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This story is part of our series, Wild Pennsylvania. Check out all of the other stories in the series here

The hemlock woolly adelgid, an invasive aphid-like insect, is destroying Pennsylvania’s state tree and its forest. It is a headache to many homeowners and conservationists, but there are ways to prevent the infestation from killing hemlocks. 

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The pest was originally introduced to North America from Asia in the 1920s, and was identified in the hemlock trees of the eastern United States later in the 1950s. It is now reported in 18 states including New York, West Virginia, Kentucky, and Pennsylvania. Generally, the impact of the hemlock woolly adelgid has done substantial damage to hemlock populations in the Eastern US.

What to Do if You See This on Your Hemlocks

Hemlock woolly adelgid is a sap feeder. This means that at an early age the insect will insert its mouth into the tree and remain there for the rest of its life feeding on the stored starches of the tree. This disrupts the flow of nutrients throughout the tree, and if left untreated, will kill the tree in four to 10 years. Because the woolly adelgid kills the tree slowly, it’s important to spot it and treat it as soon as possible. 

Woolly adelgide on a hemlock tree.

Woolly adelgide on the underside of hemlock tree.

“It kind of looks like a snowy deposit on the branches, and oftentimes we see it on the lower branches,” said Doug Oster, a local gardening expert and blogger. “If you see that white kind of fuzzy stuff on your hemlock tree, it’s time to take action.” 

Oster is fighting his own battle with the woolly adelgid on at least 100 hemlock trees on his property. He suggests using horticultural oil, which can be purchased at a hardware or gardening store.

“It basically smothers the pest and that white part that we see. It’s the hardest one to treat, because it’s kind of a waxy covering that protects the pest,” he said. “But right now, there are things called crawlers hatching out, and this is the time to treat the tree where you can really make a big difference and get those young ones before they can form that waxy coating.” 

In the spring or fall, systemic Insecticides containing imidacloprid or dinotefuran can also be applied to the trunk of the tree, or on the soil.

Oster urges people to call an ISA-certified arborists, who will do free consultations if the infestation is out of reach or any other advice is needed.

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Hemlocks Under Threat Across Pa.

This invasive species isn’t just a problem for homeowners. It is pervasive throughout many of Pennsylvanias’ parks, forests, and nature reserves. This can lead to broader problems.

“One of the worst parts about it is the fact that our hemlocks are in some areas where they’re essentially the keystone species. The entire ecosystem in a certain area may be built around those hemlocks,” said Ryan Reed, a natural resource program specialist at the Pennsylvania Department of Conservation & Natural Resources Bureau of Forestry. 

Unfortunately, climate change makes this situation worse.  

“With the warming of Pennsylvania, especially in winter, where we see the majority of warming in terms of climate change, it enables the hemlock wooly adelgid to gain more territory and advance northward and affect more and more of our forests,” he said 

Reed said typically, during the colder months, the woolly adelgid would die off. 

Hemlock woolly adelgid only requires a light wind to be rapidly spread and on average spreads at a rate of 7.6-7.8 miles each year. 70 percent of Pennsylvania’s forests are owned privately, putting the onus on the citizens to make sure that the hemlock is protected.

According to Reed, the DCNR is trying to fight the infestation in the forests that they manage. Due to budget constraints, the DCNR is unable to treat all of the trees that need it. Reed said that they create areas of priority based on conservation value, such as those most visited by tourists, spots with special wildlife, or spots where the forest is unique in another way – for example Cook Forest State Park and Heart’s Content Scenic Area. 

The Bureau of Forestry is treating pests with methods that minimize chemicals, so that other species are not harmed through the process. “If you have a pest out there, one thing that you can identify is the timing [of spraying pesticides as to] not affect other insects and organisms that would be feeding on that tree, organisms that are native and part of the natural food chain,” Reed explained. “So what if we avoid the window for when they’re actively feeding? Well, now we have completely avoided that non-target type of collateral damage.”

Another control they are trying is introducing predatory beetles that eat the hemlock woolly adelgid. Success has been limited. Reed hopes that these methods can lead to long-term control but, unfortunately, he believes that the hemlock wooly adelgid is here to stay.

These Hungry Beetles Could Save Pennsylvania’s Hemlocks