Factors influencing oocyte development in Mythimna convecta (Lepidoptera: Noctuidae) and their possible impact on migration in eastern Australia

Abstract

AbstractThe effects of crowding and photoperiod during rearing of Mythimna convecta (Walker) on oocyte development in adult females were measured in the laboratory and field. Most larvae reared under isolated conditions in a Light 12 h: Dark 12 h photoperiod had mature oocytes within 72–84 h of eclosion. Females were slower to mature when reared under longer daylengths. The mean oocyte width in 3–10 days old females was significantly lower for moths reared under long (Light 16 h: Dark 8 h) than equal (Light 12 h: Dark 12 h) daylengths. Crowding of larvae also prolonged the pre-reproductive period. In the laboratory, 47% of females had arrested oocyte development of 3–6 days after eclosion and, in the field during late winter/spring, 48% of females required 3–9 days to mature. Larvae were collected from late spring/summer populations and caged under high and low density conditions. All females emerging from both treatments had arrested oocyte development of at least seven days. In catches from baited moth traps run over the same period, the proportion of immature female M. convecta was lower during spring (18–30%), than in summer (60%) and, in both seasons, mating commenced before oocytes had matured, but after oocyte arrest. Long daylength appears to be only one of several contributing factors causing delays in ovarian development during summer. Delays in achieving reproductive maturity, induced by either crowding or summer conditions, may assist M. convecta to emigrate from unfavourable habitats. This is particularly relevant in the winter rainfall areas of south-eastern Australia where most food plants senesce in summer and suitable habitats are usually confined to the coast.