Back Issue
March/April 2000

Controlling Hemlock Woolly Adelgid

We have a hemlock hedge on our property and have been battling hemlock woolly adelgid for two years. We’ve been to several different garden centers, asking for help; the last one recommended a two-step program — applying dormant oil in March and malathion in May. Is this the best plan for combating this pest? And, we were told that as it is impossible to eradicate hemlock woolly adelgid, we’d have to settle for just controlling them. Is this true?

It depends on coverage, how thorough the application is and how accessible the insects are on the hemlock. If one could examine the base of every needle on the tree, and determine that the oil had covered every single insect, then hemlock woolly adelgid could be eradicated at that time on that hemlock. However, given time, another population of hemlock woolly adelgid will infest the hemlock again.

The oil can be applied any time there is no danger of the oil freezing on the tree before the carrier has had a chance to dry off. This can vary from 4 or 5 hours to all day, depending on the weather. Don’t spray on drizzly or humid days. Apply the oil in the morning of a nice dry day.

Mr. Kenneth A. Welch is the entomologist at the Connecticut Agricultural Experiment Station, New Haven

I recommend either of two spray programs: once in early April and again during the first half of June, or once during the first half of June and again during last half of September. Use horticultural oil or insecticidal soap. The malathion doesn’t add any advantage and is apt to kill many more non-target insects; it’s the thorough coverage that’s important whether you’re using the malathion or the oil or soap.

It is possible to control hemlock woolly adelgid systemically, by applying an insecticide, imidacloprid, that will be taken up by the plant roots and moved into the young branches of the plant where the adelgid is feeding.

Imidacloprid can be very effective. How effective depends on two factors: the health of the plant and how well the insecticide will be picked up by the roots and moved up into the branches. If the tree has been infested by adelgid and is starting to decline, then it’s probably not healthy enough to translocate the insecticide. If the tree is still in good shape, then imidacloprid can be very effective.

Here at the Windsor Station we’re conducting experiments on various application techniques for imidacloprid including soil injections, soil drenches and granular applications.The most effective method of applying imidacloprid is by injecting the soil with a hydraulic needle; this must be done by a licensed arborist or certified pesticide applicator. Soil drenches and granular applications can be effective depending on how well the plant roots can pick up the material.

Start with moist soil and loosen it so that the soil surface is rough; this will discourage the imidacloprid from running off. If it’s not going to rain, water well after applying to work the imidicloprid down to the roots of the plant. How much imadcloprid to apply depends on the size of the plant; follow the application rate on the insecticide label.

Dr. Mark S. McClure is the entomologist and chief scientist at the Connecticut Agricultural Experiment Station, Windsor.