Published online by Cambridge University Press: 10 July 2009
Many mature larvae of the khapra beetle, Trogoderma granarium Everts, enter a facultative diapause and leave stored food to cluster in crevices in the storage buildings over the whole range of temperature in which larval development occurs (20 to 40°C.). The effects of changing temperature and food conditions on the behaviour of these diapause larvae have been studied in the laboratory.
After an increase from one constant temperature to another, fewer diapause larvae move from the food to the crevices, and those already dormant in crevices tend to emerge, feed and pupate. Some larvae emerge but do not pupate and later return to the crevices. These activities increase with temperature: they are slight at 20 to 25°C., intermediate at 30°C. and great at 35 to 40°C. However, if the temperature is held at 30°C. without change from the hatching of the egg (controls), there is little activity during diapause, although early larval development is fast at 30°C. Large jumps in temperature, e.g., 20 to 30°C., are more effective in breaking diapause than small ones, e.g., 30 to 32·5°C., and changes in the upper part of the range are effective, e.g., 35 to 37·5°C. There is much variation between individual larvae in the time elapsing between the increase in temperature and pupation. Pupation is probably delayed slightly by heavy accumulation of faecal pellets in the food. Sex and the length of the diapause period do not influence the breaking of diapause, which can take place within a month, and probably much less, of the onset of diapause.
Diapause is broken by gradual as well as abrupt temperature increases, but regular daily fluctuations, such as between 25 and 40°C., are ineffective unless preceded by a period of cooling (e.g., 1 month at 20°C.). The renewal of food badly fouled with faecal pellets at 30°C. breaks the diapause of some larvae, but changes in temperature regarded as moderate are more effective.
These results provide additional evidence about the degree of diapause in T. granarium. Some features, including the breaking of diapause by cooling followed by a return to a favourable temperature, are typical of classical diapause, but the breaking of diapause soon after it begins and the effect of a jump in temperature in the upper half of the developmental range without prior cooling are not typical, suggesting that diapause is broken by a trigger effect starting or stopping some physiological process. It can be concluded that mature, dormant larvae of T. granarium undergo a fairly weak facultative diapause with some unusual features before becoming prepupae. This diapause is compared with that of other insects.
The diapause appears to be particularly suited to the environment in dry food stores. It helps larvae to survive cool periods and food shortage, until warming combined with loading of new food breaks the diapause and allows breeding at the beginning of favourable periods. It appears likely that in grain stores in the Punjab (Pakistan) the life-cycle of T. granarium is largely governed by the climatic seasons. In the very erratic, non-seasonal environment in British malt stores, the ease of breaking diapause at almost any time after it has begun, with or without cooling, enables T. granarium to breed as soon as conditions are favourable, without seasonal control. Some other features of the diapause are well suited to this environment.