Hemlock Needles

Asked May 29, 2020, 4:13 PM EDT

I have several 40’+ hemlocks on my property. They were recently treated with insecticide for HWA and EHS. Not that long after the first treatment the needles started turning colors of one particular. They are yellow/orange and some rust colored. See pictures. Could this be a result of the treatment or do i have another pest problem? When would I expect a notice difference in the HWA on the branches? Will the woolly bits go away eventually with treatment?

Washington County Maryland

1 Response

We cannot tell if the needle discoloration is coincidental with the timing of the treatments or not. Without knowing what the chemical used was or how it was applied, we cannot say how it may have impacted the tree. Some insecticides can allow for secondary outbreaks of pests such as mites, which feed on and can discolor foliage. There are also a few rust diseases that spend part of their life cycle on Hemlock and part on Blueberry, Hydrangea, or other hosts depending on the species. We cannot discern if any of these diseases are causing this browning.

Evergreens tend to shed old and damaged foliage in spring as new growth is developing. It is possible that this is just excessive needle shed due to the tree's prior seasons of stress due to the pests and environmental conditions. Hemlocks prosper in partially shaded sites that have cool, moist soil, such as by mountain streams. Western MD is a good location to try to grow Hemlocks in gardens since the temperatures and soil type are more to their liking, but last summer's drought may have caused them stress unless they were getting supplemental irrigation.

If the treatment is effective, the white adelgid wax will disappear in time as the insects die, desiccate, and fall off the stems.