Colourful or Tasty Insects - or Both?

Scientists are researching whether a caterpillar's diet makes it distasteful to birds. Biologists at Stirling University, Scotland believe that if it can determined why predators avoid certain insects, crops in many parts of the world may be saved.

The research group is focusing on the African armyworm, a crop pest in East Africa. The moth larvae is thought to develop an unpleasant taste as protection against birds, a defence it acquires from eating star grass that contains cyanide.

Researchers will also try to discover whether or not predators associate colours with an unpleasant aftertaste. The armyworm changes its hue from green to jet-black when in large groups, another possible mechanism against being eaten.

In many insect species, crowding results in a colour change. An example is the desert locust that appears green, then changes to yellow and black when crowded.

The colour-change theory is not new but has been dismissed in the past because avian and reptilian predators have frequently been observed eating both yellow and black locusts. The important aspect of the idea is that the distastefulness of the armyworm and other insects could relate to diet. If they feed on certain host plants, the chemicals ingested could make them distasteful to predators.

If it is found that birds learn to associate black with distastefulness faster than green, then this could explain why caterpillars change colour in crowded conditions. The armyworm's diet might be relevant because caterpillars in the UK that feed on grass, are constantly eaten by birds.

By examining the armyworm's lifestyle, researchers hope to discover an effective way of controlling them.