Variation in the number of larval instars of the brownheaded leafroller,Ctenopseustis obliquana(Lepidoptera: Tortricidae) at constant laboratory temperatures

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Abstract
Larvae of a laboratory population of the brownheaded leafroller (BHLR), Ctenopseustis obliquana (Walker) (Lepidoptera: Tortricidae) developed through 5 or 6 larval instars for both sexes when reared on an artificial diet at 20°C, 50–60% RH, and 18 h photoperiod. Comparison of the life cycle of the two developmental groups showed that the 6 instar group had a greater head-capsule width in the final instar, a longer larval developmental period, heavier pupae and female adults, and a higher egg fertility. The lower the total number of instars, the greater the increase in the head capsule widths between instars. Proportions of larvae in the two instar groups were influenced by the rearing and storage temperatures in the egg and first instar stages. Compared to the incidence of 6 instars in larvae reared at 20°C, eggs developed at 10°C produced 19% more 6 instars, and first instar larvae held at 25°C produced 32% more 6 instars.