Elongate hemlock scale is becoming a serious pest of hemlock trees in the Greater Boston area. Native to Japan, the Elongate hemlock scale (EHS) is an armored scale insect targeting eastern and Carolina hemlocks. Occasionally Fir, Spruce, and Yews become secondary hosts, usually when growing adjacent to infested Hemlocks.
EHS injures plants by inserting piercing mouthparts into the lower surface of the foliage withdrawing vital nutrients necessary for plant growth. The underside of needles may appear whitish from the white, waxy covering produced by the insects. Excessive loss of plant sap reduces the plant vigor causing needles to turn yellow to brown before prematurely dropping, giving the crown of an infested tree’s canopy a thin appearance.
Dieback often starts low in the canopy progressing upwards after density reaches about 10 scales per needle. Unmanaged trees often die within 5-10 years of initial infestation, but some survive longer in severely compromised conditions with only a sparse amount of foliage. Infestations of EHS weaken trees, predisposing them to other environmental stress or successful attack by secondary organisms.
Populations may build slowly on healthy trees, but rapidly on stressed plants. Outbreaks of EHS often intensify following infestations of hemlock woolly adelgid, drought, or other stresses that have weakened the trees. Therefore, maintaining trees in healthy condition will discourage the buildup of elongated hemlock scale populations.
Effective management of EHS is possible but difficult. Timing of controls is critical since all stages of the elongate scale (eggs, crawlers, and adults) occur throughout the growing season. Multiple applications are required for moderate to severe infestations.
BTSE has had success managing EHS with appropriately timed horticultural oil applications and with the selected use of systemic insecticides. If you have concerns with the health of your hemlocks, contact us to see if an elongated hemlock scale may be a factor.