An entomologist examines a branch of hemlock infested with hemlock woolly adelgid. The invasive insect plunges its needle-like mouth deep into the branches of hemlock trees and slowly sucks out the nutrients and kills the plant. John Bazemore/Associated Press

Over the past year, I’ve written about several invasive species, a couple of diseases, and whether or not to take down your bird feeders to stop the spread of disease. I was surprised to realize that I’ve never covered hemlock woolly adelgid and have been asked about it again recently. So this week, let’s make sure we know what they are and what you can, or can’t, do to help combat this invasive insect.

First, while hemlock woolly adelgid (HWA) is a new topic to my column, this destructive insect has been in Maine since 1999, when it was inadvertently shipped here on an untreated stock of plants coming up from Connecticut. HWA took its first leap (via human shipment) from its native range of east Asia to the United States back in 1951, near Richmond, Virginia. HWA are a “true bug” in the order hemiptera, or “half-winged” bugs, like aphids, leafhoppers or bed bugs. You may be familiar with the common trait of this order, their “piercing-sucking” mouth parts – think of aphids biting plants in your garden, or (hopefully not) bed bugs biting you.

This is the problem with HWA: the adults will use their stylet – the fancy name for their straw-like mouth parts – which is three times the length of their body, to pierce into a hemlock’s tissue and extract the nutrients. Finding the small, 0.8mm adults is difficult, but the egg sacks they leave on the underside of hemlock branches is much easier to detect. These patches, often with hundreds of eggs, look like white tufts of cotton along the branches.

In the spring, the larvae emerge from these eggs and then will look to spread to new trees. And here’s our question for this week. A few readers are wondering if having bird feeders up increases the spread of HWA. The speed HWA is spreading, and the amount of damage they leave behind, is a major cause of concern given how important hemlocks are to a healthy forest. The tiny HWA larva are known to spread by blowing in the wind, moving around on the fur of mammals, and even being carried long distances by catching a ride on birds’ feathers. A paper published in January 2019, by researchers at the University of Connecticut and the U.S. Forest Service, showed that the nymph stage of HWA – called “crawlers” – are most often picked up (passively) by birds during their northward spring migration, and that they are capable of settling into new hemlocks.

This study showed how a small, relatively immobile insect was able to travel and expand its range so quickly in the U.S., above and beyond the unfortunate human-caused spread. It is important to realize the reason for the spread here to illustrate that bird feeders don’t play any role in the spread of HWA. The seed you are putting out in your feeders does attract birds, but only to your feeder, making them easier for you to see. The hopefully healthy hemlock forest would be hosting so many native insects that it would be a greater attraction to the birds.

I’ll take this opportunity to remind you that keeping your bird feeders clean is important to prevent spread of diseases, especially avian conjunctivitis. Use a diluted bleach solution (or vinegar) and let it dry completely before putting it back out. When it comes to dealing with HWA, I recommend checking the Maine Forest Service’s website on HWA, which includes several control methods and a form where you can report areas where you find HWA in Maine.



Roost or nest? What’s the difference? It’s become increasingly popular to put out roosting boxes in the winter to provide shelter for birds. I want to thank Maggie from Topsham for writing in with questions about roosting boxes, and making sure there will be places for her wrens to nest come next spring. Let’s clarify exactly what they do, how and why they help birds in the winter, and how they’re different from the birdhouses we provide in the summer.

Whether they are roosting or nesting boxes, the premise is the same: We are adding supplemental shelters to the environment for birds to increase their chances of survival or successful nesting. The lack of snags, or old dead trees, in the landscape means that many species that use cavities don’t have enough of those shelters, thus we are helping with these artificial offerings.

A couple of key differences between roosting and nesting boxes is in their size and construction. When it comes to size, nesting boxes are generally smaller and only need to hold the nest a bird will build, and that nest only needs to be large enough to hold its eggs. The chicks from that nest will often fledge before they are even full grown. Roosting boxes, on the other hand, need to be larger because you’ll often have several adult (or at least full grown) birds all roosting together: more bodies equals more warmth. Most constructed roosting boxes will also have perches inside for those birds to sit on, while nesting boxes are otherwise empty (leaving space to construct a nest). The entrance hole, or cavity for entering the box, is often at the top of a nesting box, so the birds can drop down into the nest, while roosting box makers often put the holes at the bottom to help keep warm air trapped inside.

Both roosting and nesting boxes make for interesting backyard observations and make great gifts. Whether you buy them ready-made or build one yourself, keep these in mind as we head into the holiday gift-giving season. They are sure to bring a smile to the receiver and the birds.

Do you have a nature question for Doug? Email questions to [email protected] and visit to learn more about birding, native plants, and programs and events focusing on Maine wildlife and habitat. Doug and other naturalists lead free bird walks on Thursday mornings, 8 to 10 a.m., at the Gilsland Farm Audubon Sanctuary in Falmouth.

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