Some new findings on factors inducing diapause in Trogoderma granarium Everts (Coleoptera, Dermestidae)

Abstract

Abstract The influence of temperature, population density and nutrition, on the incidence of diapause in a genetically heterogeneous population of Trogoderma granarium was investigated. At 30°C, about 35 per cent of the larvae entered diapause when reared in isolation on wheat flour or maize flour, but no diapause occurred at 35°C. When larvae were fed on wheat or maize flour which had been simply wetted with water and dried subsequently, and kept in isolated cultures at 30°C, most showed severe retardation in growth rate and eventually entered diapause. This effect was averted by a higher temperature (35°C) and by supplementation with yeast, showing that at a diapause-promoting temperature, suboptimal diet can induce a greater proportion of diapause. Treatment of wheat flour with a mixture of chloroform and methanol produced a similar effect, but treatment with ether did not. Even when kept isolated on normal flour supplemented with yeast some larvae entered diapause at 30°C. This diapause exhibited in isolation is recognized as a density-independent diapause (DID) distinct from the well-known crowding-induced density-dependent diapause (DDD). In crowded cultures, about 90 per cent of the larvae entered diapause in normal flour at 30°C. Supplementation with yeast did not avert the DDD although it improved the rate of growth. Neither accumulation of faecal pellets nor presence of old food was a necessary condition for the occurrence of diapause in crowded cultures. A temperature of 35°C tended to avert the typical diapause under crowded conditions, but pupation of a certain percentage of larvae was considerably delayed. Based on some experimental results it is suggested that the diapause induced experimentally by mixing faecal components, such as ether extracted lipids or linoleic acid, with food may merely represent a nutrition-dependent DID rather than a true DDD, and that an explanation of the mode of action of crowding may have to be sought in a sensory or pheromonal type of interaction among individuals in a crowded population.