Hemlock woolly adelgid infestation probably begins in the upper crown of dominant hemlocks where it arrives with birds or wind, so it is difficult to detect visually at early stages. The insect subsequently spreads down to lower parts of the crown and to smaller trees.

Hemlock woolly adelgid detection

The Canadian Food Inspection Agency (CFIA) has been conducting surveys for this pest since 2007, and continues to be the lead agency for this insect. This page is an overview of methods to monitor for HWA, but any suspect finds should be reported to CFIA.

Currently detection options include either ball sampling or visual surveys for the adult/woolly mass stage, or crawler surveys for the newly emerged nymphs.

Developing a search image

Ovisacs of the overwintering generation are the easiest life stage to see.They are visible  on terminal and lateral twigs at the base of the needles. There may be few ovisacs at early stages of infestation, but numbers increase as the infestation progresses.

The crawler stage, nymphs newly hatched from the eggs, are the only mobile stage of the insect. Crawlers are tiny (less than 1/2 mm long) and are reddish-brown in colour. At this stage, they may fall from the tree during rain or wind events.

Detection methods

Formal presence/absence survey methods include visual survey, ball sampling and crawler surveys. Informal trunk and ground sampling are simple methods that anyone can easily incorporate into their routines. These latter two methods can also be combined with ball sampling to provide a more robust survey, which, according to MacQuarrie et. al. (2021) shows a high probability of detecting HWA

Visual survey: This method calls for examining two branches from the lower crown (within easy reach) of each tree for woolly masses, until either an infested tree is found or 100 un-infested trees have been examined. The survey proceeds along a random path through the stand. Details of the visual survey protocol.

Ball sampling: The ball sampling method was designed by Canadian Forest Service Scientists for early HWA detection. This technique calls for launching a Velcro-covered racquetball into the upper crown with a sling shot or ball launcher to sample for woolly masses not visible from the ground. The woolly masses are caught in the Velcro and can be more easily identified using this strategy vs the visual survey. The advantage of this method is that one can sample the upper crowns of very large hemlock. See details about ball sampling or watch a video about how to do it.

Crawler sampling: This method calls for sticky traps to be deployed under hemlock trees. Crawlers dislodged from the tree by wind or rain are caught on the traps and can be identified. With this method, a large area can be cost-effectively sampled. See details about crawler sampling.

Informal approaches: Super-canopy or edge trees that catch the wind are most likely to be infested first, as are trees along waterbodies. When you are in a stand with hemlock, have a look at the most vulnerable trees for signs of HWA either via an informal visual survey above, or scan the trunk and forest floor for dislodged ovisacs or the presence of them on downed branches or twigs.

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

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