Insecticide For Hemlock Woolly Adelgid

Hemlock woolly adelgid is an invasive species that is currently posing a very serious threat to the Eastern hemlock tree in North Carolina. This pest sap-sucking insect can cause hemlocks to die within five years after infestation. The good news about this threat is that nationally, the United States government has designated $4 million for a hemlock woolly adelgid research project. This means that there will be a lot more attention and research dollars dedicated to understanding what type of insecticide is best for eliminating a hemlock woolly adelgid infestation. Hemlock woolly adelgid is a tiny sap sucking insect that lays its eggs inside of hemlock trees. This may not sound like a bad thing, but when the eggs hatch, the hatched nymphs suck the tree’s sap and weaken it. If left unchecked for several years, this infestation can completely destroy a hemlock tree. So what happens if your client needs help with these pests?

Insecticide For Hemlock Woolly Adelgid

The first generation of overwintering adults lay eggs in large clusters in the woolly masses during warm weather in late winter and early spring. Eggs start to hatch in early April, depending on spring temperatures. A second generation lays eggs in June, hatching in July. The newly hatched nymphs, or “crawlers,” are only mobile for about a week while they look for a place to settle on a hemlock branch before entering a period of dormancy. By mid-September, dormancy is broken and they begin to feed again. First-instar nymphs begin producing wool in late September and spend the winter feeding on trees as mature females.

Under the right conditions, HWA can kill eastern hemlock trees in as few as four years. Cold, hard winters lower HWA survival rates. Extreme cold temperatures, below -15 degrees Fahrenheit, cause widespread HWA mortality. Rising temperatures, due to climate change, will likely allow HWA to expand northward throughout the hemlock’s range and pose a threat by reducing suitable habitat for hemlocks.

Integrated Pest Management Strategies

There are three main tools used to mitigate HWA and its impacts. These include insecticide treatments, biological control agents such as predatory insects, and cultural practices such as breeding hemlock for resistance and regenerating other desired tree species where hemlocks have died. While HWA has caused significant hemlock mortality across much of the tree’s range, many trees are surviving and in need of treatment.

Although HWA is difficult to treat and there are challenges to protecting hemlock stands not yet affected, conservation of this species is still possible. Insecticides are effective in settings from urban landscapes to managed forests. Strategies that focus on chemical control for the short term and biological and cultural control (natural enemies and host resistance) for the long term likely have the best chance for success.

Evaluate Trees

Not all hemlocks can be saved. It is too expensive to do so, as insecticide treatments are necessary every few years. Treat hemlocks that are the healthiest, most vital to the ecology of the landscape or forest, and easiest to reach and treat. Consider removing nearby untreated, symptomatic, declining hemlock trees to eliminate HWA sources that may reinfest treated trees. Do not remove healthy hemlock trees not designated for treatment as these trees may be genetically resistant superior trees capable of surviving HWA.

If trees do not have HWA, they do not require treatment. Insecticide applications are only necessary once a light infestation is present or if adjacent trees are infested. Retreat only if evidence of HWA reappears. Treating infested trees aggressively, while they are in good health, is the best way to maintain them. Severely defoliated trees are unlikely to recover, even with treatment.

Try to minimize tree stress. If possible and practical, water trees during periods of drought, properly mulch around trees, and prevent compacting soil around roots with heavy equipment. Do not fertilize infested trees. This only aids the survival and reproduction of HWA. Prevention alone does not always work. Infested trees will usually die in several years unless an insecticide treatment is applied before the tree declines.

Insecticide Treatments

Insecticide application methods include foliar broadcast sprays, soil and stem injections, soil drenches, soil tablets, and basal trunk sprays (Table 1). They are applied on an individual-tree basis, costly, and labor-intensive. Foliar sprays, soil drenches, trunk sprays, and soil tablets are described here, as these treatments do not require complex, expensive equipment.

Although chemical controls have proved effective in controlling HWA in yards, parks, and forests, the cost and effort associated with insecticide treatments make them impractical for treating large, forested areas, although it can be done. In some state and national parks and forests, hundreds—even thousands—of trees have received treatment.

Until long-term solutions involving biological control agents and/or host resistance are available, insecticide treatments are recommended. Hemlocks should be treated with either horticultural oil, insecticidal soap, imidacloprid, or dinotefuran. The specific application method used depends on several factors, including tree vigor and size, soils, accessibility, and proximity to water.

Hemlocks are not insect pollinated. Toxicity to pollinators following imidacloprid or dinotefuran treatment is not critical because bees do not collect hemlock pollen and few flowering plants grow in the shade under hemlocks. Photo credit: Dave Jackson.

Horticultural Oil or Insecticidal Soap
Oils and soaps are the safest insecticides for controlling HWA. They are not toxic but kill the insect by smothering it, dissolving its exoskeleton, and/or disrupting cell membranes as the spray dries on the pest. Applications are most effective during two treatment windows when crawler stages of HWA with no protective wool covering are active. The first is following egg hatch from late March through early April; the second is from early September to October before crawlers start producing wool. Treating at other times of year will result in poor control and may result in foliage burn.

For either product, use a 2 percent solution (2.6 ounces of spray material per gallon). There is no residual control with these materials; once dry they will no longer control HWA. Using these materials effectively is difficult if you do not have adequate spray equipment, especially for large trees. Using backpack or handheld sprayers, it is possible to treat trees 10 feet or smaller in height. Larger trees require a high-pressure sprayer, which may involve hiring a commercial pesticide applicator.

For complete coverage, spray until runoff. Drench the entire tree, including the undersides and tops of all limbs. A forceful spray is necessary to obtain adequate coverage. Be sure oils stay well mixed with the water carrier during application. Trees will likely require repeated treatments.

Insecticide For Hemlock Woolly Adelgid

Hemlock Woolly Adelgid is a small insect that feeds on hemlock trees. The adelgid was first discovered in the United States in Virginia in 1951, and has spread to 13 states.

The Hemlock Woolly Adelgid has a protective coating of white wax that gives it a white, waxy appearance. In reality, however, the insect is dark brown or black in color. It feeds on the sap of hemlock trees, which causes the needles of the tree to start dying off and turning yellow.

If left untreated for long periods of time, this can cause a tree to die completely within one year.

List Of Insecticide For Hemlock Woolly Adelgid