Evidence of a Polymorphic Life Cycle in the Hemlock Woolly Adelgid, Adelges tsugae (Homoptera: Adelgidae)

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Abstract
Hemlock woolly adelgid, Adelges tsugae Annand, has a polymorphic life cycle which involves hemlock, Tsuga, and spruce, Picea. Studies in Connecticut revealed that about half of the offspring produced by the sistens during June of two consecutive years became wingless adult progrediens. These closely resembled their mothers and remained on hemlock. The other half became winged adult sexupara and migrated to spruce where they produced eggs of the sexuales. However, none of the sexuales nymphs that hatched survived on any of 12 spruce species tested. Adelges tsugae, a cool-loving species, completed most of its development on hemlock from October through June. During the warm summer months, this insect aestivated as a first-instar nymph on the young branches at the base of the needles. All three generations of A. tsugae on hemlock had six stages of development, which included the egg, four nymphal instars, and the adult. Eggs and first-instar nymphs of the three generations were indistinguishable from one another. However, many of the later instars and the adults were readily distinguished from one another by the presence or absence of thoracic sutures and wing bud notches, by their degree of sclerotization, and by the size and shape of the body, thorax, and antennae.