Tufted Apple Budmoth (Lepidoptera: Tortricidae): Effects of Constant Daylengths and Temperatures on Larval Growth Rate and Determination of Larval-Pupal Ecdysis 1

Publisher Website
Google Scholar
Abstract
The relationship of head capsule width (HCW) to instar number in larvae of tufted apple budmoth, Platynota idaeusalis (Walker), was determined under different conditions of constant daylength and temperature. Growth rate (increase in HCW with instar) decreased markedly under the short-daylength, low-temperature condition, which has previously been shown to induce larval diapause. Variation in both growth rate and number of instars increased under diapause conditions. Under all conditions, growth rate was lower in larvae which experienced more instars before larval-pupal ecdysis, yet mean size of final-instar larvae increased with total number of larval instars. The hypothesis of constancy of growth ratio was rejected for all conditions. There was not an absolute threshold of larval size at which larvae metamorphosed, but discriminant analysis demonstrated that larval size was more important to form determination than was instar number. Field-collected overwintering tufted apple budmoth larvae, which experienced short-daylength, low-temperature conditions, did not have discrete size classes representing specific instars.