Hemlock Woolly Adelgid Treatment and Control

Hemlock Woolly Adelgid

Native to Asia, the hemlock woolly adelgid (Adelges tsugae) is a small, aphid like insect that threatens the health and sustainability of eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana) in the Eastern United States. Hemlock woolly adelgid was first reported in the Eastern United States in 1951 near Richmond, Virginia. By 2005, it was established in portions of 16 States from Maine to Georgia, where infestations covered about half of the range of hemlock. Areas of extensive tree mortality and decline are found throughout the infested region, but the impact has been most severe in some areas of Virginia, New Jersey, Pennsylvania, and Connecticut.

Hemlock decline and mortality typically occur within 4 to 10 years of infestation in the insect’s northern range, but can occur in as little as 3 to 6 years in its southern range. Other hemlock stressors, including drought, poor site conditions, and insect and disease pests such as elongate hemlock scale (Fiorinia externa), hemlock looper (Lambdina fiscellaria fiscellaria), spruce spider mite (Oligonychus ununguis), hemlock borer (Melanophila fulvogutta), root rot disease (Armillaria mellea), and needlerust (Melampsora parlowii), accelerate the rate and extent of hemlock mortality.

Here’s a good overview and description and research on predators.

The most common control treatments for Hemlock Woolly Adelgid are Horticultural Oil and Merit (Imidacloprid).

Horticultural Oil is an ultra refined oil that is almost non-toxic to other organisms and is not hard on insect predators.


  • Horticultural Oil will give you quick results.
  • Is relatively cheap.
  • Is available at most stores that sell gardening supplies.
  • Can be easily applied with hand or backpack sprayer if the plant is not too large.


  • As oil works by smothering, any insect not covered with oil will not be killed.
  • Oil has no residual and must be re-applied if new insects reach the plant.
  • Can burn if it is used when the weather is hot and sunny, or if sprayed before new growth has hardened.
  • Can be hard to apply on tall trees, or plants that are not accessible to application equipment.

Imidacloprid is a neonicotinoid, which is a class of neuro-active insecticides modeled after nicotine and acts on the nicotinic acetylcholine receptor. Imidacloprid is notable for its relatively low toxicity to most animals other than insects due to its specificity for this type of receptor.


  • Imidacloprid is a systemic (a chemical that is taken up by the plant) and will travel within the plants vascular system to all parts of the plant where it will kill all the Hemlock Woolly Adelgid.
  • Will stay in the plant and kill the pest for two years (some tests have shown that this can be a lot longer).
  • Is usually injected into the soil, or sometimes directly into the tree trunk, reducing the chance of drift to non target areas.
  • Can be applied by digging small holes around the plant and placing the product in the hole, thus allowing plants to be treated that can not be reached by an oil spray.
  • Has a low mammalian toxicity, compared to some other common pesticides.
  • Breaks down quickly when exposed to sunlight


  • Imidacloprid is expensive.
  • Takes time to travel from the roots to the branches, thus Hemlock Woolly Adelgid can feed for several weeks and in a heavy infestation may have time to kill the plant before the chemical can control them.
  • Under certain conditions has been known to travel through the soil to water sources. ( Before applying, read the label and pay attention to situations where this may happen. )
  • Will kill beneficial insects including predators of Mites, another common problem on Hemlocks, though these pests rarely will kill Hemlocks.
  • Breaks down quickly when exposed to sunlight

I will thoroughly evaluation your trees and property before deciding which method will be the most appropriate for your situation.