Capable of reaching up to 100 feet in height and living for more than 500 years, hemlocks are some of the most iconic trees found in the eastern portion of Tennessee. They not only make beautiful additions to many landscapes but also serve as a keystone species in natural habitats.
But unfortunately, these beloved trees are under attack from the invasive hemlock woolly adelgid (HWA).
Capable of killing hemlocks within a few years’ time, HWA is not something to take lightly. Fortunately, with early diagnosis and prompt care, many — if not most — treated hemlocks can survive an infestation.
We’ll outline everything you need to know to keep your hemlocks safe below, from pest identification to treatment options.
The Hemlock Woolly Adelgid Basics
To understand the threat HWA presents, as well as the best ways to help protect Tennessee’s hemlocks, it’s helpful to learn some basic information about the insects.
History and Spread of the Hemlock Woolly Adelgid
Hemlock woolly adelgids (Adelges tsugae) are small insects that are native to East Asia but have been introduced to both U.S. coasts. They were first discovered in Oregon back in the 1920s, but they were only found living on the East Coast in the 1950s.
Interestingly, HWA do not cause serious problems in either their native range or the Western U.S., where they primarily feed on western hemlocks (Tsuga heterophylla). It is only in the Eastern U.S., where they feed on eastern (Tsuga canadensis) and Carolina hemlocks (Tsuga caroliniana), that they are a significant threat.
This is thought to be due to a combination of factors, including natural predators of the insects in Asia, as well as the fact that western hemlocks appear to be partially resistant to the insects — advantages that eastern and Carolina hemlocks generally don’t enjoy.
The Hemlock Woolly Adelgid Life Cycle
HWA has a complex, two-generation annual life cycle in the Eastern U.S. Both generations consist entirely of females that reproduce without mating.
One generation — called the sistens generation — hatches in the late spring or early summer. This generation becomes dormant by mid-summer, then resumes development in the fall. They’ll then deposit eggs the following spring.
The other generation is called the progrediens generation. It hatches from the eggs deposited by the sistens generation and then develops quickly, producing eggs in the late spring, which will hatch and become the next sistens generation.
Biology of the Hemlock Woolly Adelgid
In addition to reproducing entirely without mating in the Eastern U.S. (both sexes are found in Asia), HWA have a number of traits that may seem strange to homeowners and property managers.
For example, the crawler stage is the primary mobile stage on hemlocks; most other life stages are completely sedentary.
The crawlers, which are small and unprotected, begin seeking new hemlock growth after hatching. Once they find a suitable location, they insert their piercing-sucking mouthparts near the bases of the needles and begin feeding on xylem ray parenchyma cells.
From this point forward, HWA do not move around. They also begin producing a waxy secretion, which makes them appear like tiny tufts of cotton or wool.
Signs of Hemlock Woolly Adelgid Infestations
HWA infestations produce several visible signs, while the insects’ feeding activity can cause affected trees to exhibit a number of symptoms. A few of the most noteworthy include:
- White, Cotton-Like Masses: The most obvious sign of an HWA infestation is the appearance of small white cotton-like masses near the base of the needles.
- Needle Discoloration: As HWA feed on hemlocks, they cause damage that triggers the normally dark green needles to become yellowish or gray.
- Needle Drop and Crown Dieback: As infestations progress, hemlocks often begin shedding large quantities of needles, sometimes leading to a thinner canopy.
- Reduced Vigor: Many hemlock trees will start to decline following HWA infestations, and their growth rate may slow drastically.
What Can Be Done About Hemlock Woolly Adelgid?
Left untreated, serious HWA infestations are often fatal, and heavily infested hemlocks may die within a few years. However, there are several strategies homeowners and property managers can employ to help protect their trees:
- Monitor your trees carefully. It’s important to catch HWA infestations early, as they’re easier to treat before the insects build up to large numbers. So, inspect your trees every spring and fall, looking for foliar discoloration, crown dieback, and white, woolly masses.
