The Abiotic Environment

of eastern North America, namely, the fall webworm (Hyphantria cunea) and the spruce 679

budworm (Choristoneura fumiferana). In eastern Canada, which is the northern limit of the range for H. cunea, the species is univoltine (has one generation per year), and summer temperatures play a major role in limiting population density through a variety of direct and indirect effects. First, late summer temperatures near or above optimum (about 32°C) permit rapid larval development, high larval survival rates, and formation of large pupae from which adults with high fecundity will emerge the following spring. Warm weather in the spring will allow pupae to complete metamorphosis and the adults to emerge early enough that their offspring can grow and pupate before the following fall. In addition, the following indirect influences operate. In cooler years larval development is prolonged, and larvae must therefore feed on older, less nutritious foliage; as a result, larval survival and the fecundity of the females that emerge are much reduced. This effect on survival is apparently cumulative; thus, if the offspring of these females have to feed on older foliage because of a second cool summer, very few survive beyond the larval stage. Finally, under cooler conditions the life cycles of H. cunea and one of its major parasitoids, the ichneumonid Sinophorus validus, are more closely synchronized so that more hosts will be infected.

Outbreaks of C. fumiferana, which despite its common name is primarily a pest of balsam fir (Abies balsamea), generally result from a sequence of warm, dry years. Such conditions, while obviously directly conducive to the insect's development, are much more influential over abundance through their stress effect on the host plant. Normally, mature balsam firs flower in alternate years; however, under these stress conditions they flower heavily each year. Ovipositing C. fumiferana lay more eggs on taller, more exposed trees which also produce more flowers. First-instar larvae, which hatch in late summer, seek out the cuplike remains of the flowers. After building a silken hibernaculum, they molt and enter diapause. Under these conditions the larval survival rate is high. In early spring the larvae begin to feed on the flowers that emerge before the vegetative buds and provide nourishing food for the larvae whose development is more rapid on flowers than on buds and needles. Also, the new foliage of flowering trees has a greater amino acid content than that of non-flowering trees and old foliage, a feature that leads to formation of larger pupae and more fecund females. Figure 22.7 summarizes the interactions that lead to population outbreaks in C. fumiferana.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook

Post a comment