The dispersion behaviour of Tribolium castaneum (Hbst.) and Cryptolestes ferrugineus (Steph.) was studied under controlled laboratory conditions. The effect of density, temperature and moisture content of grain on surface numbers was studied using groups of insects initially placed centrally in jars containing 500 g. wheat and vertical dispersion was studied by observing groups of insects in a thin plate of grain held vertically between two glass sheets. The underlying behaviour of group dispersion was studied, using isolated individuals. Over the range of conditions used (15–35°C, and 9–17 per cent. moisture content), 14–49 per cent. of groups of Tribolium and 0·2–4·0 per cent. of Cryptolestes appeared on the surface. Surface numbers in both species were depressed at each end of the temperature range. In the middle of the temperature range, more individuals of Tribolium were found on the surface in grain at 17 per cent. moisture content than at 9 per cent., the reverse being found with Cryptolestes.

Analyses of locomotory activity in Tribolium showed that in all conditions females moved around less than males. For both sexes, activity was relatively depressed in the driest grain and at the lowest temperatures. Change in position within the grain was almost totally absent at 35°C. although locomotory activity remained relatively high. In Cryptolestes, locomotory activity was likewise depressed at the lowest temperature but both sexes moved around apparently at random in grain at 14 per cent. moisture and 30°C. In grain at 9 per cent. moisture, nearly all individuals became inactive within 24 hours. These results are discussed in relation to the onset and detection of infestation in grain and to the associations of grain-infesting species.