Imidacloprid, the insecticide most commonly used to treat hemlock trees for hemlock woolly adelgid, is relatively safe in the context of hemlock trees. It is a systemic insecticide, meaning that it works within a tree to fight off infestation. Imidacloprid is in a class of insecticides known as neonicotinoids, which are often in the news because they are associated with increased risk to pollinating insects such as bees. This concern is lessened in the context of hemlock woolly adelgid because hemlock trees are wind-pollinated, and bees therefore do not rely on them as a food source. Furthermore, there are risks associated with losing hemlocks by choosing not to treat them; though imidacloprid can impact species besides hemlock woolly adelgid, these species would not survive in the absence of hemlock trees, either. Biological control is a critical long-term solution to hemlock conservation, but in the meantime, hemlocks that are infested with HWA can only be saved using chemical treatments. You can read more about imidacloprid here.

You can do this using the tool below. Please select the county in which you would like to contract a pesticide applicator and hit “Submit.” A list will be generated of applicators who operate in your county and in neighboring counties and their contact information. Please bear in mind that we cannot recommend any particular applicator and that this list may not be exhaustive; the applicators on this list represent applicators who we know possess the relevant licensure and have opted in to being listed.

It can be tricky to tell if and when chemical treatments work because the wool that HWA produce can stick around long after the insects have died. In scientific studies, HWA mortality is often determined by taking samples from hemlock trees and looking at the adelgids under a microscope, but this method is impractical for homeowners, particularly for assessing whether a treatment worked on a whole tree or many trees.

The easiest way to check whether your treatments worked is to wait until the next time that your hemlocks put out new growth – this is the bright green needles developed at the ends of twigs in the spring and early summer. Usually, HWA prefers to settle on these bright green, newer needles (pictured on the right). If you check your trees and only see HWA wool on the older needles, that is a good sign that the wool is from old and likely dead HWA. If your trees were in poor condition prior to treatment, the presence of new growth in and of itself is an excellent sign that they are recovering!

Something else to bear in mind is that different chemicals can take different amounts of time to become effective. The most common options for hemlock tree treatment are: imidacloprid, dinotefuran, and a tank mix of both. There are pros and cons to price and timing of application for each of these options.

Bearing in mind the probable timing of protection from each of these options, you may see signs of improved hemlock health in as little as 3 weeks or it may take as long as 1 year.

Neither Laricobius nigrinus nor the Leucotaraxis species are commercially available at this time, as both are limited and reserved for targeted releases and research on public lands. However, we have done a lot of releases on public property in various parts of New York State, so it is possible that they may spread to your property on their own. Their rate of spread is dependent on multiple factors, such as temperature, mating success, and availability of prey.

There is no empirical evidence that supports the efficacy of Sasajiscymnus tsugae as a biocontrol agent of hemlock woolly adelgid on the east coast. Though S. tsugae has become established in some locations, verified recoveries of the beetles are inconsistent, and no field impacts due to its release have been empirically demonstrated. In response to the lack of evidence of this beetle’s establishment and efficacy, all university and federal research laboratories have stopped working with this beetle (Mayfield III et al. 2023, Havill et al. 2016, Onken and Reardon 2012, McCarty and Addesso 2019).

NYSHI is a publicly funded research program, meaning we are obligated to release predators only on land that falls under public protection, like parks and preserves, for research purposes. We have released predators in about 83 locations in New York State as of 2024 and expect them to spread on their own, but it will take time for predator populations to build to the point that they are able to save hemlock trees. You can refer to the map below to see NYSHI’s predator release sites and estimate how far away they are from your land. In the meantime, the only way to save hemlocks is with insecticide treatment. If you haven’t already, we recommend taking a look at our HWA management page.

We have been releasing Laricobius nigrinus since 2008 and silver flies (Leucotaraxis spp.) since 2015 in New York. In this time, we have released these species at many sites in New York State and a few sites outside New York. The map below depicts all our releases. Leucotaraxis release sites are depicted by blue bubbles, and Laricobius release sites are depicted by yellow bubbles. The size of each release corresponds to the size of the bubble by which it is depicted (i.e., larger bubbles correspond to releases with a greater number of flies or beetles).

A number of factors go into determining each year’s release sites for HWA predators, or biocontrol agents. We work with partners across the state, such as state and county agencies and land trusts; to determine where predators are released, we line up permissions and partners in advance. We also take into account how many predators we are able to secure from the Pacific Northwest each year. Because those numbers are variable, we maintain some flexibility each year. Finally, our priorities for release sites depend on suitable conditions for predators and the leading edge of HWA’s spread. It is difficult for us to guarantee the timing and location of predators too far in advance given the variability of all these factors.

We are still investigating silver fly establishment after releases. Because they are better fliers than the Laricobius beetles, it is turning out to be more difficult to confirm establishment for the flies than for the beetles! That said, we’ve already shown that one of the fly species can survive the cold winters here and emerge as adults at the right time to control HWA (Dietschler et al. 2023, Preston et al. 2023). And in Virginia, they have been able to recover new generations of flies. In the meantime, we’re looking for signs of establishment at a lot of our release sites using foliage collection methods and eDNA.

In eastern North America, HWA has 2 generations. The first, called the “sistens generation” or the overwintering generation, starts as eggs that hatch in the early summer (June/July). HWA are only mobile just after hatching, when they are in their “crawler” life stage. This stage lasts until mid-summer, at which point they will settle at the base of new hemlock needles and become dormant (this is called “aestivation”) for the rest of the summer. From mid-summer until they break aestivation, they are nearly impossible to see. The dormant nymphs are smaller than a sesame seed and do not begin to produce wool until after they break dormancy in mid-autumn. They grow and produce wool from that time until late winter/early spring. They lay eggs in the spring that will hatch into the second generation.

The second generation of HWA is called the “progrediens generation” or spring generation. They skip the dormant period, completing their development much faster than the summer generation. They enter their mobile “crawler” life stage in April/May and settle nearly right away. They reach adulthood in the summer, laying the eggs that will become the summer generation. In New York, the life cycle approximately follows the patterns outlined below:

To read more in depth about the life cycle of Hemlock Woolly Adelgid, check out our HWA Phenology Page

HWA is only mobile during one life stage: the crawler stage. They are only in this life stage in the spring and early summer, making that the only risky time to prune hemlocks. It’s best to prune your hemlocks at any time except the spring and early summer. To further minimize the risk of spreading HWA, be sure to compost your branches on site whenever possible. That way, any HWA that manage to survive until the crawler stage on the pruned branches will not have an opportunity to spread to a different site.