Influence of temperature, photoperiod, and food on the speed of development, longevity, fecundity, and other qualities of the Diamond-Back moth Plutella Maculipennis (Curtis) (Lepidoptera: Tineidae).

Abstract

Larvae of Plutella maculipennis (Curtis) were reared on food of different qualities at different temperatures and were exposed during their development to different photoperiods. The plants on which the larvae were reared were also exposed to different photoperiods during their development to see whether this influenced the quality of food for the insect. It was found that although caterpillars developed more quickly at 25ºC than at l8ºC, they weighed less at pupation at 25ºC than at l8ºC, but the ratio of dry matter to water was the same at both temperatures. Exposure of caterpillars to different photoperiods influenced their speed of development. There was an indication that the influence of light was different at different temperatures. The influence of quality of food was more evident. Larvae reared on white or young green leaves of cabbage developed more quickly than those reared on green but mature or senescing leaves; the slower speed of development was associated with a higher death-rate and a lower body weight combined with a higher water content of the pupae. Irrespective of temperature and the quality of the food, females developed more slowly than males and also had more dry matter and a higher liveweight in the pupal stage. Insects bred at a lower temperature produced more eggs than those at a higher temperature; the difference was thought to be due to a lower body weight in the latter. Photoperiod also influenced fecundity but the effect was not so clear. Caterpillars reared on plants grown in a long photoperiod were more fecund in the adult stage than those reared on plants grown in a short photoperiod, but the difference was not quite significant. Insects reared on white or young green leaves of cabbage laid significantly more eggs than those reared on green but mature leaves. Simarly when larvae were reared on the better quality food they produced more silk, the resulting moths lived longer, and a higher proportion of the males were fertile. The general conclusion is drawn that for normal activities of Plutella a balance of nutrients is required. For the normal activities of the insect body, like healthy development and reproduction, nutrients may be required in certain proportions. If owing to some cause (as probably, with change in season) that proportion is upset, certain abnormalities may appear in the metabolism or form of the insect, such as deformities in wings and shape of body, slower development, reduction or impediment in egg-laying capacity or diapause, etc. In the author's opinion this general conclusion may be a useful hypothesis on which to base further work. In the light of the above observations and observations on other insects, the seasonal activity of Plutella is discussed. It is stated that although apparently its behaviour and activity seem to be nicely adjusted to seasons, showing a beautiful picture of adaptation, yet the recurrent heavy reductions in numbers which this insect and many other species suffer in the field suggest that the adaptations are not as efficient as may at first sight appear.