20 SEPTEMBER 1940, Page 10


A White Migration

The group of biologists who have been pioneers into that odd corner of science, the migration of insects, took some suggestive observations during the immense inroad of cabbage white butter- flies this summer. For four continuous weeks a stream of these butterflies flew southward across a Hertfordshire common. Where they came from is not certain, but previous experience suggests that they had flown the North Sea. Why this strange movement? No answer is forthcoming, but some observers of the species say that if it were not for immigration the insects would be cleaned out by the host of parasites that show a particular fondness for the caterpillars. It happens that this week I was sent a victimised caterpillar with a selection of its murderers. They were the off- spring of the smallest of the parasitic wasps, which are about a quarter the size of a small apple pip. They destroy innumerable grubs.