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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 are chemicals that are carried in the cells inside the tree that transport water from the roots to foliage. Systemic insecticides are applied by stem injection and basal bark spray application. Direct, systemic injection into trees greatly decreases environmental exposure to the chemical as well as total product amounts required. Currently there are four insecticides approved for use in Canada through Health Canada’s Pest Management Regulatory Agency. These insecticides and their various pros and cons are summarized in the table below:
Chemical insecticide | Application method | Time to take effect | Years of protection | Cost/cm diameter at breast height |
Imajet | stem injection | 6-9 months | 4-7 years | $1-2 |
Xytect 2F | stem injection | 6-9 months | 4-7 years | $1-2 |
TreeAzin | basal bark spray | c. 1 month | 1-2 years | c. $0.75 |
Starkle 20SG | basal bark spray | c. 1month | 1-2 years | c. $0.75 |
To find out more about potential impacts of chemical treatments go to our resource page.
To inquire about chemical treatment for hemlock on your property please contact a licensed applicator. Contact list will be updated as more licensed applicators become available.
Scott Robinson
Cell: 902-875-7686
Email: arborcom@gmail.com
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 the Canadian Forest Service. 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.