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Non-Toxic Methods for Saving Hemlocks and Other Trees from Invasive Insects

Early spring is the perfect time to inspect your valued trees, and if need be, consult with a tree specialist if you find signs of damage from invasive insects such as the Hemlock Wooly Adelgid, the Emerald Ash Borer and the nematode which results in Beech Leaf Disease. If so, make sure that you ask your arborist about all treatment methods, including non-toxic and biological controls. This way you can ensure outcomes that will provide for the health of your tree and the diversity of your yard. Here are some common backyard tree species and the pests they may be afflicted by: 


HEMLOCKS: Our valued native hemlocks, both Eastern and Carolinian (Tsuga canadensis and Tsuga caroliniana), respond very well to treatment. Those treatments options include:

1.    Horticultural oil or insecticidal soap: Horticultural oil or insecticidal soap applications, once or twice annually in May and late June, depending on the level of HWA infestations, are the preferred method of control with the least impact on non-target and beneficial species.

The oils and soaps are sprayed onto the limbs and shoots and are most effective if the adelgids are thoroughly covered. This method can be done by the homeowner for smaller trees; for larger trees a professional should be consulted. Timing is important to avoid affecting any pollinators who may be in the area.

2.    ST Ladybeetles: Also available to the homeowner is a biological control, a tiny ladybeetle known as ST (Sasajiscymnus tsugae). It is the only HWA predator that is reared commercially and is now available to the public from Tree-Savers. This species feeds on all stages of HWA and studies have shown its ability to adapt and overwinter in the northeast. 

3.    Neonics: The use of systemic chemicals (including the neonics imidacloprid and dinotefuran) should be the last treatment method of choice. If your arborist immediately recommends a neonic, you should ask why a less toxic remedy would not work. There may be situations where spraying with oils or soaps would not be effective such as 1) inability to reach all of the of the tree due to its height, 2) the proximity of a building making it difficult to use necessary equipment, and 3) extent of infestation making it unlikely to cover all adelgids with oils or soaps.

Hemlock wooly adelgid infestation. Photo by Steven Katovich,

Check to see if your state requires a licensed applicator to apply a neonic. Ten states have this restriction. The best application method would be the trunk injection method since it minimizes exposures to other species. However, remember that any use of a neonic should be done with careful consideration of timing and following all label directions to protect pollinators, other beneficial insects and adjacent vegetation.

Also, it has been reported that an unwanted outcome of using a neonic is the possible infestation of spider mites, due to killing of the beneficials insects that control spider mites.

ASH TREES: The Emerald Ash Borer is another pest that has already done much damage to ash trees in New England. Trees that are heavily infected should be removed and destroyed by a professional. An ash tree infested with EAB may show the following signs:

  • "blonding" or large strips of bark falling off due to increased woodpecker activity;

  • tree canopy dieback, yellowing, and browning of leaves;

  • distinctive D-shaped exit holes in the bark of the branches and the trunk left by adult beetles emerging from the tree.

A good rule of thumb is to treat a tree that still has half or more of its canopy. The most effective treatment is a chemical called emamectin benzoate. Another practice that may be recommended, but should be a last resort if used at all, is using the neonic imidacloprid as a trunk injection. In either case, caution to protect pollinators and water sources should be taken as both of these chemicals are toxic to bees, fish and aquatic species.

The authoritative publication about pesticide use against EAB is “Insecticide Options for Protecting Ash Trees Against Emerald Ash Borer;” by some of the leading experts on EAB.

BEECH TREES: Another newer pest is a foliar nematode which results in Beech Leaf Disease, defoliating many beech trees in New England. For small trees, 2-4” diameter, drenching the soil around the tree with potassium phosphite, a fertilizer, several times a year may improve the health of the tree. Be careful to drench over a wide area so as not to subject the roots to a high dose of this salt. For details see Beech Leaf Disease Management Options.

Close up of a beech leaf showing symptoms of beech leaf disease. Photo courtesy of CT Agricultural Experiment Station via by Yonghao Li.

In conclusion, have trust in your arborist and ask questions to make sure all options, including non-toxic ones, are considered when making a treatment decision.


Written by Mary Wilson. Mary has lived in Newtown, Connecticut for over 50 years. Her love for the New England landscape has led to advocacy work for the protection of habitats and the wonderful creatures they support. She has degrees in education and chemistry.



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