Hemlock Woolly Adelgid Control

Hemlock Woolly Adelgid: Tom Coleman, US Forest Service, Bugwood.org

The hemlock woolly adelgid (HWA) has caused significant damage to eastern hemlock, Tsuga Canadensis, in ornamental plantings and forests across much of Pennsylvania. However, many trees are still surviving and in need of treatment.

Description:

The most obvious sign of a HWA infestation is the copious masses of white filaments of wax produced by the females. These "cottony" masses normally persist throughout the season and into the following year, even after the insects are dead. The overwintering females are black, oval, soft-bodied, and about 2 mm long. They are concealed under their characteristic white waxy mass. HWA populations are usually located on the underside of the twigs at the base of the needles.

Life History:

The overwintering adult females begin laying eggs in large clusters in the cottony masses during warm weather in late winter and early spring. They continue to lay eggs into June. Eggs start to hatch in early April, and depending on spring temperatures, hatching is completed by late June. The newly hatched nymphs or “crawlers” become mature by late September and spend the winter on trees as mature females.

Damage:

Host plants are injured by the adelgids inserting their piercing-sucking mouthparts into the base of the needles and removing plant fluids. Moderate HWA populations may cause a reduction in tree health. Severe infestations may result in premature needle drop, reduced twig growth, dieback, or death of trees.

Evaluate Trees:

Not all hemlocks can be saved. It would be too expensive to do so, as insecticide treatments need to be applied every few years. Consider the cost! Also consider the cost of removing dead trees that are near homes and other structures. Treat hemlocks that are the healthiest, most vital to the landscape or forest, easiest to reach with a sprayer, and furthest from sources of water. Remove trees that will NOT be treated to eliminate nearby sources of the insect to re-infest treated trees.

If trees do not have any HWA on them at all, they do not need treating. Insecticide applications are necessary once you see a light infestation or if adjacent trees are infested. This will keep the tree from going into decline. Treating HWA aggressively, while the tree is still in good health, is the best way to maintain a healthy tree. Severely defoliated trees will likely not recover even with treatment.

Treatment:

Try to minimize any stress on the tree. If possible and practical, during periods of drought, water the tree. Do not fertilize infested trees. This only aids the survival and reproduction of the adelgids. Prevention alone does not always work and trees which are infested will usually die unless an insecticide treatment is applied.

Horticultural Oil or Insecticidal Soap: These are the safest insecticides for controlling HWA. They are not toxic, but kill the insect by smothering it as the spray dries on the pest. These treatments are made in the fall from August until it gets too cold to spray. Treating other times of year will result in poorer control of the adelgids and may result in foliage burn. The entire tree, including the bark of the trunk and limbs, is thoroughly sprayed (drenched actually) with this material. A forceful spray is needed to get adequate coverage.

Both of these products are used at a 2% solution (2% solution = 2.6 ounces of spray material per gallon of water). There is no residual control with these materials; once they dry they will no longer control HWA. Trees will probably need to be treated annually. Be sure oils stay well mixed with the water during application. Using these materials is difficult if you do not have adequate equipment, especially on large trees.

Homeowners may be able to spray trees eight feet or smaller with a backpack sprayer or other types of hand held sprayers. For complete coverage, spray until droplets are observed running off. Be sure to spray on the undersides of limbs as well as the top. Every branch must have thorough coverage to get control. Larger trees require a high pressure sprayer and may require hiring a commercial arborist/pesticide applicator.

Soil Drenches: This treatment is effective for large trees that cannot be completely sprayed. The insecticide is applied to the soil surrounding the roots of the tree. The tree roots take up the product and move it into the foliage where the insect is killed. Soil drenches must be applied when there is adequate soil moisture in either the spring or fall. The best time to treat is in March or April. Do not use in areas near streams or ponds or where the soil is exceedingly rocky.

Most soil drenches are made with an imidacloprid product (Merit 75 WP, Malice 75 WSP, Zenith 75 WSP). Any of these can be purchased by the homeowner and applied to their own property. There are other imidacloprid products coming on the market. Any may be used as long as they have a landscape label. Imidacloprid products mentioned above all have 75% active ingredient. An imidacloprid product specifically for homeowners, Bayer Advance Garden Tree and Shrub Insect Control, is more readily available and can be purchased from home improvement stores. The rates of these products are based on the size of the tree trunk or height in the case of a hemlock hedge.

