HWA COLD-HARDINESS: TOWARDS DEFINING LIMITS OF RANGE EXPANSION

Evidence of the cold-tolerance of HWA overwintering Cold temperature is probably the most important factor limiting the range of insects in temperate regions (Salt 1936 and others since then). The relationship between insects and low temperature is dynamic, making it difficult to assign specific temperatures at which an insect won't survive, or more importantly, become established in a particular region. Steinhaus considered cold to be one of several agents of noninfectious disease (Steinhaus 1962)—a useful paradigm for evaluating effects of biotic and abiotic factors on insect susceptibility to cold and changes in fitness. Table 1 lists some factors to consider when evaluating insect responses to cold stress. The hemlock woolly adelgid (HWA), Adelges tsugae Annand, responds uniquely to the change of season in the progression of its life cycle. The sisten generation aestivates or rests as a first instar during summer months and resumes development in fall, which is heralded by development of their woolly coat. As temperature permits, the adelgids develop through the winter into adults. The sistens surviving overwinter to adulthood begin egg production in early spring, producing the progredien and sexuparae generation—sexuparae are a winged form that presumably don't contribute to population growth or expansion in the northeastern U.S. because they lack a required alternate host. The progredien generation in turn produces a new sisten generation in late spring that will ultimately overwinter.