Labobatory studies on dispersion behaviour of adult beetles in grain. I.—The grain weevil,Sitophilus Granarius(L.) (Coleoptera, Curculionidae)

Abstract

The dispersion of groups of adult grain weevils (Sitophilus granarius(L.)) 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 effect of population density, and temperature and moisture content of the grain, on numbers at the surface was studied by using groups of weevils in 500 g. wheat, and on dispersion in the vertical plane by confining weevils, isolated or in groups of twenty, within a thin layer of 200 g. wheat contained between vertical glass plates, through which the positions of individuals could be recorded.In grain at 25°C. and 14 per cent. moisture content, the proportion of the population at the surface was about 17 per cent. for densities of 2–100 insects in 500 g., and was only temporarily changed by adding or removing weevils at the surface; the proportion was higher at nine per cent. moisture content or at 15 or 35°C.In grain at 25°C. and 14 per cent. moisture content, the typical behaviour pattern of individuals consisted of random vertical and lateral movements, so that there was a constant turnover within a group, but at nine per cent. moisture content and 25°C. they tended predominantly to move upwards. At 15 and 35°C., isolated individuals were stationary, but upward movement was elicited by increase in density; relatively static behaviour at 35°C. is thought to be associated with the need for water conservation. The possible relevance of these results to the detection and control of pest populations and their spread in storage conditions is discussed.