Understanding the Hemlock Woolly Adelgid

 Hemlocks are one of Pennsylvania’s most beloved landscape trees (and our official State Tree), valued for their evergreen foliage, shade, and beauty. But these iconic trees face a serious threat from a tiny invasive insect: the hemlock woolly adelgid (HWA). Recognizing the pest early can be the difference between saving a tree and losing it.

What Is the Hemlock Woolly Adelgid?

The hemlock woolly adelgid (Adelges tsugae) is a tiny, aphid-like insect native to East Asia. In its home range, natural predators and coevolution with host trees keep populations in check. However, after being introduced to the eastern United States in the 1950s, the insect found a perfect combination of abundant hosts and no meaningful predators. Our native hemlock trees simply have no natural defenses against the pest. HWA was first detected in Pennsylvania in the late 1960s in the southeast and has since spread throughout much of the state. It is now common across the state, with infestations noted in many state parks and forests.

Recognizing HWA

On hemlocks, HWA appears as small, white, cotton-like tufts clustered at the base of needles on twigs — often most visible in late winter through spring. These woolly tufts protect the insects’ eggs and are usually the first sign of an infestation.

How HWA Damages Hemlocks

HWA feeds on tree sap by inserting their mouthparts at the base of the needles and sucking out plant fluids, slowly starving the tree. Signs of an infestation include:

• White, woolly ovisacs along the underside of branches
• Yellow/grayish foliage
• Branch dieback
• Thinning canopy

Without intervention, HWA can kill a hemlock tree within four years. 

Treatment

Many professionals use harmful neonicotinoids to treat HWA. You can read more about why neonicotinoids are harmful here. At Champion Tree & Plant Health Care, we time horticultural oil applications to target the crawler phase when the insects are most vulnerable. This helps protect your trees with minimal environmental impact compared to harsher chemicals.

How to Keep Your Hemlocks Healthy Long-Term

Even after treatment, maintaining tree health helps them withstand stress:

• Water during dry spells, especially in late summer
• Mulch to keep soil cool and moist (but keep mulch away from direct contact with the trunk)
• Limit root disturbance from construction, digging, or heavy equipment
• Avoid nitrogen fertilizer. Studies show the addition of nitrogen to hemlocks that are infested with the HWA worsens the infestation.

Healthy trees respond better to treatment and recover faster.

For questions, consultations, or to schedule treatment, reach out to the team at Champion Tree & Plant Health Care.