USA NPN National Phenology Network

Taking the Pulse of Our Planet

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Hemlock Woolly Adelgid Forecast

Hemlock woolly adelgid can be deadly to hemlock trees and, in the eastern United States, lacks enemies that keep their populations in check. Researchers wish to identify the optimal window to release insect predators; you can support this effort by observing hemlock woolly adelgid life cycle stages using Nature’s Notebook

Hemlock Wooly Adelgid Current Day Forecast.

Hemlock Wooly Adelgid 6-Day Forecast.

Pheno Forecast maps show when management actions should be taken for key pest species. These maps are updated daily and are available 6 days in the future.

Pheno Forecasts are based on published growing degree day (GDD) thresholds for points in pest life cycles when management actions are most effective. Forecasts are currently available for five insect pest species.

Help us improve these maps! Our Pheno Forecast map products are still in development, and we seek input on their performance in your area. Give your feedback on the sidebar on the right side of this page. 

Species Background

Hemlock woolly adelgid (Adelges tsugae) damages hemlock trees by piercing the twig at the base of the needles and sucking sap from the trees. It occurs in forests in the eastern US as well as in the Pacific Northwest. In the Northwest, it is kept in check by natural predators; in the east, where predators are few, this pest is causing major damage to hemlock forests. Researchers are working to identify the best time of year to release predator insects or biocontrols, that would limit the rapid spread and damage of the hemlock woolly adelgid. You can help with this effort by checking hemlock trees for hemlock woolly adelgid eggs and reporting your findings in Nature’s Notebook.

Observing Details

HWA eggs, Photo: Lorraine Graney, Bartlett Tree Experts,

Help scientists better understand when hemlock woolly adelgid insects are active and most susceptible to treatment by reporting when you see this pest in various life cycle stages. The Pheno Forecast map indicates when you will be most likely to see hemlock woolly adelgid eggs and active nymphs. GDDs are accumulated from a base temperature of 32oF.

1. Join Nature's Notebook. If you haven't already, create a Nature's Notebook account. See our specifics of observing if you need more details on getting started. You can set up a phenology monitoring site in your backyard or another location you frequent.

2. Take observations. For this effort, we are seeking observations on the following life cycle stages of hemlock woolly adelgid:

Phenophase Definition Photo (click to enlarge)
Eggs One or more eggs are seen. The small, brownish-orange eggs are present when the white, woolly sac around an adult female appears puffy, generally starting in the early spring, and can be seen by gently scraping the "wool" aside.  Hemlock woolly adelgid eggs, photo: Nick Dietschler
Active nymphs One or more nymphs are seen moving about or at rest. Nymphs ("crawlers") are reddish-brown in color and move around before settling in one spot on a plant. Hemlock woolly adelgid crawler Photo: Kelly Oten, North Carolina Forest Service, 
Inactive nymphs One or more nymphs are seen in a dormant state. Nymphs settle in one spot, usually at the base of a needle, turn black in color with a small halo of white "wool", and enter dormancy (aestivation) between midsummer and fall.  Hemlock woolly adelgid inactive nymphs, photo: Nick Dietschler
Post-dormant nymphs One or more nymphs have come out of their dormant state. The post-dormant nymphs are still settled in one spot but are increasing in size and wooliness, and changing color from black to grey. Do not include individuals that have molted and shed their exoskeleton, often appearing as if another adelgid is riding on top of them. These individuals are "wool"-covered instars that can be considered "Active adults."  Hemlock wooly adelgid post-dormant nymphs, photo: Nick Dietschler

Observation tip: Nymphs are tiny! See how they compare to the size of a penny here.

3. Report your observations. As you collect data during the season, log into your Nature's Notebook account and enter the observation data you record. You can also use our smartphone apps to submit your observations.

More information on map development and re-use policy. 

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