Instars of laboratory reared Epiphyas postvittana (Lepidoptera: Tortricidae): a new perspective

The light brown apple moth (LBAM), Epiphyas postvitanna, is an invasive pest that was recently found in California and Oregon. Like many other Lepidoptera, instars can be distinguished by the width of the head capsule. LBAM instars differ in their susceptibility to cold and parasitism; as a result, instar identification can be critical to predict population dynamics, for example. Literature indicates LBAM has six instars, but all six instars were difficult for us to find in a colony reared at 22°C. We measured head capsule widths for 2,049 larvae that were reared in the laboratory on a bean (Phaseolus vulgaris)-based diet at 22°C or 15°C. We analyzed frequency distributions to identify instars. Our data suggest that LBAM only had five instars at either temperature. Larvae from 22°C had significantly wider head capsules than larvae of the same instar from 15°C, except for first instars, which did not differ between temperatures, and fifth instars, which were significantly wider from 15°C. Width distributions differed somewhat from those reported in the literature. Future studies, field and laboratory, may need to consider variation in LBAM instars for accurate instar identification.