Hemlock Wooly Adelgid Invades State
The N.H. Division of Forests and Lands and the Plant Industry Division
of the N.H. Department of Agriculture, Markets & Food, have been monitoring
for the presence of hemlock wooly adelgid (adelges tsugae), a
serious pest of hemlock species in nurseries, landscapes and native trees.
The hemlock wooly adelgid (HWA) is a small, aphid-like insect native to Japan and China . First detected in the U.S. on the West Coast in the 1920s, it appeared in Virginia in the 1950s. Since then HWA has continued a northward migration in the Eastern United States , devastating stands of Eastern hemlock, T. Canadensis, and Carolina hemlock, T. Caroliniana.
The first New Hampshire infestation was discovered on naturalized stands of hemlocks in Portsmouth ’s Elwyn Park in the fall of 2000. Since then, HWA has been found in other areas of the state, showing up in Peterborough in 2001, Bedford and Epsom in 2002, in Jaffrey in 2003, and this year in Nashua and Hollis. So how did it get here?
According to Jen Bofinger, forest health specialist/Entomologist with the New Hampshire Division of Forests and Lands, these insects can be dispersed by wind, birds or by forest-dwelling mammals. Humans also have contributed to the spread of this devastating insect by transporting infested hemlock trees.
To prevent the spread of this insect into our state, the New Hampshire Division of Forests and Lands and the Department of Agriculture, Markets & Food, have restricted the flow of hemlock products imported into the state. A quarantine restricts importing and transplanting hemlock nursery stock into the state from infested areas, unless hemlock seedlings and nursery stock are inspected and certified free of HWA.
HWA feed on eastern hemlock, T. Canadensis, and Carolina hemlock, T. Caroliniana, attaching themselves to the stems at the base of the needles, killing the needles and hindering the development of new growth. Feeding damage first appears as needle discoloration (from a deep green to grayish green to yellowing), followed by premature needle drop/defoliation, branch desiccation, and finally, loss of vigor. Major limbs may die back within two years on heavily infested trees. Dieback usually occurs and progresses from the bottom of the tree upwards, even though HWA infestation may be evenly distributed throughout the tree.
HWA are all females and complete two generations of development per year on hemlock. The adelgids display several different forms during their life history, including winged and wingless. Adults are brownish-reddish in color, oval in shape and are about 0.8 mm in length. The adelgid lays between 50 and 300 brownish-orange eggs under a cottony wax. The eggs will hatch over an extended period of time (March through April).
Half the eggs will develop into a winged, migratory form. Unable to reproduce on hemlock; the winged adelgids migrate in search of spruce to complete their development. Because there is no suitable spruce host available in New Hampshire ( North America ), these adelgids soon die.
The other eggs develop into wingless adults that remain on the hemlock tree. Newly hatched woolly adelgids (nymphs, or immature stage) emerge from the cottony egg mass as the new hemlock growth expands in May and June. These crawlers are black, oval and flat and are extremely small, making them difficult to see with the naked eye.
Once hatched, these insects migrate to the base of a needle and begin to feed. Here they will become dormant until the middle of October when they resume feeding. These nymphs continue to feed and develop during the winter and mature by spring.
There are a number of preventive actions you can take to help manage the spread of this devastating pest:
Monitoring: Frequent visual inspection of trees is one of the most effective means of determining infestations. Look for the dry, white “wool” that is produced. It will be located on the underside of the young twigs. Depending on the size of the trees binoculars may prove to be very helpful in locating infestations. Infestations will be hard to detect from July through September when the dormant nymphs produce very little wool. Other signs of potential infestations include thinning of needles, grayish-green needles, branch desiccation, and loss of vigor.
Improving Tree Health: Trees growing in poor sites or those experiencing stress from drought and other factors succumb to HWA attack more quickly than those growing under optimal conditions. Therefore, maintaining good growing conditions can play an important role in the survival of hemlocks in home landscapes.
Their shallow roots leave hemlocks vulnerable to drought stress. Provide one inch of water per week (including rainfall) during droughts. Roots on an established tree extend beyond the spread of its branches, so apply water to the entire root zone. Water deeply as needed and avoid frequent shallow irrigation.
Pruning dead and dying branches may also help improve the health of hemlock. This may help promote new growth by allowing more light to reach the foliage, and may reduce the likelihood of attack by other insect pests and diseases.
Cultural Controls: One way to reduce the risk of an HWA invasion is to practice caution when moving plants, logs, firewood, or bark chips from infested areas to ones that are uninfested.
If you believe you’ve seen the hemlock wooly adelgid, please contact the N.H. Forest Health Program at 271-7858.
by Rachel Maccini, Coordinator, UNH Cooperative Extension Family, Home & Garden Education Center
Photo courtesy of USDA Forest Service
For more information:
- NH Forest Health Program (271-7858)
- UNH Cooperative Extension’s Family, Home & Garden Education Center’s Toll Free Info-Line, 1-877-398-4769, M-F, 9:00 AM - 2:00 PM
- US Department of Agriculture’s comprehensive HWA Web site
- HWA photo gallery
