Dispersion behaviour of the adult of the lesser grain borer, Rhyzopertlia dominica (F.), was investigated under controlled conditions by observing the numbers occurring on the surface, or at various depths below it, in small bulks of grain. The effects of population density, temperature and moisture content of the grain on numbers at the surface were studied by using groups of insects in 500 g. wheat, and on dispersion in the vertical plane by confining beetles within a monogranular plate of wheat held between vertical glass sheets, through which the positions of individuals could be observed.

In grain at 25°C. and 14 per cent, moisture content (70 per cent. R.H.), the proportion of a group on the surface averaged approximately six per cent, for densities of 5–100 insects in 500 g., and the proportion was higher than in grain at the same temperature and 9 or 17 per cent, moisture, or at the same moisture content and 15 or 35°C.

In grain of 14 per cent, moisture content, the level of individual locomotory activity increased from 15 to 35°C. At 25°C., locomotory activity was depressed in grain of 9 and 14 as compared with 17 per cent, moisture content. These results are discussed in relation to group formation and dispersal of R. dominica and its contact with Sitophilus granarius (L.) if both species are present in the same grain bulk.