Studies on the Dermestid Beetle, Trogoderma granarium Everts. II.—The Occurrence of Diapause Larvae at a Constant Temperature, and their Behaviour

Abstract

The development of the khapra beetle, Trogoderma granarium Everts, and the cycle of movement of insects between the food and hiding places has been studied at 30°C. The results have been correlated with observations at maltings, its chief habitat in Britain.In populations at 30°C., many larvae delay pupation and enter diapause, the remainder spending most of their time in the food and completing development quickly. The diapause larvae leave the food in the fourth and in later instars in search of hiding places. Disturbing the food stimulates the larvae to leave it a little earlier. Larvae resting in diapause moult occasionally, and at long and irregular intervals they emerge, presumably to replenish their food reserves by feeding, after which some pupate and the rest hide again.The hidden diapause larvae can persist for a very long time; in cultures, 16 per cent, remained after four years. Apparently, as the food stored in the fat-body is depleted, they become readier to emerge from the crevices to feed. Replacing stale food by new food causes some larvae to emerge and the effect of disturbing the larvae is variable. After feeding, some of these larvae complete development and the resulting adults breed, presumably producing both quick developing and diapause larvae among their offspring.When denied a hiding place, the diapause larvae remain inactive in the food without increasing their rate of pupation. However, they feed more, producing heavier beetles, an effect also obtained by regularly providing fresh food.It is concluded that when a malt store is loaded, some diapause larvae remain in the fabric of the store and others emerge and feed; after feeding, some larvae re-enter crevices and others complete development and breed; the offspring contain some diapause larvae, which accumulate in the crevices and on the walls just below the malt surface; after the malt is unloaded, diapause larvae may persist in empty stores for a number of years, because they can restore their food reserves by occasionally eating grain residues.