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Wrecking havoc on the Eastern hemlock is the 2mm long aphid-like insect woolly adelgid. Observed as early as the 1920s in the US and Canada, this menacing pest has spread from Massachusetts to Maryland and Virginia to eastern Kentucky and Tennessee. Although innocuous in its native Japan and China, it has decimated North Atlantic forests.
Adelgids are parthenogenetic: all are female and complete two generations of reproduction beginning in March/April. Females lay approximately 100 eggs protected by a cotton swab-like sac attached to the underside of a branch (see photo). In late April to early May, the eggs hatch and offspring disperse. Those born with
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wings must fly away to find a suitable spruce host as they cannot reproduce on the hemlock. Their search is often in vain as they usually die unable to find a suitable spruce in the east. Those born without wings lay their own eggs within two months. Between July and October, adelgids generally dormant. In October actively feed again.
They feed through their piercing mouthparts near the needle base. They secrete a toxic saliva which causes needles to desiccate, turn a grayish-green and eventually drop off. Most buds are killed and there is little or no growth. An infestation can cause dieback on the limbs, from the ground upward. Trees face mortality
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