Is something giving your hemlocks the woolies?

By Dawn Pettinelli - UConn Home & Garden Education Center
Feature Article - posted Fri., Mar. 11, 2011
- Contributed Photo

Canadaor eastern hemlocks are a beautiful and desirable component of many suburban landscapes, as well as native forests. Over the last 20 years or so, a serious insect pest - the hemlock woolly adelgid - has been taking up residence in many of our hemlock trees with disastrous results. Native to Asia, the hemlock woolly adelgid was first detected in 1924 on the west coast. It has since made its way east and was brought up to southern parts of New England by hurricanes in the mid-1980s.

The hemlock woolly adelgid is a small, soft-bodied insect related to aphids. Adelgids have piercing-sucking mouthparts which are inserted into young twigs to feed on the plant’s sap. Severe infestations result in the cessation of plant growth, needle discoloration, premature needle drop, branch dieback and eventually death of the tree. While feeding, adelgids may also inject a toxin into the tree which injures the tree’s vascular system.

Tiny blackish-gray female adelgids overwinter on trees. Eggs are laid on branches from late March through May and are covered with white cottony egg sacs. Usually, the egg sacs are what first causes tree owners to realize something is amiss. From early April through early June, reddish crawlers or nymphs emerge from the eggs and begin feeding. They mature into adults in just a few weeks.

Most adults are wingless and stay on the hemlock, producing a second generation. Others develop into winged forms or can be transferred to nearby hemlocks by wind, birds or animals, including man. The second generation crawlers go through a dormant phase from late summer until early autumn when they resume feeding. They will develop into the overwintering adults.

Check hemlocks frequently for the presence of white egg sacs, as this is the most obvious sign of woolly hemlock adelgid infestation. It is important to detect infestations early, as hemlocks can rapidly decline, especially if the trees are stressed by drought or other problems. Small, new infestations can often be knocked off with repeated applications of a strong stream of water from a hose. Most of these insects will die before they can crawl back onto the tree.

Fortunately, hemlock woolly adelgids are susceptible to several insecticides. Either summer or dormant horticultural oils will provide good control. Dormant oils are generally applied in April, while lighter weight summer oils are used from July through September when insects are most vulnerable. Insecticidal soap is another less toxic means of control also best applied mid to late summer.

Thorough coverage is essential. Many homeowners may be able to achieve good coverage on small specimens but usually tree companies are needed to spray large trees. Try to enlist the support of your neighbors if they have hemlocks on adjacent properties. Otherwise, your plants will be continuously reinfected.

Chemical controls include the insecticide imidacloprid, which can be injected into the tree by a professional or placed in the soil around the tree for the tree to take up through its roots. It is a systemic insecticide and will take several weeks to translocate throughout a large tree. Be aware that this control may not work with trees that have compromised vascular systems.

Keep your hemlocks in good health by watering during times of drought. Avoid compacting the soil under the trees by minimizing foot and vehicle traffic, especially during times when the soil is saturated. Mulching around hemlocks is also beneficial. Do not fertilize adelgid-infected trees with nitrogen. The lush foliage stimulated by nitrogen applications encourages even more adelgid colonization.

For more information on hemlock woolly adelgids or other home and garden information, contact the UConn Home and Garden Education Center, toll-free, at 860-486-6271, visit the website, or contact your local Cooperative Extension Office.

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