Hemlock Woolly Adelgid Management Strategies

HWA is an invasive, aphid-like, sap sucking insect found at the base on hemlock needles which attacks and eventually kills hemlock trees dependent on population levels. It has few natural predators and can grow significantly during the late fall and early spring when hemlocks produce abundant quantities of sugar and amino acids. HWA was identified in 2017 in southwest areas of Nova Scotia and a quarantine zone has been applied to the 5 western counties.

For more information about HWA in Canada, visit the Canada Food Inspection Agency website via this link

Woodlot owners with hemlock groves and/or mixed forests with a composition of hemlock are faced with a management dilemma since HWA threatens the long-term existence of this ecologically important species in terms of the unique environment it creates for wildlife and other organisms, societal and recreational benefits, and for overall ecosystem health. For woodlot owners whose objectives are mainly focused on ecosystem health and biodiversity, it is a reasonable goal to investigate and consider available management strategies to maintain hemlock composition in perpetuity for our next generation of society.

There are 3 management strategies to consider: chemical control, biological control, and/or applying silvicultural methods.

Chemical control involves the use of insecticides to provide short-term protection for hemlock and help to conserve genetic diversity of existing hemlock until a longer-term strategy such as a biological control program is developed. In Canada, the only method of application that is currently permitted is the tree injection method, and further research is needed to explore different options of application. Additionally, the cost of the tree injections can be high, and it may not be feasible for a landowner to pursue at the stand-level, so may be better suited for treating a few, select trees. This method is currently being explored by various agencies including government regarding its application and feasibility.

Biological control through use of natural insect predators or ‘biocontrol agents’ can control HWA populations since they hold the best possibility of a successful long-term approach to saving eastern hemlock forests. US scientists have been studying adelgid natural predators for decades and have made significant gains in selecting and rearing several predators that demonstrate abilities to keep the adelgid population in check. HWA is also found on the west coast of Canada-US where it feeds on western hemlock but causes only minor damage due to natural insect predators that keep adelgid populations in check.  Scientists at the Canadian Forest Service (CFS) are building upon the foundation of existing US research and considering the natural predator on the west coast to hasten development of HWA biocontrols here. It is possible that biocontrols may be available in 5-10 years.

Silvicultural methods such as thinnings and creating various size canopy openings is another tool being used to address HWA impacts on woodlots. Early research suggests that silvicultural thinning to increase light penetration in infested hemlock stands can increase the photosynthesis potential of hemlocks to increase their vigour and help to decrease HWA survivorship. Woodlot owners may want to diversify a hemlock-dominated forest in anticipation of HWA-related mortality. Creating openings in dense, shaded hemlock canopies will allow more light to reach the forest floor that aids in the establishment and growth of a wider range of tree species. When an insect outbreak occurs, not all trees may be affected at once and some may be more resilient than others.

Landowners will be informed once more information becomes available on each of these possible HWA controls. In the meantime, if you are interested in applying any of the control measures please contact us at info@westernwoodlotcoop.com