Laboratory studies on dispersion behaviour of adult beetles in grain. VI.—Three-dimensional analysis of dispersion of five species in a uniform bulk

Extract

The three-dimensional pattern of dispersion of five species of grain-infesting beetles was analysed in a 12-in.-cube bulk of wheat, kept in darkness at 25°C. and 14 per cent, moisture content, and built up of four layers, each of 16 three-in.-cube bags of Terylene net; the net retained the grain but allowed passage of insects. One hundred adults were placed in the centre of the upper surface of the topmost layer and after two weeks the entire bulk was broken down and the number of insects in each bag was determined. Five replicates were used for each species. More recoveries of Sitophilus granarius (L.), Tribolium castaneum (Hbst.), Rhyzopertha dominica (F.) and Cryptolestes ferrugineus (Steph.) were made from the top quarter of the bulk than from any of the lower layers. The vertical dispersion of Oryzaephilus surinamensis (L.) was homogeneous. Lateral dispersion was homogeneous with T. castaneum, R. dominica and C. ferrugineus, but with S. granarius and O. surinamensis relatively more recoveries were made from the corner units of the bulk than from elsewhere. The dispersion pattern assumed by a species in a uniform bulk is affected by locomotory activity, boundary response and intragroup stimulation. Thigmotaxis was demonstrated in S. granarius, persisting at relatively high densities; thus, following dispersal, individuals collect along habitat boundaries and then move up them as a result of intragroup stimulation. The positive response to boundaries shown by individuals of T. castaneum in isolation breaks down in crowded conditions, producing movement back into the bulk of grain and thus homogeneous lateral dispersion. O. surinamensis shows a positive response to habitat boundaries but no movement up them. Dispersal is slow in R. dominica, but the ultimate pattern of vertical dispersion is homogeneous. C. ferrugineus shows a tendency to form clumps, even in conditions free from marked physical gradients. The data are discussed in relation to ecology of the species, low-density populations, aggregation behaviour and a dynamic concept of population dispersion.