The hemlock woolly adelgid has a complex life cycle, involving two different tree host species as well as asexual and sexual life stages. On eastern hemlock, HWA produces two generations a year, an overwintering generation (sistens) and a spring generation (progrediens); these two generations overlap in the spring. The progrediens has two forms, a wingless form that remains on the hemlock and a winged form (sexuparae) that flies in search of a suitable host spruce tree upon which to start a sexual reproductive cycle (McClure 1995). In New York, there are no suitable spruce, thus the winged HWA are not successful. Each generation has six stages of development: egg, four juvenile (nymph) stages, and the adult. Overwintering adult females are black, oval, and soft-bodied (approximately 2mm long). They are usually concealed under the white woolly masses of wax (ovisacs) they secrete from special glands on their back-side. From March through May, these females lay 50 to 300 eggs in the woolly masses. The eggs are brownish-orange and very small (0.25mm long by 0.15mm wide). Depending on spring temperatures, eggs hatch from April – June.
Newly hatched nymphs – also known as crawlers – are reddish-brown with a small white fringe near the front (less than 0.5mm long). Crawlers search for suitable sites to settle, usually at the base of the hemlock needles, where they begin to feed and will remain attached to the tree with their specialized sucking mouthparts for the rest of their lives. Crawlers, an important dispersal phase of HWA on hemlocks, can be spread by wind, on the feet of birds, or in the fur of small mammals (McClure 1990). Once settled, these HWA crawlers quickly develop through the four nymph life stages, and mature in June.
Some of the adults of the spring generation (progrediens) are wingless and remain on the hemlock tree, feeding and producing eggs protected by woolly masses just like the overwintering generation, but during June-July. Their offspring hatch into crawlers, quickly settle onto hemlock branches, begin to feed and then enter a dormant period for several months until late October when feeding and development resumes. These nymphs become the next overwintering generation (sistens). The other portion of spring adults has wings and leaves the hemlock trees in June in search of spruce trees to complete the sexual phase of HWA reproduction. However, in North America, no spruce species (Picea spp.) are suitable hosts and any offspring produced die within a few days of feeding. Thus, the winged adult form can be a significant source for HWA population reduction. This is particularly important considering the number of winged adults produced in the spring generation increases with the density of overwintering adelgids, likely a result of changes in nutritional quality in the hemlock host tree.
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