Marjorie A. Hoy

Diapause in the mite Metaseiulus occidentalis: Stages sensitive to photoperiodic induction

Abstract Stages of Metaseiulus occidentalis sensitive to photoperiod induction of diapause were determined by transferring various stadia into diapause-inducing conditions, and rearing them until adult females could be scored for reproductive condition. When eggs were transferred to 10 hr light at 19°C from 24 hr light at 25°C and the mites reared to adults, 92 per cent entered diapause. When larvae and all subsequent stages were kept under the inductive conditions, 62 per cent of adult females diapaused. Mites transferred as protonymphs into inductive conditions yielded only 10 per cent in diapause, and mites transferred as deutonymphs or newly emerged females did not enter diapause. However, adult females reared from eggs at 19°C under 12 hr light (which is near the critical photophase of 11·2 hr at 19°C) showed an unexpected sensitivity to photoperiod. Some newly emerged females oviposited upon transfer to an 8 hr photophase at 19°C. Some then stopped ovipositing and apparently entered diapause; these females resumed ovipositing after intervals ranging from 34 to 100 days. This was termed ‘switching’ into diapause. Some females reared under a 16 hr photophase at 19°C ‘switched’ also upon transfer as adults to shorter photophases—either 8 or 12 hr at 19°C. Thus, ‘switching’ may be due to transfer to shorter photophases. Promptness of mating vs delayed mating allowed ‘switching’ to be more easily detected.

Diapause in the gypsy moth: environment-specific mode of inheritance

Preliminary investigation of the genetic basis of diapause in the gypsy moth, Lymantria dispar, involved a comparison of reciprocal crosses between wild-type moths and a selectively bred non-diapause strain with the parental stocks, where half of the eggs resulting from each of the four mating types were exposed to one month of chill at 5 °C. The presence of chill dramatically altered the phenotypic expression of diapause-dependent characteristics. Early hatching was completely recessive in unchilled eggs, while hatching time was intermediate in chilled eggs, and there was no difference between reciprocal hybrids. Proportion of eggs hatching and, therefore, number of larvae produced was also influenced by chill. Unchilled hybrids did not differ substantially from wild-type eggs, while chilled hybrids were closer to the performance of the selected line. In this case, a significant reciprocal difference indicated some involvement of sex-linkage in the inheritance of diapause.