An established population of hemlock woolly adelgid (HWA) was detected in Nova Scotia in 2017 and in the Niagara Region of southern Ontario in 2019. Following is an overview of management approaches used to conserve hemlock in the US. An inter-agency technical advisory committee is tasked with assessing management options for HWA in Canada.

General resources:

• NEW: HWA management plan for Canada   English   French

• Biology and Control of Hemlock Woolly Adelgid This USDA publication provides a general overview of HWA biology and management; control measures are from a US perspective.
• The HWA documentary gives an overview of HWA and biocontrol in the US.
• Eastern Hemlock Forests: Guidelines to Minimize the Impacts of HWA includes management decision making approaches.
• Website compiling HWA resources (U.S.)

Silvicultural approaches

In general, silvicultural management designed to improve hemlock health shows promise - vigorous hemlock appear to be less vulnerable to HWA. Forest thinning is found to improve hemlock carbon balance and is associated with lower HWA abundance. There is evidence that leaf chemistry is associated with susceptibility to HWA. In particular, high nitrogen and potassium levels were associated with increased vulnerability to HWA, so nitrogen fertilizers are not recommended. There are a number of resources, listed below, that may be helpful in forest management planning.

Resources:

• Management Strategies for Eastern North American forests Threatened by HWA, a webinar by USDA scientist Mary Ann Fajvan. Access through CIF or contact us for connection details.
• Silvicultural Options for Managing Hemlock Forests Threatened by HWA.
• Role of Silvicultural Thinning in Eastern Forests Threatened by HWA.
• Factors Associated with Hemlock Vulnerability to HWA.
• Elevated Light Levels Reduce HWA Infestation.
• Thinning Effects on Foliar Elements in Eastern Hemlock: Implications for Managing Spread of HWA.
• Foliar Chemistry Linked to Infestation and Susceptibility to HWA.

Biological control (US)

There are no know parasitoids of HWA, so work on HWA biocontrol in the US has largely focussed on prey-specific predators. Generalist fungi have shown promise in early lab trials, however, the non-target impacts of generalist fungi need to be considered. The search for predators has been conducted both in western North America and in Japan. Several predators have been identified for HWA control, and five species have been released to date (2018). However, results have been inconsistent and release of some species have since been discontinued. Two species of Laricobius beetles, L. nigrinus from western North America and L. osakensis from Japan show promise. On their own they are unlikely to provide HWA control; a broader suite of predators is needed. Leucopis, a silver fly native to western North America, may compliment predation by Laricobius beetles, and is being prepared for release.

Resources:

• Biology and Control of Hemlock Woolly Adelgid This USDA publication provides an overview of biological control agents and status to 2014
• Biological Control of Pests in Forests of Eastern US USDA and university scientists provide a detailed history of biocontrol programs to 2009.
• Silver fly Predators of the HWA the USDA describes early results on the newest HWA biocontrol agent.

Chemical control (US)

In the US, insecticides are used for short-term protection of hemlock trees from HWA. Imidicloprid is most often used, sometimes in combination with Dinotefuran, providing up to seven years of protection; both are neonicotinoid insecticides. They are applied to individual trees by methods such as basal bark spray, tree injection, soil drench, soil injection or time-release soil tablets. Currently, available treatments registered for use in Canada are systemic imidacloprid insecticides from Confidor and Ima-Jet, and landscape oil. The use of insecticides for HWA in Canada is being evaluated.

Resources:

• Insecticide Treatment of Hemlock Trees for HWA Cornell University's Hemlock Initiative gives an overview of chemical control options in the US.

Think you have found HWA?
Contact the CFIA Plant Health Surveillance Unit for assistance.