Vermont Bans Importation of Hemlocks

While only the size of a pinhead, the presence of woolly hemlock adelgid is given away by its white wool, which collects around egg masses and is present at all stages of adelgid development. Photo: USDA Forest Service

The hemlock woolly adelgid (Adelges tsugae Annand) is a small aphid-like insect that feeds on several species of hemlock (Tsuga spp.) in Asia, its homeland, and in North America where it was introduced in the 1980s.

 

APPEARANCE. A fully grown HWA adult is only about the size of a period on this printed page. However, this insect is easily recognized during most of the year by the presence of a dry, white woolly substance on the young twigs. The "wool" is associated with all stages of the adelgid, but it is most abundant and conspicuous during spring when egg masses resembling cotton swab tips are present at the bases of needles.

 

HABITAT. HWA has only been found on hemlock, but an unknown spruce (Picea sp.) probably also serves as a host in Asia. The adelgid is a harmless inhabitant of T.chinensis in Taiwan, T. diversifolia and T. sieboldii in Japan, and on western hemlock (T. heterophylla) and mountain hemlock (T. mertensiana) from northern California to southeastern Alaska. However, the situation has been very different in eastern North America where the HWA has caused extensive damage to forests and ornamental plantings of eastern hemlock (T. canadensis) and Carolina hemlock (T. caroliniana) from Virginia to New England.

 

LIFE CYCLE. HWA completes two generations of development per year on hemlock. During March and April, overwintering adults of the lay 50 to 300 eggs in a cottony mass on the young twigs. Nymphs hatch from these eggs during a period of several weeks in April and May, settling on the twigs near the base of the needles where they insert their piercing and sucking mouthparts. There they feed and remain throughout their development. This spring generation matures by the middle of June. Wingless adults are able to reproduce on hemlock, while adults without wings search for another suitable host, such as spruce, but soon die for lack thereof. In the middle of June, wingless adults lay 50 to 300 eggs each in a cottony mass on the twigs. Crawlers hatch in early July and settle on the new growth but enter a non-feeding dormancy until October, when feeding resumes. Nymphs feed and develop during the winter and mature by spring.

 

The eggs and nymphs, which are not attached to the plant via mouthparts are abundant from March through June and can be readily dispersed by wind, birds, deer and other mammals. Humans can also disperse the insect through various activities including moving infested plants.

 

DAMAGE. The HWA injures susceptible hemlocks by sucking sap and probably also by injecting a toxic saliva while feeding. This causes the needles on infested branches to desiccate, turn a grayish-green color and then drop from the tree ,usually within a few months. Most buds are also killed, so little new growth is produced on infested branches. Dieback of major limbs usually occurs within two years and progresses from the bottom of the tree upwards, even though the infestation may be evenly distributed throughout the tree. Trees often die within four years, but some survive longer in a severely weakened condition with only sparse foliage at the very top of the crown. These weakened trees are unsightly and have little chance for recovery, often falling victim to secondary pests.

 

CONTROL METHODS.

