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Thursday, 13 January, 2000, 16:08 GMT
Why caterpillars can taste bad

caterpillars Caterpillars can cause devastating crop losses

By Environment Correspondent Alex Kirby

Why does the taste of some caterpillars deter birds from eating them? It is a subject that is puzzling researchers at the University of Stirling, UK.

The team, headed by Dr Ken Wilson, of the university's faculty of natural sciences, has been given a grant of almost 35,000 by the Natural Environment Research Council.

The researchers suspect that one factor affecting the insects' taste may be their diet. They are concentrating on the African armyworm, which they think may absorb tiny amounts of cyanide from one of its staple foods, African star grass.

The armyworm is a huge problem in east Africa, where it can devastate the grain harvest.

The Stirling team is also looking at the role of colour changes in the caterpillars, to see if these also play a part in deterring predators.

Pressure of numbers

Armyworms are normally green, but under certain conditions - chiefly when they find themselves in a crowd - they change to jet black, possibly as a defence mechanism.

This is a characteristic of locusts, another species which the team has studied, and which are normally green at low densities but change in a crowd to a vivid yellow and black.

locust Locust behaviour is like the armyworm's
There is speculation that the colour change has evolved to make crowds of locusts more conspicuous to potential predators, reminding them that the insects taste unpleasant, so that they will avoid eating yellow and black locusts in future.

The only flaw in this theory is that predators have often been seen eating yellow and black locusts. So it is now being suggested that the colour change alone is not enough - predators may be deterred only if the locusts have fed on certain host plants containing chemicals which themselves taste nasty.

Many insect species change colour in response to crowding, and when armyworms do so it seems to give them no automatic protection, as birds eat them just the same. The team will see whether the caterpillars' diet is the crucial factor, as it appears it may be with locusts.

Helping predators

"The grant will help us determine whether this density-dependent colour change has evolved as a warning signal to stop them being eaten", said Dr Wilson.

"If that is the case, we might be able to find a way to stop them becoming black, and then the birds might help in a natural process of control."

masai woman Masai folklore could have a scientific basis
Dr Wilson told BBC News Online: "We think it's possible that the plants the armyworms feed on don't normally produce cyanide, but only in response to a mass onslaught by the caterpillars".

"We hope to show that this is what makes them distasteful to predators.

"According to the folklore of the Masai people of east Africa, cattle which graze on land where armyworms have been will themselves die of poisoning.

"If we're right about the plants, that story could turn out to be true - the mass of the armyworms may stimulate the plants to produce cyanide, and what's left over may then kill the cattle."

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23 Aug 99 |  Sci/Tech
Insect spotters count the cost

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