Studies on the Diapause of the Indian-meal Moth, Plodia interpunctella HÜBNER

II. The Effect of Population Density on the Induction of Diapause

Abstract

Investigations have been made of the occurrence of diapause in the fully grown larvae of the Indian-meal Moth, Plodia interpunctella HÜBNER reared in various larval densities under a constant temperature condition of 30°C, using rice bran as a food of the larvae.
No larvae enter diapause when they are reared in the condition of low larval densities (50 eggs per 30g rice bran) under this temperature condition.
On the other hand, when they are reared in high larval densities (100-400 eggs per 30g rice bran) even under this temperature condition, some individuals wander out of the food at the fully grown stage.
The pupation of the wandering fully grown larvae is not interrupted at 30°C. The wandering fully grown larvae, however, show characteristics such as the interruption of pupation at about 20°C, the resistance to cold, the chilling effect of low temperature (10°C) condition upon their pupation at 20°C, the increased body weight and decreased water content, and the lowered rate of development at their young larval stage.
The characteristics described above are just the same as that of the diapausing larvae of this species, as previously reported as a new type of diapause, so the wandering fully grown larvae must be the individuals which have entered diapause depending upon the high larval densities but have no actual diapausing period as they are under the higher temperature.
Increase of mortality within this range of density (50-400 eggs per 30g rice bran) is not so remarkable, whereas the diapausing larvae appear and their occurrence apparently depends upon the density.
Some of the larvae enter diapause when 2 or more individuals are reared together in a tube with a very small amount of food, whereas the solitary culture never results in diapause.
In other solitary culture in which the rice bran contaminated by the larval excrement is used, and also in the 35°C culture, no larvae enter diapause.
Rise of temperature and increase of water content of rice bran in these experiments of higher larval densities are not so great.
In view of the results described above, it can be suggested that the occurrence of wandering fully grown larvae, the diapausing larvae at 30°C, in this case, may be mainly due to the mutual stimulation between the larvae.
It seems to be important that the pattern of influence of larval density upon the population of the insects having this type of diapause may show that the density effect on occurrence of diapause does not necessarily lead the population to elapse a long period of arrested development but only to show other physiological and behaviouristic changes such as mentioned above.