by Lindsay Bond Totten
Scripps Howard News Service
Don't squash that ladybug on the windowsill! She may be one of a special new breed of predators on whose tiny shoulders rests the future of our North American hemlock trees.
Just open the window. Or gently show her the door. Then wish her God speed.
If she's a Pseudoscymnus tsugae ladybug--only experienced entomologists can tell for sure--the hemlock woolly adelgid is on the menu for lunch. And you definitely don't want her to miss a meal.
This amazing little bug has come all the way from Japan to help us out of a mighty serious jam: Hemlock woolly adelgids--fuzzy white aphid-like insects--are ravaging hemlock populations all over the East.
From an epicenter in Virginia, the adelgid has spread south into North Carolina and north as far as Massachusetts. Maine, New Hampshire and Vermont are bracing for invasion (hemlocks are an important component of northern forests), and trying desperately to delay the inevitable by banning uncertified hemlock nursery stock from crossing their borders.
Both species of hemlocks native to the eastern United States--Tsuga Canadensis (Canada hemlock) and Tsuga caroliniana (Carolina hemlock)--are attractive targets for this voracious pest.
Hemlock woolly adelgids suck the sap--and eventually the life--from the twigs of their defenseless victims. The bark of heavily infested trees can be nearly white with the bodies of multiple generations. When the host dies, young adelgids move on to nearby hemlocks.
Interestingly, if hemlocks decline drastically in Eastern forests and landscapes in the coming decades, it won't be the first time North America has experienced such loss. Pollen records indicate a crash of hemlock populations across the continent about 5,000 years ago. Whether their rather sudden disappearance (up to 80 percent of the hemlock trees died within a 30-year period) was due to insects or disease, experts aren't sure. But hemlocks nearly perished, and it took the species nearly 1,000 years to recover.
Perhaps this time around we can mitigate nature's fury, though to what extent remains to be seen.
Imported from Asia several years ago--where the hemlock woolly adelgid is not a threat--Pseudoscymnus tsugae has been released on public lands and in state parks throughout the East.
She's a hard worker, but early results suggest she's going to need some help. Even under ideal laboratory conditions, the beetle consumes just 75 percent of adelgids on heavily infested hemlock branches; real-life averages are bound to be lower. Hemlock trees will continue to suffer damage.
Three other predatory beetles and four pathogenic fungi are being studied for effectiveness and compatibility with Pseudoscymnus tsugae. One or more may soon join her in battle.
While the future of hemlocks in our forests ultimately depends on the success of biological controls, gardeners can wage hand-to-bug combat by protecting individual landscape specimens. Horticultural oil, an environmentally friendly pesticide applied to trunks and twigs of infested hemlock trees, suffocates the adelgids without poisons. There's no danger to other tree species. For very tall trees, consult a certified arborist with access to spray equipment that will reach the tops.
Stronger pesticides are labeled for use in some states, but these should be reserved for use by qualified professionals on individual specimens of significant size or value. Indiscriminate use of toxic chemicals could lead to resistance in isolated adelgid populations. They reproduce so quickly, adelgids may be able to devise defenses against our chemical arsenal faster than we can develop new ones. Because horticultural oil works in a different way, suffocating rather than poisoning them, resistance is more difficult for the insects.
Close monitoring of the adelgid population in your yard and neighborhood is the best way to determine the need for, or frequency of, sprays. Horticultural oil can only be deployed once the adelgid shows up. Oil sprays have no residual effect and no ability to prevent further attack.
Eventually, bio-controls will catch up to the hemlock woolly adelgid, just like they conquered whatever mysterious pest devastated hemlocks 5,000 years ago. Hopefully, scientists can speed up the recovery process this time. If they can't, many of us will have to search for alternate evergreens, and Pennsylvanians will have to choose a new state tree.
(Lindsay Bond Totten, a horticulturist, writes about gardening for Scripps Howard News Service.)
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