The following is information on Hemlock Woolly Adelgid which has hatched at this time.
In most areas, Hemlock Woolly Adelgid (HWA) sistens have completed 100% egg hatch. Now is the time to control this pest with horticultural oil. 100% egg hatch has been recorded from 505-676 (Avg. 591) GDD. HWA crawlers appear as small black flecks along the needles, settling and feeding at needle bases. The white cottony sacks that were the adults will now fade away.
Very little growth and development of HWA occurs in the heat of the summer, and the pest often appears dead. With cool temperatures in September, the sistens nymphs resume feeding and swell up. You can manage adelgid populations successfully with sprays of horticultural oil at 1-2% (v:v). Soil-applied neonicotinoids like Merit (imidacloprid) or Safari (dinotefuran) are also used. Merit is typically soil injected in the fall, while Safari can be spring applied and found in the foliage in as little as three weeks. Remember only HWA is controlled--elongate hemlock scale is not generally affected by imidacloprid treatments.
Water mature hemlocks during the summer season. Do not fertilize infested hemlocks, as nitrogen will increase mite, scale, and adelgid populations. In fact, Spruce spider mite and hemlock rust mite populations have found to be stimulated by imidacloprid applications to hemlock. Most biological controls for HWA in forested situations have not established widely yet and are cost prohibitive for landscape use. Ironically, since it is a home-invader, nuisance pest, the multi-colored Asian ladybeetle (Harmonia axyridis) is an effective predator of HWA.
Information from Casey Sclar, IPM Coordinator, Longwood Gardens