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Detecting hemlock woolly adelgid

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.

There are no attractants or traps currently developed for HWA detection. Currently detection options include either ball sampling or visual surveys for the adult/woolly mass stage, or crawler surveys for the newly emerged nymphs. Before employing any of these detection methods, develop a search image for the key life stages of HWA you will see - ovisacs and/or crawlers.

Hemlock woolly adelgid infestation is thought to begin in the upper crown of dominant hemlocks, 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. Dispersal is by rain, wind, birds and animals. Wind and rain typically cause local dispersal, which can be a few centimeters or a few metres. Because of this, HWA distribution within the tree and within a stand will be clumped at early stages of infestation, so keep this in mind when surveying for the pest.

Developing a search image

Ovisacs of the overwintering generation are the easiest life stage to see. Once the adelgid begins feeding, around late September to early October, it begins to produce a white waxy woolly covering which becomes more pronounced through the winter. These woolly ovisacs are visible on the undersides of twigs, at the base of the needles. Look for these on the underside of terminal and lateral branches. There may be few ovisacs at early stages of infestation, but numbers increase as the infestation progresses. The woolly patches are easiest to see in late winter or spring when they are newly formed (March to May in southern Ontario).

patchy ovisacs on hemlock branch heavy hemlock woolly adelgid infestation
Patchy ovisacs on bottom of hemlock twigs.
Images from left: William M. Ciesla, Forest Health Management International,; Michael Montgomery, USDA Forest Service,

Especially from a distance, spider sacs, spittle bugs and scale insects can be mistaken for HWA ovisacs. However, spittlebug foam is not found in winter or early spring, and scale insects are found on the needles and not the twigs. Spider egg sacs are more fibrous than the woolly HWA ovisac. If in doubt, take a photograph and report the find to CFIA.

Prior to the development of the white woolly ovisac over the fall and winter, HWA is even more difficult to see. During the summer and early fall, the immobile adelgid aestivates as an nymph and doesn't feed or develop. The nymph is very tiny and at this stage has a light ring of wool around it. The nymphs will be easier to see using a hand lens. Residual wool from previous generation of HWA may also be seen.

nymph with ring of wool
Hemlock woolly adelgid nymph with wool ring.
Images: Kelly Oten, North Carolina Forest Service,

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. They can be surveyed using the crawler survey approach.

hemlock woolly adelgid crawler stage hemlock woolly adelgid crawlers on ovisacs
Hemlock woolly adelgid crawler stage left, crawler on ovisacs right.
Images from left: Kelly Oten, North Carolina Forest Service,; Pennsylvania Department of Conservation and Natural Resources - Forestry, .

Detection methods

Visual survey approaches:

Presence/absence survey: The CFIA conducts presence/absence surveys for HWA. It 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 CFIA's survey protocol.

Informal approaches: When in a stand with hemlock, examine trees and branches along waterbodies for ovisacs. These are the trees most likely to be infested first. Another informal approach is to examine upper branches from felled hemlock to look for the life stages described above.

Read more about early detection of HWA using visual methods.

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 horizontally oriented 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.

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

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