Hemlock Woolly Adelgid Control

Hemlock Woolly AdelgidThe Hemlock Woolly Adelgid is a sucking insect that is like an aphid. It has no natural enemies and left untreated will kill Hemlocks. You can find Hemlock Woolly Adelgid by looking for small cottony masses at the crotch of the needles. The good news is that Hemlock Woolly Adelgid is easily controlled in the landscape and treatment can be applied any time of year. Late winter or very early spring treatments are most effective in New Hampshire and New England. The bad news is that Hemlock Woolly Adelgid is almost impossible to control in a forest situation.

The most common control treatments for Hemlock Woolly Adelgid are Horticultural Oil and Merit (Imidacloprid). Here I discuss the benefits and problems of each.

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, especially on humid or overcast days, or if sprayed before new growth has hardened. Oil can also damage if frost occurs within 48 hours after treatment.
  • 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 five years (some tests have shown that this can be a lot longer).
  • Has usually been injected into the soil, or sometimes directly into the tree trunk, reducing the chance of drift to non target areas. The most recent and now preferred treatment is to apply this product onto the bark.
  • Even larger trees can be treated by the homeowner if strict attention to rates are followed.
  • Has a low mammalian toxicity, compared to some other common pesticides.
  • Breaks down quickly when exposed to sunlight


  • Imidacloprid takes a long time to travel from the roots to the branches, thus Hemlock Woolly Adelgid can feed for months and in a heavy infestation may have time to kill the plant before the chemical can control them. It can take up to 18 months for Imidacloprid to reach the top of large trees, and for some reason not yet understood the lowest set of branches on very large ones.
  • 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 if the application rates are too high, including predators of Mites another common problem on Hemlocks, though these pests rarely will kill Hemlocks.
    The amount of active ingredient needed to control Woolly far less than on the label of the product and different sizes of trees require different rates. If you plan to do this yourself contact IPM Of New Hampshire for instructions and rates.
  • There is a maximum amount of imidacloprid allowed per acre per year so not all trees on a property may be treatable in a single year.
  • Breaks down quickly when exposed to sunlight.


NOTE about bees. Imidacloprid is in the class of chemicals called neonicotinoids that have been blamed for Colony Collapse Disorder of bee hives. This has been disproved by science. Imidacloprid will; as many other pesticides do, kill bees if they come in contact with it. However bees do not forage Hemlocks so have little chance of contacting this product if properly applied. By looking out for flowering plants when treating trees and removing the flowers, or treating when they are not in bloom the chances of killing a bee when treating for Woolly Adelgid are close to zero.


For smaller accessible trees oil can be a good choice if you are willing to treat at least once a year. For larger trees that are not heavily infested, and those that are hard to access I recommend the Imidacloprid. Where a heavy population of Woolly Adelgid exists I recommend an oil treatment for quick knockdown and the Imidacloprid for long-term control.
Even for the largest trees knocking down a heavy Adelgid population as high as possible on the tree with oil can give the tree time while the Imidacloprid gets time to work.



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