AN EXPERIMENTAL-STUDY OF EARLY STAGE SURVIVAL OF HELICOVERPA-ARMIGERA (LEPIDOPTERA, NOCTUIDAE) ON COTTON

The pattern and cause of mortality of the early stages of Helicoverpa armigera (Hubner) on cotton was investigated in both field and glasshouse situations for two seasons. In 1987, egg losses were high (32-88%) over days 1-3 following oviposition, more so from the lower than upper leaf surface. The mortality of first instar larvae was 93-100% over days 4-5 following oviposition, and losses were higher from seedling stage than squaring cotton. Wind, rain (or watering), aerial predators, dispersal of first instar larvae, and host plant effects (including leaf orientation) were important mortality factors. In 1988 trials, egg losses over days 1 to 3 after oviposition on plants in glasshouse and field situations, respectively, were 0-1% and 5-6% due to changing leaf orientation alone; 4-22% and 16-32% due to rain/watering and changing leaf orientation combined; 14-31% and 12-31% for wind and changing leaf orientation combined; and 4-38% and 23-52% for the combination of wind, rain/watering and changing leaf orientation. Mortality of first instar larvae on day 4 was 32% and 29% on plants in glasshouse and field, respectively, from which the action of such mortality factors had been excluded. Mortality on plants with changing leaf orientation alone was 30% and 68%, with rain/watering and changing leaf orientation combined 48% and 74%; with wind and changing leaf orientation combined 40% and 63%; and with all factors combined 45% and 69%, respectively. The highest losses (of eggs) occurred on day 1 after oviposition, and similarly for first instar larvae on day 4, which coincided with hatching.