1. Integrated Pest Management. Hemlocks growing in ornamental landscapes can be protected from HWA by using an integrated pest management (IPM) approach. Frequent visual inspection is the most effective means of determining whether or not a hemlock is infested. Pay particular attention to trees located downwind from or near infested hemlocks. For most of the year the dry, white "wool" produced by the HWA on the twigs is quite conspicuous contrasted against the dark green needles. Binoculars may prove to be helpful in locating infestations in tall trees. An infestation may be difficult to detect during July through September when dormant nymphs have very little wool associated with them. Further evidence of a HWA infestation is the thinning or grayish-green (not red or yellow) color of the needles on some branches. Usually by the time these symptoms appear, the tattered "wool" of a previous HWA generation is also present on the branches.
2. Cultural Control. Discouraging birds, squirrels and deer from visiting hemlocks will reduce the risk of these animals transporting the insects to the trees. Take care when moving plants, bark chips, etc. from infested areas onto an uninfested property, especially from March through June when HWA eggs and crawlers are abundant. Check clothing during this period since it is possible that crawlers could be transported on clothing.     Infestations often start on large hemlocks that intercept the prevailing wind or that are especially attractive to birds and other wildlife. When heavily infested these trees may serve as an effective "launch pad" for HWA eggs and crawlers. Selective removal of these heavily infested reservoir trees from the immediate vicinity will retard the establishment of new infestations.
3. 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. Maintaining good growing conditions can play an important role in the survival of hemlock. Because it is shallow-rooted, hemlock is prone to stress when precipitation is low. During periods of drought stress, provide one inch of water per week (including rainfall). Roots on an established tree extend beyond the spread of its branches so it is important to apply water to the entire root system. Avoid frequent shallow irrigation.     Pruning may also be of some value in improving the health of hemlock. Remove all dead and dying branches to help promote new growth by allowing more light to reach the foliage, thereby reducing the likelihood of attack by other insect pests and diseases.     Although applying fertilizer may improve the growth and vigor of uninfested hemlock, fertilizing infested hemlocks with nitrogen also enhances HWA survival and reproduction. A fertilized hemlock becomes more heavily infested and more severely injured than an unfertilized one. Fertilizing a tree after HWA has been controlled may encourage growth and stimulate recovery. The potential risks and benefits of applying fertilizers that do not contain nitrogen to HWA-infested hemlocks are unknown. Avoid fertilizing lawn areas within the root zone of infested trees and hedges.
4. Mechanically removing adelgids. Eggs and crawlers of HWA are readily dislodged from the young hemlock twigs by wind and rain and are unable to find their way back onto the tree and die. Therefore, intentionally dislodging these by directing a strong stream of water at infested branches periodically during April through June may be of some value in an integrated approach to managing HWA populations. Clipping heavily infested twigs from branches will also reduce HWA populations on a tree. However, extensive clipping may have undesirable effects on the appearance and health of the tree.
5. Planting Resistant Species. Of the four resistant hemlocks previously mentioned, T. heterophylla is the most similar to eastern hemlock, T. canadensis. Planting T. heterophylla as a replacement should reduce the impact of the HWA in the ornamental landscape. Also use alternatives to hemlock which have similar qualities and characteristics.
6. Biological Control. Native species of predators unfortunately have not had a significant impact on HWA populations. Research is currently being conducted on the effectiveness of the oribatid mite, Diapterobates humeralis and the ladybird beetle, Pseudoscymnus (a new species), which are effective predators in Japan. The ladybird beetle has been recently released in Connecticut. Eventually these predators may be included in an integrated program for managing HWA.

 

CHEMICAL CONTROL METHODS

Deciding whether or not to use insecticides. Infested hemlocks are usually unable to survive for more than a few years unless insecticides are applied and the trees will need to be protected from HWA as often as necessary until the danger has passed. This may be for a period of several years. It may be advisable to identify individual trees or groups of trees that have special value or significance on the property and to concentrate control efforts on those trees. This may be more successful than the overly ambitious approach of trying to save all the trees at the outset.

 

Management Options. Fluvalinate*#, horticultural oil (Spray Oil 10E#), imidacloprid (foliar or soil application), insecticidal soap. When to treat: Dormant application with horticultural oil (Until research provides clearer guidelines in regard to the safety of a dormant application of horticultural oil in the fall this application is best left for early spring.) Foliar sprays late April to early May. Again in mid-July. Fall and spring soil applications with imidacloprid have been effective. Addition of adjuvant will improve penetration and coverage when using foliar sprays.

 

Note: Merit (imidacloprid) soil injection not for use in Nassau and Suffolk Counties, Long Island.

 

* = restricted-use pesticide; may be purchased and used only by certified applicators or used by someone under the direct supervision of a certified applicator.
# = 2(ee) recommendation based on quantitative efficacy data. Any user must have the 2(ee) recommendation in his or her possession at the time of application, e.g., "Cornell Guidelines."

Source: Cornell University Cooperative Extension. Read the full text of this fact sheet: http://www.cce.cornell.edu/suffolk/grownet/tree-insect/hemwool.html