the normally camouflaged pests are not eaten by birds when they change colour and become much more conspicuous.

The team's leader, Ken Wilson, a NERC research fellow in biology, says that when there are few caterpillars in a given area, they are camouflaged by their green colour. But during the season when they produce eggs and numbers rise, they turn jet black.

"For decades people have wondered why this colour change occurs, but nobody knows," he says.

Dr Wilson found the black caterpillars infesting even the gardens of his hotel in Tanzania and the landing strip. They were not eaten by birds despite apparently offering a plentiful food resource.

The Stirling team speculates that the colour change has evolved to make the caterpillars more conspicuous to potential predators, acting as a warning signal to remind the predators that they taste bad. This hypothesis has been dismissed in the past because predators have been seen eating locusts, even when they have changed colour.

But Dr Wilson says one of the caterpillars' main food sources is star grass, from which they build up traces of cyanide. While the caterpillars are resistant to this, cattle grazing in star grass areas have been known to fall ill or die, with plant chemistry suggesting this is from cyanide poisoning.

"Our hypothesis is that the reason you don't see birds eating the caterpillars is because of this cyanide and other chemicals inside them. They're turning black in order to say to the birds 'Don't feed on me'," he says.

"The best way to avoid being found is camouflage. But in plague conditions, you're found everywhere, so the best thing is to ensure that birds remember that caterpillars were distasteful the last time they fed on them."

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