What is hemlock woolly adelgid?
Hemlock woolly adelgid (HWA), Adelges tsugae, is a tiny sap-feeding insect from East Asia. It settles at the base of hemlock needles and feeds on the tree’s stored starches. Each insect covers itself in a white, waxy fluff, which is what you see on infested branches.
It reached Massachusetts in the 1980s and has been spreading through New England ever since. Eastern hemlock, the native hemlock in Andover’s woods, has almost no natural defense against it.
Where are hemlocks in Andover?
Hemlocks grow naturally in shaded, cool spots: along brooks, on north-facing slopes and in mixed stands with white pine, which you’ll find on many wooded lots in West Andover and South Andover. They were also planted widely as hedges and screens, because they tolerate shade and can be clipped into a dense wall. A row of hemlocks along a property line is a common sight across town, and a hedge is often where an infestation is noticed first.
How do you spot it?
Check in late fall, winter or early spring, when the white tufts are largest and easiest to see.
- White woolly tufts at the base of the needles, on the underside of twigs. They’re about the size of a pinhead to a small grain of rice.
- A grayish, dull look to the foliage instead of deep green.
- Needles dropping and little or no new growth in spring.
- Thinning from the bottom and inside of the crown, with dieback moving up over the years.
Look at branches on several sides of the tree, especially lower branches you can reach. On a hedge, check the inside of the row as well as the outer face.
Don’t confuse HWA with spittlebug froth, which is wet, or with spider egg sacs, which are usually single and larger. Another pest, elongate hemlock scale, often turns up on the same trees. It shows as small, flat, brownish or white specks on the underside of the needles themselves rather than at the base. Our tree symptom checker can help match what you’re seeing to the likely cause.
How fast does it harm a tree?
Decline is slow at first, then speeds up. A lightly infested hemlock may look fine for a couple of years. Once the adelgids are widespread, the tree stops putting out new shoots, and with no new growth the adelgids have to settle on older wood, which weakens the tree further. Untreated trees can die within several years to a decade or more, depending on the tree’s health and the weather.
Cold helps. Very cold winters kill a large share of the overwintering insects, which is one reason damage has been slower in northern areas than further south. Mild winters let populations build quickly, and drought makes infested trees decline faster.
A thin hemlock near the house needs a lookHemlocks that have lost most of their needles become brittle and can drop limbs or fail at the base. A badly declined tree close to a house, driveway or play area should be assessed rather than left to see if it recovers.
How does adelgid spread?
The insects are spread by wind, birds, deer and people. Moving infested nursery stock, branches or brush from one property to another carries them to new trees. If you’re pruning infested hemlocks, bag or chip the clippings on site rather than dumping them at the edge of the woods, and avoid hanging bird feeders in infested hemlocks, since birds pick up the crawlers on their feet.
What treatments work?
The good news is that hemlocks respond well to treatment when it starts early, while they still have most of their needles.
Systemic treatments
Systemic insecticides applied to the soil around the base or as a trunk treatment are taken up by the tree and reach the insects through the sap. They work well on trees of any size, including tall forest hemlocks that can’t be sprayed effectively, and a single treatment can protect a tree for several years. Because they move through the tree slowly, it can take months before the effect is visible. In Massachusetts, treatments applied for hire must be done by a licensed pesticide applicator.
Horticultural oil and soap sprays
Horticultural oil or insecticidal soap kills adelgids on contact. It’s a good choice for hedges and small trees, where the whole plant can be covered thoroughly, and it’s often timed for spring or fall. It doesn’t protect new growth, so it may need to be repeated.
Biological control
State and university programs have released small beetles that feed only on adelgids. Biological control is a long-term effort in forests rather than a treatment for a single yard tree, but it may help slow damage in larger woodlands over time.
Care that helps
- Water during drought, since dry summers speed up decline.
- Hold off on nitrogen fertilizer, which feeds the adelgids as much as the tree.
- Prune out dead branches once the tree is treated, so you can see whether new growth is returning.
Our plant health care program includes inspection, treatment and follow-up checks for hemlocks, and tree disease management covers the other problems that often show up in the same stands.
A yearly check for hemlock owners
- Late fall to early spring: turn over branches on every side of the tree and look for white tufts. This is when they’re easiest to find.
- Spring: check whether the tree is pushing out new shoots. A hemlock with no new growth is under stress, with or without visible adelgids.
- Summer: water during dry spells, and look for needle loss and a grayish cast.
- After treatment: compare the amount of new growth each spring. Steady new shoots over two seasons are the best sign that treatment is working.
When should a hemlock be replaced?
A hemlock that has lost most of its needles and has a lot of dead wood is unlikely to recover even with treatment. If it’s near something it could hit, removal is the safer choice. For screens and hedges, replace a lost hemlock row with a mix of species, so one pest can’t wipe out the whole screen again. Our guide to the best trees to plant in Andover suggests evergreens that suit local yards.
Hemlocks are one of several trees in town with a serious pest problem. Our guide to common trees of Andover covers the others, and if your oaks were stripped this summer, see our guide to spongy moth in Andover.
Call Shawsheen Tree Co. to have the trees checked and a treatment plan put together while they still have their needles.
Hemlock woolly adelgid FAQs
What does hemlock woolly adelgid look like?
Small white woolly tufts at the base of the needles on the underside of twigs. They’re easiest to see from late fall through early spring.
Can an infested hemlock be saved?
Often, yes, if treatment starts while the tree still has most of its needles. Badly declined trees with a lot of dead wood rarely recover.
How long do treatments last?
Systemic treatments can protect a hemlock for several years. Oil and soap sprays kill adelgids on contact and may need to be repeated.
Should I fertilize an infested hemlock?
No. Nitrogen fertilizer can help the adelgids more than the tree. Focus on watering during dry spells instead.
Should I check the hemlocks on my Andover property?
Yes. The adelgid has been spreading across Massachusetts since the 1980s, so hemlocks in Andover yards, hedges and woods are worth checking every winter.
