Biology

The Hemlock Woolly Adelgid is an aphid-like insect that targets hemlock trees. The insect obtains nutrients from storage cells at the base of the needles which ultimately kills the needles and eventually the tree.

Life Cycle

In its native range in Asia, HWA undergoes both sexual and asexual reproduction. To reproduce sexually they must find a suitable species of host tree which is not present in North America and thus are limited to asexual reproduction in North American where only female adelgids are present. The first of two HWA generations per year hatches in late spring and are known as crawlers. This is the most mobile stage of HWA and although the crawlers are wingless, they may be dispersed by other means such as wind and birds until they settle at the base of a healthy hemlock needle. Once settled they remain at that location for the summer and winter and will, indeed, feed throughout the winter. In early spring, this generation gives rise to the second, and shorter, generation which consists of both wingless and winged offspring. It is the winged offspring that are known to be capable of sexual reproduction, however in North America this stage is not completed. 

Geographic Extent

Although this pest is new to Nova Scotia, it has a long history throughout North America and Asia. As a native to Asia, the HWA made its way to eastern North America in the 1950s, where it was first found in Virginia, US, likely originating on nursery stock from Japan. The HWA has been established in western North America for a significant period of time. This pest is not considered a threat in western North America and Asia because its populations are kept in control by natural predators.

The potential for devastation at the hands of the HWA in the east is due to the lack of natural predators, and the speed at which it can spread and establish a population. Severe damage has recently been inflicted on hemlock populations throughout eastern USA, and the establishment of HWA in Nova Scotia is suggestive of a similar impending fate in the northernmost range of the eastern hemlock. 

Impact to Nova Scotia

The HWA targets all ages and sizes of  hemlock and eventually this pest causes a significant loss of needles and impairs the tree from producing new growth. Within as little as four to 10 years following HWA infestation, hemlock trees often become discoloured and slowly die. Contributing factors such as drought can accelerate decline in affected trees.  Since the HWA undergoes asexual reproduction the transfer of just one individual to an unaffected hemlock stand can establish a pest population, and with two generations per year, the population size of the HWA can expand rapidly. Although the Nova Scotian winter can sometimes feel endless and inhospitable to those of us living through it, the HWA is resilient and feeds on hemlocks throughout this cold season. One factor that may limit, or at least slow down the spread of HWA northward in Nova Scotia are temperatures colder than the pest has encountered previously. Within Nova Scotia, HWA is currently contained in the southwestern region of the province. The first appearance of this pest in Atlantic Canada was confirmed in July 2017 in Digby County, Nova Scotia. As of now, HWA has been confirmed in four additional regions of Nova Scotia: Queens, Shelburne, Yarmouth, and Annapolis counties. Neighbouring counties, which include Kings County, should be prepared for the possible invasion of HWA to their own hemlock stands. The spread of HWA is mediated by several factors including birds, mammals, storms, and human transport. Within the infected counties, priority is currently placed on the containment of the pest, while organizations such as the Canadian Food Inspection Agency (CFIA), the Canadian Forest Service, and the Nova Scotia Department of Lands and Forestry work to determine the most effective strategy to eradicate or control pest populations.

​adapted from H. Roberts BNS Newsletter (2018) Vol 45 No. 3 pp34-37