Hemlock Woolly Adelgid:
Harming Eastern Hemlocks, Waiting for Predator Ladybugs

Hemlock Woolly Adelgid (HWA) is a small insect pest from Japan that sucks plant sap from eastern hemlock (genus Tsuga) twigs at the base of the needles and injects toxic saliva while feeding. As a result, needles lose color and fall off the tree. Entire trees may die in as little as two to four years. It also injects toxic saliva while feeding. The insects are brownish-red, ovular, and less than a millimeter long (smaller than the period at the end of this sentence). Wingless nymph "crawlers" produce white masses 3+ mm in diameter that appear on hemlock bark, foliage, and twigs, surrounding the insects' bodies.

Horticultural oils that smother the insects, which most desire fertilized trees, have been the best insecticidal treatment, causing their mortality. The oils are non-toxic to the trees, as opposed to soap, which is an otherwise effective treatment. However, the least harmful cure may be introduction of Japanese ladybugs to eat the aphid-like creatures. As a pilot program, this solution was approved and funded by the U.S. government in July 2000 for Hemlock Gorge in southern Massachusetts.

According to Brian Yates, president and founder of Friends of Hemlock Gorge, a Connecticut researcher traveled to the adelgid's native Japan in 1992 to find out why the hemlocks there were healthy. The researcher discovered that a poppy-seed-sized black ladybug, previously unknown, was keeping the adelgid population under control by eating the aphids' egg sacs, which look like strings of fluffy little white balls. The Japanese hemlocks "never got to such a stage that they would be vulnerable to dying because there were natural predators there," Yates says. "In Japan, they're in perpetual balance. We're trying to establish the balance."

Other control methods include planting species of hemlock that are resistant to HWA and breeding certain other Japanese insects that effectively prey on them. Two Japanese hemlock species, Tsuga diversifolia and Tsuga sieboldii, and two western North American hemlock species, Tsuga heterophylla and Tsuga mertensiana, are resistant to HWA. According to the Connecticut Agricultural Experiment Station, "Although HWA infests these resistant species, it rarely reaches high enough densities to cause injury. Therefore, planting these resistant hemlocks should reduce the impact of HWA in the ornamental landscape. Of the four species, Tsuga heterophylla is most similar to eastern hemlock in appearance, growth form, and utility. However, the likelihood for long term success of these hemlocks in the eastern United States in unknown."