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Monitoring for HWA: how and when

HWA on a branch. Photo: Mark Whitmore


Hemlock woolly adelgid (HWA) is visible anytime from November to July, but is most obvious in late winter through early summer. HWA is nearly invisible after mid summer, when the insect is in a resting stage.

What to look for

Look for white woolly masses (ovisacs) on the underside of twigs at the base of needles in winter/spring through early summer. In the early stages of the infestation, there may only be a few of the woolly masses, typically on the new growth. As the population grows HWA will also occur on older twigs, and at the base of nearly every needle. Flip over and inspect the underside of twigs on the lower branches of hemlock trees; walk around the tree inspecting several branches at different points. Note that HWA is always on the twig, not the needle – and while it’s possible there could be only one HWA on a twig, it’s more likely that there will be several, which helps distinguish it from spider ovisacs that are usually solitary.

Freshly fallen branches and twigs are an opportunity to survey the tops of trees.
Freshly fallen branches and twigs are an opportunity to indirectly survey the tops of trees.

Any monitoring effort is worthwhile, even if you only have time to inspect one tree.  If you have limited time, focus your efforts around streams and water bodies where birds are more likely to land and spread the insect. Anecdotally this is where infestations seem more likely to start, and also there are often low branches along shorelines that are easier to inspect.

When walking through the stand watch the ground for fallen twigs or branches and inspect them; HWA often starts high in the canopy, so fallen twigs are a valuable opportunity to see what’s happening above you.

We’ve produced a handout which you may download as a pdf. Paper copies are available on request. If you’re watching for HWA, even on just a few trees, please let us know (

In larger stands if you’re feeling serious you can use the sampling methodology from the US Forest Service to randomly distribute your efforts. Inspect multiple trees at different locations, since infestations are initially patchy. As a guideline, in mid-sized stands sampling 100 trees gives you a good probability of detecting HWA even at light infestation levels, and 10-20 trees would be needed to detect moderate infestations. But if 500 people each inspect one tree, across a wide geographic range, it is likely to be better than one person inspecting 100 trees (and ideally, all of the above!) – so whatever you can find time to do will help.

Look alikes

  • Spider sacs: are made of much stronger fiber than the wool of the HWA and are usually not closely appressed to the twigs.
  • Spittlebugs are found on twigs but make watery, white foam and are not found in winter .
  • Scale insects are found on the needles, not on the twigs
  • Other potential look-alikes are bird droppings and pine pitch.

If in doubt consult this look-alike fact sheet. HWA will usually (though not always) have multiple egg sacs on the same branch, often in a row, which can help differentiate it from look-alikes such as spider egg sacs.

If you find it

Sightings in Ontario can be verified or reported online or using your smartphone via Or sighting can be reported directly to the CFIA Plant Health Surveillance Unit, call 1-800-442-2342, or contact your regional CFIA office.

In NY State, you may contact your local Cornell Cooperative Extension office or the New York State Department of Environmental Conservation at: 1-866-640-0652, e-mail In Vermont call 802-879-5687, in New Hampshire 603-464-3016, and in Maine 207-287-2431.

Take photos of suspected infestations and detailed notes, including GPS coordinates, and the date. Although the risk of transporting HWA is low, don’t go from a suspected HWA-infected site to any other hemlock forest without first changing your clothes.


Cornell HWA early-detection fact sheet

HWA sampling methodology

Hemlock Woolly Adelgid images