Q. I have a row of 16 – three year old “Emerald Green” arborvitaes. I suspect they have arborvitae leafminer (cypress tip moth). I have noticed the moths before, but now there are more and I just recently noticed brownish-yellow tips on a couple of the trees. All of them have lots of needles falling from the interior. My problem is that I have received conflicting reports about the proper time to spray for them, and is there anything I can do in the meantime to lesson the damage? Thank you! Linda Brieger – Tacoma, WA
A. The way to gain control over any pest population is to know its life cycle. Spraying is geared toward eliminating the adult form of the insect, which is a moth as the second of the two common names indicates. The most likely reason you may see conflicting reports on when to spray the moths is because of varying times the moth may emerge in different regions where they are present. They are generally active in our area from April to June with a peak of activity in May. The moths lay their eggs during this period and the eggs hatch and then burrow into the needles of the host plant. According to the WSU extension the adult moths are silver-tan and approximately 1/4″ in length. External sprays won’t have an effect on the larvae once they burrow so you need to spray weekly during this period to catch the larvae as they hatch. Systemic insecticides are able to kill the larvae once they are in the host. You can limit systemic insecticide spraying to one application near the beginning of the activity since they generally remain effective for some time (see labels for instructions). As far as “in the meantime” a sprayless solution is to prune out and destroy infected parts of the host now so that there are less moths in the spring. You can also keep an eye out in the spring for the white cocoons that form after the larvae exit the host to become adult moths. You can remove these as well.