Calling behaviour and pheromone titre of the true armyworm Pseudaletia unipuncta (Haw.) (Lepidoptera: Noctuidae) under different temperature and photoperiodic conditions

Abstract

True armyworm females that were exposed to temperatures of 20, 15 or 10°C following their first night of calling at 25°C significantly advanced their mean onset time of calling compared to those maintained continuously at 25°C. The mean time spent calling also increased significantly following a 10 or 15°C decrease from 25°C, while a 5°C decrease had no effect. Only temperature conditions during the scotophase affected the overall expression of the overt calling behaviour, with those during the photophase significantly modifying neither the onset nor the duration of calling. Regardless of the time at which a 15°C decrease in temperature occurred during the first half of the scotophase, more than 45% of the females were already calling by the 5th hour of the scotophase, compared to 0% in the 25°C controls. This suggests that the absolute ambient temperature determines the time at which calling will be expressed, with the calling gate opening earlier under low night temperatures. However, when the temperature was increased from 10 to 25°C at 2 and 4 h after lights off (following a decrease from 25°C the first night of calling), some females started calling much earlier than would be expected if the expression of calling behaviour was uniquely related to ambient temperature. Data from other experiments indicated that temperature conditions during the previous calling period may, under certain conditions, modify calling behaviour. Both the proportion of glands containing pheromone and the pheromone titre/female varied in a periodic fashion during the photoperiodic cycle immediately following the first night of calling at 25°C, 16 h light:8 h dark. Both values decreased during the photophase, falling to zero by the onset of the scotophase, before increasing again during the latter half of the scotophase when overt calling occurred. Results obtained from females held under continuous darkness following their first night of calling confirmed that the periodicity of pheromone titre, like that of calling behaviour, is circadian. A good correspondence between the pheromone titre and calling behaviour was also observed under 12 h light:12 h dark at 25°C, even though the mean onset time of calling was significantly different than that at 16 h light:8 h dark, 25°C. Furthermore, pheromone-gland analyses of females subjected to a decrease in temperature from 25 to 10°C at different times during the photoperiod following the first night of calling confirmed that overt calling behaviour and increases in pheromone titre are closely linked.