Hemlock woolly adelgid is a tiny, aphid-like insect from Asia that has killed large numbers of eastern and Carolina hemlocks across the eastern United States. In a yard, it is treatable, especially if you catch it early. The first step is knowing what you are looking at.
The short answer: small white, cottony tufts at the base of the needles on the underside of hemlock twigs, most visible from late fall through spring, are the classic sign of hemlock woolly adelgid. Untreated hemlocks gray, thin and can die within several years. Systemic insecticides applied by a certified arborist or licensed applicator protect landscape trees for years. In some states you are asked to report new finds.
What it looks like
The adelgid itself is barely visible. What you see is the white, waxy wool it produces to protect itself and its eggs. According to Penn State Extension, look for:
- White, cottony masses about the size of a cotton swab tip, at the base of needles on the undersides of the twigs.
- Needles that turn grayish green instead of deep green.
- Needle drop and few or no new shoots in spring.
- Gradual thinning of the crown, starting on lower branches.
The wool is most noticeable from late fall to early summer. In midsummer the insects are dormant and much harder to spot.
| Sign | Hemlock woolly adelgid? | Other possibilities |
|---|---|---|
| White woolly tufts under twigs at needle base | Yes | Spider egg sacs or spittle are usually scattered and not at the needle base |
| Gray-green needles, little new growth | Often | Drought, scale insects |
| Brown needles in late summer, mostly lower branches | Sometimes | Drought, elongate hemlock scale |
| Whole tree brown within a season | Less likely | Root damage, drought on a young tree |
How to check your trees
Start with the lower branches you can reach. Turn over the tips of several branches on different sides of the tree and look at the base of each needle on the newest growth. Use a hand lens if you have one. Check again after a few weeks if you are not sure, since the wool becomes more obvious as winter goes on.
Why it matters
The insect feeds on stored starches at the base of the needles. Over time the hemlock stops producing new growth and loses its needles. Penn State Extension notes that trees can die within four to ten years of infestation, although that varies with tree health and climate. Stressed trees, especially in drought, decline faster.
The adelgid spreads on the wind and on birds, deer and people, and on nursery stock. That is why Michigan State University and the state of Michigan treat it as a regulated pest and ask the public to report suspected infestations. Check your state’s rules; some states have quarantines on moving hemlock material.
How treatment works
For landscape trees, the main tools are systemic insecticides that move through the tree to where the adelgids feed. Penn State Extension describes the two most common:
- Imidacloprid, applied as a soil treatment or a trunk injection. It is slow to take effect, often a year or more on large trees, but protection lasts for several years.
- Dinotefuran, usually applied as a bark spray on the lower trunk. It acts faster, within weeks, but protection is shorter. It is often used on trees that are already heavily infested, sometimes with imidacloprid for long-term control.
On small hemlocks that can be covered completely, horticultural oil or insecticidal soap sprayed on the foliage can reduce adelgid numbers. Coverage has to be thorough, and it needs to be repeated.
In forests, researchers are releasing predatory beetles that feed only on adelgids, as a long-term biological control. These are not a quick fix for a yard tree.
Many of these products have restrictions on where and how much can be used, especially near water. That is one reason treatment of larger trees is usually done by a certified arborist or a licensed applicator. Always follow the product label.
What you can do yourself
- Inspect your hemlocks in late fall, winter and early spring. Turn over branch tips and look at the underside.
- Keep trees healthy. Water deeply during droughts and maintain a layer of mulch over the roots.
- Do not fertilize an infested tree with nitrogen. Studies cited by extension specialists suggest it can help the adelgid population more than the tree.
- Avoid moving hemlock branches, logs or nursery plants from infested areas.
- Remove bird feeders near hemlocks, since birds can carry the crawlers.
When to call a professional
Call a certified arborist as soon as you confirm adelgids on a hemlock you want to keep, or if a hemlock is near the house and already thin. Early treatment is far more effective than trying to rescue a tree that has lost most of its needles. The ISA Find an Arborist directory lists certified professionals. Your county extension office can confirm a sample and tell you about any local reporting requirements.
For other conifer problems, see why junipers turn brown, or browse the Tree Problems & Diagnosis hub.
Sources
- Hemlock Woolly Adelgid, Penn State Extension
- Hemlock Woolly Adelgid, Michigan State University
- Find an Arborist, International Society of Arboriculture
Top photo: Michael Montgomery, USDA Forest Service, CC BY 3.0 us, via Wikimedia Commons