 Elongate Hemlock Scale:Pennsylvania DCNR - Forestry , Bugwood.org

IMPORTANT: If your hemlocks are also infested with elongate hemlock  scale then you must use another similar product, with slightly faster uptake, called Safari 20 SG (dinotefuran). This may also be applied as a soil drench as well as a bark spray. It will control both the scale insect and the adelgid. Safari trunk applications should be made before bud-break or shoot elongation in the spring.

Homeowners: (READ the LABEL!) Use Bayer's Advance Garden Tree and Shrub Insect Control for soil drenches.

·         Measure at chest height the total number of inches in circumference around the tree (if you use Merit, Malice, Zenith, or Safari – measure the circumference at chest level and divide by 3 to get the diameter in inches).

·         Note: Bayer's Tree and Shrub Insect Control will treat 32 inches of trunk circumference cumulatively, (equals approximately a 10 inch diameter tree.

·         One ounce of either Merit, Malice or Zenith will treat 15 inches of trunk diameter at the high rate or 30 inches at the low rate. If your trees are highly infested with HWA, use the high rate, otherwise consider using the lower rate. This can be then be mixed in a bucket with any amount of water to pour around the tree.

·         Safari 20 SG can be used at 1.0 to 4.2 ounces of product per 10 inches trunk diameter. This can be mixed in any amount of water to pour around the tree. It is also labeled for trunk spray on the lower 4-5 feet of the trunk. This is currently the only product labeled as a bark spray. A carefully applied trunk spray may be appropriate where surface water is nearby.

·         Use the following method - dig a shallow trench around the circumference of the tree, 1 foot away from the trunk. Be sure to remove all mulch and other organic material, the insecticide must be applied directly to the soil. Pour product in trench at the correct rate.

For soil drenches to work, the trees must be healthy enough to move the product from the roots up into the foliage. If trees are already in a state of decline, due to HWA, spray as much of the tree as possible with either horticultural oil or insecticidal soap to kill as many of the pests as possible. Then treat the following season with imidacloprid.

Determining Control:

It is not easy to know if HWA has been successfully controlled. The white filaments of wax may still be on the foliage following insecticide treatments. In many cases, it takes examination of the insect under magnification to see if it is dead. The best way to learn if the trees are recovering is to wait until the next flush of hemlock growth to determine if growth has improved. Even when working properly, a soil drench of imidacloprid may take a year or longer to show control. Do not expect instant results, be patient.

Follow-Up Treatments:

Any of these treatments can last anywhere from 1-5 years depending on their success and proximity to infested hemlocks. Successful treatments are usually a function of the initial health of the tree and the amount of soil moisture when treatments were made. Keep monitoring new growth for the white, waxy wool of the adults. Re-treatment is necessary when adelgids are found on many of the branches.

Large Tracts of Hemlock:

In wooded areas with many hemlocks it is not possible to save every tree. If deciding to treat with a soil drench, determine budget for materials and treat trees that are most important to the landscape or important as seed trees. Measure and mark with paint the trees to be treated. Small understory trees can be sprayed with horticultural oil in the fall.

Precautions:

Always read and follow pesticide labels. The label contains other important information not included in this article. The use of any insecticide can have unwanted consequences to the environment. Be sure to follow all label directions. Do not apply insecticides to the soil near surface water such as streams or ponds or where the soil is exceedingly rocky. Only spray trees if the material will not drift into open water such as streams or ponds or onto adjacent property. Do not exceed labeled rates of products. Applying a higher rate than what is labeled will not increase control.

Additional Sources of Information:

Fact Sheet: Hemlock Woolly Adelgid, Penn State Extension

Fact Sheet: Elongate Hemlock Scale, Penn State Extension

Fact Sheet: Recommendations for Hemlock Woolly Adelgid Control in the Landscape, North Carolina State Cooperative Extension Service

Video: Hemlock Woolly Adelgid, Michigan State University Extension

Video: Invasive Species: Hemlock Woolly Adelgid - David Orwig, Harvard University

Reference:

Recommendations for Hemlock Woolly Adelgid Control in the Landscape, North Carolina State Cooperative Extension Service, by Dr. Jill R. Sidebottom, Area Extension Specialist and Christy Bredenkamp, Swain and Jackson Horticultural Agent, June 2009.