The Woolly Adelgid And The Hemlocks
With our trip to the shore behind us, we are spending a lot of time wandering around our yard. As always, the madman spends a lot of time at the conifers, especially the hemlocks. He’s not really looking at the hemlocks, he’s looking at the Hemlock Woolly Adelgid (HWA).
A Serious Problem
“To steal from Shakespeare,” the madman says. “Though she be but little, she is fierce.” This tiny bug is capable of taking down entire stands of hemlock trees. As most troublesome insects are, the HWA is an invasive species. Scientists feel it got its eastern start in 1951 around Richmond, Va. Someone imported hemlock trees that were infested. In its native range in Japan and Asia, this insect is kept in check by natural enemies, mostly beetles. But the Hemlock Woolly Adelgid is a clever species with no desire to die out. Over time it developed a survival strategy to beat the beetles. The HWA evolved a bi-annual reproduction cycle and the ability to reproduce asexually as well.
Dangerous Reproduction
This reproductive strategy is what makes it so dangerous to the eastern United States. Unlike most other pests we deal with, the HWA is active during the winter. They come out of aestivation (summer hibernation) around October and begin feeding and growing. They reach adulthood by early spring and lay up to 300 eggs per individual throughout April. This new bunch hatches during April and May and develops much quicker than the previous generation, reaching adulthood by June. Eggs that are laid (around 75 per individual) hatch in early July and are settled into new hemlock growth by August. These then go into aestivation until early October, then the cycle begins again.
In their native lands, the HWA reproduce sexually as well as asexually. In these areas, winged adults hatch and fly to Tigertail Spruce trees where they lay their eggs in galls. This hatching contains winged females and males who reproduce sexually.
No Tigertails
Here in the eastern United States, we have no native Tigertail Spruce, and the spruce trees we do have are not compatible with the little monsters. Over generations, they seem to have stopped producing the winged varieties here – after all why waste the time and energy on something that’s not going to happen.
In 2021 an interesting phenomenon occurred. Beachgoers along the Maine coast reported a black gunk washed up on the beaches – a gunk that turned bare feet black. This substance was found from southern Maine to eastern Massachusetts. About the same time, a boater in Beverly, Ma. reported a massive swarm of insects that coated his boat and nearby waters for hours.
Related Issues
Scientists quickly determined that the Beverly swarm and the beach gunk were related – both consisted of winged insects. But they couldn’t identify the species exactly – an educated guess pointed to a winged form of the HWA. The theory was that because of successive mild winters, the HWA population mushroomed creating an overabundance of the winged variety. Lab tests positively identified the insect. With no natural enemies and warmer winters (subzero temperatures will kill them), their numbers are growing, and their range is spreading.
Concern For The Forests
This causes forest managers a great deal of concern. Hemlock trees are important species in the eastern United States. Since the HWA appeared in Richmond, Va. 70 years ago, it has been killing hemlocks in Shenandoah National Park, and spreading south to the Great Smoky Mountains and north to Pennsylvania and New England. And the spread is continuing westward aided by wind, birds, animals and, of course, man.
Loss of hemlock trees is a blow to our ecology. Hemlocks grow on 19 million acres and are the dominate tree species on 2.3 million of those acres. They are a keystone species – a species that supports a wide range of other species. Its disappearance would seriously hurt the forests in our region. The southern stands of hemlocks mostly belong to the species Tsuga virginiana, while the northern ones are Tsuga canadensis. Both have similar functions within the forest.
Hemlocks Are Important
Stands of hemlocks provide dense shelter for White-tailed deer and other wild animals. In the winter, the thick boughs keep snow from falling to the ground beneath the trees. And, in summer these same boughs block sunlight and keep the forest floor cooler while limiting the understory growth. Their branches support over 120 different species of wildlife.
Hemlocks also provide important benefits for our waterways. Their overhanging branches keep the water temperature cooler benefitting populations of native fish and other aquatic vertebrates.
They often grow along the edges of streams where they stabilize the banks and prevent runoff of nutrients and soil. They take up excess water during heavy storms making flooding less severe. Hemlocks are truly an important specimen in the forests.
Nice Tree
On top of that, they are really nice to look at. In his Manual of Woody Landscape Plants, Michael Dirr says of the hemlock, “If I were forced to select one conifer for my garden, it would certainly be Tsuga canadensis.” The madman agrees, and long before he heard of Dirr in the Master Gardeners’ Program, he incorporated hemlocks into our landscape at both our house and the lake. With their thick branches and conical shape, they are excellent for screening and tolerant of pruning. Over the years, he noticed that his Hummingbirds often choose these trees for nesting. Because of this these trees are very dear to the madman, so he needs to keep them healthy.
Time To Act
Over the years, our HWA populations have fluctuated. Much of the fluctuation is weather dependent. Cold temperatures can kill large numbers of the HWA, but they thrive in winters as warm as this one has been. If you have hemlocks, now is a good time to look for white woolly spots on the underside of needled branches. For the madman, this is a year that needs action.
There are a number of ways to control the HWA. Some are available to homeowners like the madman, others require the services of a licensed arborist. The scope of your infestation and the size and location of your hemlocks will help you decide whether to go it alone or call in the cavalry.
Horticultural Oil
The madman’s first choice is either horticultural oil or insecticidal soap, applied in the spring and the fall. Before application, he’ll do any needed pruning to shape the trees, remove dead branches and maybe eliminate a few of the insects.
The horticultural oil will coat and smother all stages of the insect. Insecticidal soap disrupts the bug’s metabolism. Just remember both have to be sprayed on the trees when temperatures are above 45˚ for at least 24 hours. The madman knows he will need to repeat the application over a couple of years. Someone told him to schedule his spraying along with the time changes in spring and fall so remembering will be easier.
Imidacloprid
At Christmas time, he read about what he thought would be an easier treatment, but then found it couldn’t be shipped to Connecticut because it contained imidacloprid. This chemical is toxic to just about everything. He started thinking about the damage he could do to beneficial insects and his birds, especially the hummingbirds. That ended that. Licensed arborists can offer a variety of treatments from soil drench to sprays to even injections into the tree itself. They are trained in the safe handling and application of chemicals like this.
Biology To The Rescue
We have only a few trees to tackle in our yard; all are reachable. But imagine the state and national forests of New England – forest managers have millions of acres of hemlock trees to protect; they can’t possibly treat them all. But, that doesn’t mean they aren’t trying. In addition to the injections, the soil drenches and the sprays, scientists are breeding and releasing insects that prey on HWA. Connecticut was one of the first states to actively pursue biological controls through research efforts led by scientists at the Connecticut Agriculture Experiment Station (CAES). A number of other institutions and research facilities have joined the fight. Eventually, the efforts of many may help bring down the fierce little insect known as the Hemlock Woolly Adelgid.
When the weather warms up, the madman will be out there with his oil and soap. In the meantime, it’s time for a cup of hot cocoa.
