Invasive Species Spotlight: Hemlock Woolly Adelgid 

The hemlock woolly adelgid has a particular palate, feeding exclusively on hemlock trees. First discovered in Virginia, this invasive species has spread throughout the forests of the Appalachian Mountains, where it poses a serious threat to forest ecosystems. Invasive species experts have attempted to prevent the spread of these aphid-like insects, but management tactics, including chemical control, have yet to suppress hemlock woolly adelgid populations.  

Hope is not yet lost for the hemlock forests of the Southeastern and Mid-Atlantic states, however. Every day brings entomologists closer to developing more effective integrated pest management (IPM) strategies, and the efforts of individuals can go a long way toward preventing these insects from inadvertently being introduced into new areas.  

Let’s take a closer look at the hemlock woolly adelgid and its destructive appetite.  

From Japan, With Love  

The hemlock woolly adelgid was first spotted in the Pacific Northwest in 1922, but this is believed to be a case of mistaken identity. The first confirmed sighting in North America was in Richmond, Virginia, in 1951. Native to Japan, the adelgid is believed to have been introduced via infested ornamental Japanese hemlocks. These insects can now be found in over 20 states and Nova Scotia. Without natural predators or hosts with natural resistances, adelgids are free to feed on eastern and Carolina hemlocks.  

Hemlock trees play a vital role in their ecosystem. From a black bear making its den in a tree hollow to a speckled trout swimming in waters cooled by tree shade, all types of animals and plants rely on the habitat provided by hemlock forests. Found on steep slopes, hemlock trees even prevent erosion by stabilizing shallow soil. Losing hemlock trees could mean losing unique plants and wildlife, which is what makes adelgid infestations so concerning.  

Four Years to Live  

Hemlock woolly adelgids feed by inserting their piercing mouth parts, known as a proboscis, into plant tissues and extracting stored nutrients. Affected hemlock trees shed their needles and lose buds and branch tips necessary for regeneration. Eastern hemlocks are particularly vulnerable to infestation, having fewer defenses against piercing-sucking insects. A herbivore-induced hypersensitive response also results in needles with higher levels of H2O2 (hydrogen peroxide), resulting in tissue death at the feeding site. Infested hemlock trees die within as few as four years, and as many as 80% of hemlocks in the Shenandoah National Park in the Blue Ridge Mountains have died due to infestation

Winter and Spring Adelgids  

Two generations of adelgids emerge during the year: the winter generation and the spring generation. In early summer, winter adelgids hatch only to enter summer dormancy at the bases of hemlock needles. They spend the rest of their lives at this feeding site. Dormancy ends come winter, and winter adelgids progress through four stages of life as wingless nymphs. During this time, they produce a waxy, woolly mass above their bodies, where they will lay their eggs through early spring.  

The lifecycle of the spring generation is only three months long and comes to a rather abrupt end. Hatching in early spring, a proportion of adults known as sexuparae develop wings and fly in search of tigertail spruce trees to deposit their eggs. There’s just one problem: There are no tigertail spruce trees native to their area. Having reached a reproductive dead end, the winged adults die, and it’s the non-winged adults that lay the eggs that will become the winter generation.   

Current Management Tactics  

Hemlock woolly adelgids are miniscule, measuring less than two millimeters. The woolly mass they produce is easy enough to spot, however. Each mass resembles the head of a cotton swab, and an infested tree will have a number on the underside of its branches throughout fall and spring.  

Once detected, there a number of potential methods for managing these invasive insects:   

Chemical Control 
Insecticides have been shown to control hemlock woolly adelgid populations, but the cost of applying pesticides to large areas makes this strategy less than desirable.  

Silvicultural Control  
Silvicultural control entails removing infested hemlock trees, which allows more sunlight to reach the forest floor, stimulating regeneration and helping hemlocks survive an infestation. Unfortunately, silviculture can lead to unintended consequences, including the removal of resistant hemlocks.  

Host Resistance 
Attempts to cross the eastern hemlock with the highly resistant Chinese hemlock have been unsuccessful. However, there are over 200 cultivars of eastern hemlock, and the possibility remains that some hemlocks have an innate resistance to the adelgid. 

Biocontrol  
Native and introduced enemies have a substantial impact on the hemlock woolly adelgid, including Laricobius nigrinus, among its most important predators. A six-year study spanning nine field sites across six states revealed that this black beetle had a significant impact on winter generations. Spring generations rebounded, however, resulting in the need for a more comprehensive approach.  

Integrated Pest Management  
IPM uses a combination of the strategies listed above and has been shown to have moderate success in forest and urban settings. IPM is the most promising long-term solution to controlling hemlock woolly adelgid populations.  

Here’s What You Can Do 

The keen eyes of entomology enthusiasts can help slow the spread of hemlock woolly adelgids. If you’re in a state actively monitoring for adelgids and you spot them in a city where they’ve yet to be identified, you can document their appearance and report it to your local government. However, there is a way to take a more active role in managing this invasive insect.  

The University of Florida offers an online entomology master’s degree and graduate certificate, each with three specialization options. Our online courses instill the principles and practices of IPM, helping working professionals like yourself become entomologists, ecologists, biologists, IPM managers, and more. Explore our program and course offerings to find the graduate credential that’s ideal for your career.   

The hemlock woolly adelgid is one of many invasive insects. Apply to one of our online entomology programs to protect the delicate balance of our natural habitats.  

Sources: 
http://entnemdept.ufl.edu/creatures/TREES/hemlock_woolly_adelgid.html  
https://www.dec.ny.gov/animals/7250.html
https://inspection.canada.ca/plant-health/plant-pests-invasive-species/insects/hemlock-woolly-adelgid/fact-sheet/eng/1325616708296/1325618964954
https://www.nrs.fs.fed.us/disturbance/invasive_species/hwa/control_management/host_plant_resistance/
https://www.fs.fed.us/forestmanagement/vegetation-management/silviculture/index.shtml

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