What to do about cherry bark tortrix attacks

Last week you learned how to identify the cherry bark tortrix, a moth whose larvae can kill cherry trees and infest woody trees and shrubs in the rose family.

Today we'll talk about what to do if you suspect the presence of this new pest in your landscape. Symptoms include accumulated orange frass (moth waste material) and "frass tubes" on the bark and large scaffold limbs of trees.

Basic tree care can help. Avoid creating wounds by careless pruning. