Keeping the Legacy Alive

Chemical Control

Chemical pesticides are widely used as a primary defense against hemlock woolly adelgid (HWA) populations in the US. Chemical control can delay adelgid impacts, essentially providing a ‘life support’ for the trees and allowing more time for nature to adapt and for the development of longer-term solutions. In the US, it has been the most successful short-term method available for protecting high-value hemlock trees and slowing the spread of HWA. Chemical treatments to save trees may take as long as two years to translocate to the tree canopy where it becomes effective against HWA. A hemlock tree may lose capacity to translocate the chemical, and recovery will not be possible if treatment is not delivered early enough.  

Because HWA is protected by its woolly covering it is not effective to spray chemicals to control HWA. Systemic insecticides have been applied by stem injection, basal bark application, and soil injection. Systemic insecticides are chemicals that are carried in the cells inside the tree that transport water from the roots to foliage. Stem injection is the only method currently permitted in Canada. Direct, systemic injection into trees greatly decreases environmental exposure to the chemical as well as total product amounts required.

The only product currently available in Canada that is registered for use against HWA is IMA-jetTM, using the active ingredient, imidacloprid. Imidacloprid is in the neonicotinoid class of insecticides and acts as a neurotoxin to insects. Although imidacloprid is highly toxic to insects it has a low toxicity to vertebrates (mammals, birds and fish). It is often used as the active ingredient in tick and flea treatments.

The stem injection of imidacloprid is effective for 5-7 years and will require future treatments to keep the tree from becoming infested again. Stem injection treatments can be costly and it is recommended that you contact Arborjet or a local certified pesticide operator for pricing. 

To inquire about chemical treatment for hemlock on your property please contact a licensed applicator. Contact list will be up-dated as more licensed applicators become available.


Scott Robinson
Cell: 902-875-7686

Less toxic control products

Insecticidal soaps and horticultural oils have been used instead of insecticides. They have a low toxicity and have been effective when properly applied. Multiple applications are required and thorough coverage on larger trees is extremely difficult. Horticultural oils function by suffocating the adelgid; infested needles and well-protected waxy ovisacs must be completely drenched. Traditional pyrethroids (an organic compound found in the majority of household insecticides) and other pesticides are considered less effective than systemic insecticides because of the difficulty in achieving adequate coverage and the larger amounts required for external spray applications, greatly increasing the toxicity exposure to other organisms. Other less toxic insecticides are also being tested by CFS.


There has been some concern about the use of neonicotinoids and its impacts on bees and insect pollinators. Neonicotinoids are a commonly used pesticide in agriculture but are of serious concern when applied over large areas where pollinators and other non-target insects can become exposed to the pesticide. There is evidence that neonicotinoids may affect non-target invertebrate species across terrestrial and aquatic ecosystems.

Please carefully consider the options available and consult with a professional certified pesticide operator if you decide to use neonicotinoids to help keep hemlock alive and mitigate the spread of HWA on your property.



Risk Assessment of Imidacloprid use in forest settings on the aquatic macroinvertebrate community.


Environmental Risks to Arthropods from Imidacloprid Applications for Hemlock Conservation.