- Support your trees to keep them healthy and vigorous. Healthy hemlocks will generally tolerate infestations better and have a better chance of recovering than stressed individuals will. So, protect your trees’ root zones and apply supplemental water when appropriate.
- Treat high-value trees with systemic insecticides, insecticidal soaps, or horticultural oils. Many hemlocks respond well to treatments when initiated at the beginning of an infestation. Discuss the best treatment strategy for your trees with an ISA-certified arborist.
- Consider removal when infestations become severe. Treatment may not be effective for hemlocks that have already suffered extensive canopy loss or branch mortality. So, have an ISA-certified arborist evaluate severely declining trees to determine if removal is warranted.
Hemlocks & Hemlock Woolly Adelgid in Tennessee: FAQ
Given the importance of protecting your hemlocks and the strange characteristics of HWA, our clients often have a lot of questions about their trees and this pest. We’ll try to help by answering some of the most commonly asked questions below.
Can a hemlock recover from hemlock woolly adelgid?
With prompt and proper treatment, yes — many hemlocks can and do recover from HWA infestations. The key is starting treatment early, before your trees have suffered significant canopy dieback or needle loss.
What is the best time of year to treat HWA?
Spring and fall are usually the best times of year to treat HWA infestations, but the ideal timing will vary based on the treatment option employed. Your AKA Tree Service arborist will be happy to discuss treatment timing during your initial evaluation.
How often do hemlocks need to be treated for HWA?
The ideal treatment frequency will depend on the method used to control the insects. Some treatments need to be repeated annually, while others are effective for four years or longer.
How quickly can HWA kill a hemlock?
Significant HWA infestations can kill hemlocks within a few years, particularly if the tree is young, stressed, or growing in a poor location.
Can I treat hemlock woolly adelgid myself?
There are a few HWA treatments homeowners and property managers can apply, such as some horticultural oils. However, professional assistance will almost always yield better results.
Will cold winters kill HWA?
Cold winters do kill some percentage of HWA in an area, but they don’t eliminate the insect entirely.
Should I remove a hemlock that has HWA?
Not necessarily. There are treatments available for HWA, and many trees that receive prompt treatment will recover and live for many years.
Let AKA Tree Service Help Protect Your Trees
Whether you have hemlocks suffering from an HWA infestation, you want help protecting your ash trees from emerald ash borer, or you’re worried about the southern pine beetle, the tree healthcare experts at AKA Tree Service are here to help.
Contact us today to schedule your free evaluation. One of our ISA-certified arborists will visit your property, assess the situation, and recommend a prudent course of action.
Sources and References
- Tennessee Department of Agriculture, Division of Forestry – “Hemlock Woolly Adelgid”
State forestry page on hemlock woolly adelgid (Adelges tsugae) in Tennessee, covering its threat to native hemlocks, signs of infestation, and treatment and biological control efforts.
https://www.tn.gov/protecttnforests/insects/hemlock-woolly-adelgid.html - American Conifer Society – “Tsuga canadensis”
Species profile of eastern hemlock (Tsuga canadensis), describing its characteristics, growing conditions, landscape use, and available cultivars.
https://conifersociety.org/conifers/tsuga-canadensis - The Nature Conservancy – “Hemlock”
Conservation story on efforts to protect Tennessee’s eastern hemlocks from hemlock woolly adelgid, including treatment and predator-beetle biological control in the state’s forests.
https://www.nature.org/en-us/about-us/where-we-work/united-states/tennessee/stories-in-tennessee/hemlock/ - University of Tennessee Extension – “SP503-G: The Hemlock Woolly Adelgid — A Threat to Hemlock in Tennessee”
Extension publication on hemlock woolly adelgid (Adelges tsugae), covering its Asian origin, spread through the eastern U.S., susceptibility of eastern and Carolina hemlock, and the extensive mortality it has caused.
https://voljournals.utk.edu/utk_agexfores/28/
