Laboratory studies on dispersion behaviour of adult beetles in grain. XII.—The effect of isolated pockets of damp and mouldy wheat onCryptolestes ferrugineus(Steph.) (Coleoptera, Cucujidae)

Abstract

The effects of pockets of damp wheat upon the spatial structure (dispersion) of experimental populations of adults ofCryptolestes ferrugineus(Steph.) were investigated in the laboratory. Isolated pockets of non-mouldy wheat of 18 per cent, moisture content (85 per cent. R.H.) and of equally moist wheat supporting a mould flora (mainlyAspergillus candidus) were placed in a larger bulk of wheat of 14 per cent, moisture content (70 per cent. R.H.) at 25°C. One hundred beetles were released at the centre of the top surface of the bulk, and their dispersion within it was observed one week later. The method used allowed the entire bulk (25 kg.) to be quickly broken down so that the relative numbers of individuals in each of the 64 cubes of which it was composed could be recorded. Using other apparatus, the responses of single, isolated individuals to these physical conditions were analysed.Insects reared at 25°C. and 70 per cent. R.H. accumulated to an equal extent in the pockets of damp wheat irrespective of whether it was mouldy or not. Insects in a preferendum arena went to the drier side,i.e., to 70 per cent, as opposed to 85 per cent. R.H., and the underlying mechanism was found to be a klinokinetic response to humidity. Maintaining insects at 40 or 85 per cent. R.H. for 14 days before testing did not alter their dispersion behaviour in bulks of grain; maintenance at 40 per cent. R.H. did not alter their response to humidity in a preferendum arena, but at 80 per cent. R.H. it was abolished.A study of oviposition behaviour showed that when there was a choice between wheat of 18 and 14 per cent, moisture content, nearly all the eggs were laid in the damper grain, both when it was mouldy and when it was not. It is considered that oviposition requirements, and to some extent trophic behaviour patterns, over-ride the hygrokinetic response when wheat is damp or damaged, but that under field conditions, where the presence of dust and broken grain throughout the bulk provide conditions suited for oviposition and feeding, accumulations due solely to hygrokinesis may occur in the drier parts of a bulk.The results are discussed in relation to the ecology of the species in grain and to its detection and control; and evidence from this and other studies is discussed in relation to the dynamic nature of spatial organisation of insect populations in grain.