UNIVERSITY PARK, Pa. – Hemlock trees in Pennsylvania and the Northeast are declining and dying, and many experts are blaming an aphid-like insect known as the hemlock woolly adelgid.
However, identifying a culprit is not that simple, according to a tree-pest specialist in Penn State’s College of Agricultural Sciences.
“Hemlock woolly adelgid gets most of the publicity, but people shouldn’t place all the blame for hemlock decline on this pest,” said Gregory Hoover, extension entomologist.
“Other insects are contributing, such as elongate hemlock scale, spruce spider mite, hemlock rust mite and cryptomeria scale.
“Elongate hemlock scale probably has caused more hemlock decline in Pennsylvania – in terms of quick death – than hemlock woolly adelgid,” he adds.
There are others.
“And elongate hemlock scale is harder to manage because, unlike hemlock woolly adelgid, it’s active throughout the growing season. So there are several pests in the landscape that we need to concentrate on to maintain healthy hemlocks.”
Nevertheless, hemlock woolly adelgid is a serious threat to hemlock trees. The tiny (2 millimeter long) insect has piercing-sucking mouthparts that it uses to remove fluids from herrilock needles, causing them to dry out and drop prematurely.
As trees thin and lose “leaf’ area for photosynthesis, they become weak and more susceptible to drought stress and attack from other pests. These stressed trees eventually may die.
As its name implies, the hemlock woolly adelgid infests only hemlocks. The most obvious sign of an infestation is small, cottony-looking masses at the base of the needles.
These masses – which conceal the black, oval pest – are fine filaments of wax secreted by the adelgids.
The hemlock woolly adelgid is native to Asia and has been present in the United States since the 1960s. The pest initially gained a foothold in southeastern Pennsylvania, and has spread north and west to an area now encompassing about half of the state.
“If you draw a line from the northeast corner of the state near Scranton and Wilkes-Barre, south and west to Bedford County, hemlock woolly adelgid may be found in the area south and east of that line,” Hoover explains.
“There also is a small, isolated infestation in Allegheny County.”
Hoover said hemlock woolly adelgid is a particular problem in forests, where management options are limited.
He said a lady beetle from Japan, the adult and larval stages of which prey on adelgids, has shown promise as a biocontrol agent for managing the pest in forests.
Homeowners can control infestations on small hemlocks by thoroughly covering the needles with a registered formulation of insecticidal soap or horticultural oil, applied according to label directions.
“You must time these measures with vulnerable periods in the life cycle of the pest – mid to late June or mid-September through October are the best times to get effective control,” Hoover said.
“For larger trees, homeowners should consult their county extension office or a certified arborist for advice.”
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