Effect of crowding on oviposition, development and mortality of Cryptolestes ferrugnieus (Stephens) (Coleoptera, Cucujidae)

Abstract

Laboratory experiments on crowding of the Rusty grain beetle, Cryptolestes ferrugineus (Stephens), were performed at 30° ± 0·5°C and 70° ± 5 per cent r.h. on two culture media, whole-wheat flour and wheat kernels. Oviposition on 0·5 g flour measured over the first 20 days decreased from 6·4 eggs per female per day for 1 pair of beetles per vial to 1·5 for 64 pairs of beetles; on 0·25 g wheat the decrease was from 5·6 for 1 pair of beetles per vial to 0·74 for 64 pairs of beetles. Developmental time from egg hatching to adult emergence on 0·5 g flour increased from 24·0 days for isolated larvae to 87·1 and 86·2 days for initial crowding levels of 32 and 64 larvae per vial, and from 21·2 for isolated individuals to 37·6 days for a crowding level of 32 larvae when prepupae were removed as they formed; on 0·25 g wheat developmental time increased from 28·0 for isolated individuals to 42·3 days for a crowding level of 32 larvae. The combined mortality of larvae and pupae on 0·5 g flour increased from zero for isolated larvae to 93·0 per cent and 96·5 per cent for crowding levels of 32 and 64 larvae per vial unless prepupae were removed as formed when it was only to 37·5 per cent for a crowding level of 32 larvae per vial; on 0·25 g wheat, post-oval developmental mortality rose from 6·2 per cent for isolated larvae to 98·4 per cent at 64 larvae per vial. The deleterious effect of crowding on oviposition, development and mortality was always greater in wheat kernels than in flour. The influence of crowding on the rate of increase of a population in flour is calculated. It is concluded that changes in developmental time caused by crowding produce the greatest alteration in the rate of increase, and that changes in mortality are more important than in oviposition.