Hemlock woolly adelgid (HWA) is an invasive insect that attacks and kills eastern hemlock trees. This sap-sucking insect is originally from Asia and was first reported in the eastern United States in the 1950s. Since then, HWA has spread rapidly and caused widespread mortality of hemlocks, which are important ecologically and economically in many areas. Getting rid of HWA is critical for saving existing hemlock stands and preventing further spread of the pest. This guide provides comprehensive information and effective methods for getting rid of HWA on hemlock trees.
Understanding HWA Biology and Effects
HWA are tiny, aphid-like insects that attach themselves to hemlock twigs and feed on sap from the tree’s newest growth. The insects secrete a white, waxy “wool” that covers their bodies and egg masses, giving infested hemlocks a snowy appearance. HWA feed on hemlocks year-round, draining the trees of nutrients and eventually leading to needle loss and branch dieback. Infested trees succumb within 4-10 years. Widespread hemlock mortality drastically alters forest ecosystems and eliminates habitat for wildlife species that depend on the dense, evergreen canopy hemlocks provide.
Monitoring and Early Detection
Carefully monitoring hemlocks for early signs of HWA infestation is crucial for effective control. Look for:
- White woolly masses on the underside of branches at the base of needles. Use a hand lens to inspect for tiny black HWA bodies mixed within the woolly masses.
- Grayish-white fuzzy spots on twigs. Scrape off some of the wax with a fingernail to check for HWA underneath.
- Needle discoloration and premature needle drop. Dead shoots with no new growth.
Catching infestations when they first get started allows you to target treatments and potentially save trees. Monitor throughout the year since all life stages can be present year-round, but focus efforts during late winter-early spring when egg masses are most visible.
Cultural Control Methods
For lightly infested trees, the following cultural control methods can help eliminate HWA:
Horticultural Oils and Insecticidal Soaps
Applying horticultural oils or insecticidal soaps directly smothers and kills HWA on contact while posing minimal risks to non-target organisms. Good coverage of the foliage is essential. Repeat applications may be needed.
Fertilization
Applying nitrogen fertilizer can help improve the vigor of infested trees. The improved health may help hemlocks better withstand effects of HWA feeding.
Irrigation
Watering trees during drought periods can reduce plant stress and deprivation of nutrients. Healthier trees may resist HWA better. Prioritize irrigation for the most valuable trees.
Removal of Infested Branches
Pruning out lightly infested branches can eliminate HWA populations before they spread. Dispose of pruned branches immediately by chipping or burning.
Chemical Control Options
When HWA populations are high or spreading rapidly, stronger chemical control measures are warranted:
Systemic Insecticides
Systemic insecticides like imidacloprid and dinotefuran are taken up by the tree and make its tissues toxic to HWA. They provide long-lasting control with one application. However, they carry higher risks to bees and other pollinators. Avoid use around flowering plants.
Horticultural Oils
Dormant season horticultural oil applications help smother overwintering HWA while minimizing risks to pollinators. Good coverage is essential. Oils must contact insects to be effective.
Insect Growth Regulators
Insect growth regulators like fenoxycarb disrupt the HWA lifecycle and prevent maturation. Multiple applications are needed for ongoing control. The effects are slower but more selective than other pesticides.
When using any pesticide, strictly follow all label directions. Consider hiring a licensed applicator for large-scale treatments. Always use proper protective equipment.
Biological Control Options
Natural enemies of HWA can provide suppression of populations without chemicals:
Predatory Beetles
Several species of predatory beetles (e.g. Laricobius nigrinus, Scymnus sp.) have been released in eastern forests and have established populations that prey on HWA. Releasing beetles from insectaries may boost control.
Predatory Flies
Flies in the genus Leucopis feed on HWA as larvae and have been shown to significantly reduce populations. Periodic releases can augment biocontrol.
Entomopathogenic Fungi
Fungi like Lecanicillium muscarium infect and kill HWA with limited effects on other species. Spray applications of fungal formulations can provide suppression but regrowth often occurs.
Parasitic Wasps
Tiny wasps (Pseudoscymnus tsugae) lay eggs inside HWA bodies. The larvae hatch and consume HWA from within. Periodic releases at proper rates are key to good control.
Integrated Pest Management
The best strategy is to use an integrated pest management (IPM) approach that incorporates multiple control tactics. Monitor trees closely and treat hot spots early before they spread. Combine lower impact cultural controls and biological controls first, and use chemical controls only where/when absolutely needed. Maintain diversity of control methods over time. Work regionally with neighbors and agencies to contain infestations.
Preventing HWA Spread
Vigilance is critical for halting the spread of HWA:
- Inspect all hemlock products (logs, nursery stock, wreaths, etc.) for HWA before transporting. Reject or treat infested material.
- Monitor trees at campgrounds, parks, and other high traffic areas. Report signs of HWA immediately.
- Do not move potentially infested hemlock wood, clipping or debris from your property. Chip infested material on site.
- Check clothing and gear for HWA “wool” after visiting hemlock stands. Clean properly before going to new areas.
With early detection, persistence, and integrated control, it is possible to save hemlocks from HWA. The collective efforts of individuals, land managers and agencies will be key to effective, long-term management of this devastating pest.
Frequently Asked Questions
What does HWA look like on hemlock trees?
HWA appears as small white woolly or cottony masses on the undersides of new hemlock twigs, often clustered at the base of needles. The insects themselves are tiny black specks embedded in the white wool.
How quickly do infested hemlock trees die?
Hemlocks deteriorate rapidly once heavily infested, typically dying within 4-10 years. The entire decline process can occur in as little as 2-3 years for severely infested saplings.
How do I properly dispose of infested hemlock branches?
Cut infested branches into smaller pieces and thoroughly chip or burn the material onsite. Never remove infested wood or debris from the property without treatment.
When is the best time to monitor hemlocks for HWA?
Monitor year-round, but focus efforts in late winter-early spring when HWA egg masses are large and most visible. This allows early detection and treatment.
How long do systemic insecticide treatments last for HWA control?
Most systemic treatments provide HWA control for 2-3 years, sometimes longer. Effectiveness depends on application rate, tree health, and environmental factors. Annual monitoring helps determine re-treatment needs.
Will predatory beetles or wasps completely eliminate HWA from my trees?
Natural enemies typically provide partial control of HWA, not complete elimination. Periodic releases are needed to maintain predator populations for ongoing suppression of HWA.
Conclusion
Hemlock woolly adelgid poses a serious threat to the health and survival of hemlock trees across eastern North America. A heavy infestation can kill a hemlock in just a few years. However, a proactive integrated pest management approach utilizing a combination of monitoring, cultural controls, biological controls, and judicious use of pesticides can successfully preserve hemlocks in both forests and landscapes. Protecting hemlock trees preserves the unique ecosystems they support and maintains their ornamental beauty. With persistence and vigilance, it is possible to effectively combat hemlock woolly adelgid.