THE OCCURRENCE AND NATURE OF DIAPAUSE-FREE DEVELOPMENT IN THE SPRUCE BUDWORM, CHORISTONEURA FUMIFERANA (CLEM.) (LEPIDOPTERA: TORTRICIDAE)

Abstract

Spruce budworm populations in Eastern Canada are effectively univoltine. After hatching, the larva follows a complex behavior pattern that leads to the building of a hibernaculum in which it molts and spends the winter months without feeding. This pattern seems to be independent of environmental conditions, except as they affect the rate at which the component events occur, and, in the absence of any evidence to the contrary, must be considered as inherent.Laboratory studies have shown that in 62% of the families produced by matings of field-collected individuals at least one larva can develop without diapause if subjected to long photoperiods (at 71° F.); on the average 3 to 4% of the second-instar larvae in such progenies can forego diapause. Selection over six generations yielded a strain that, if subjected to continuous light in the laboratory, is virtually free from diapause. This stock has been reared for a further six generations and still retains this photoperiod dependence. Behavior is usually normal during hibernaculum construction and molting, but then the larvae vacate their shelters, feed readily, and develop without any delay. No associated differences have yet been found between non-diapause (N-D) and normal (D) insects. The ability to develop without diapause is apparently determined by multiple genes; it is not sex-linked but may be to some extent sex-controlled.Diapause-free development in this strain results only when first-instar larvae are exposed to a photoperiod of at least 15 hours, shorter times resulting in almost universal diapause and longer times giving a progressively greater proportion of N-D insects, approaching 100% in continuous light. This response is reduced by lower temperatures. Potential N-D insects deprived of a light-treatment behave as normal D insects; under appropriate conditions of light their progeny in turn develop without diapause.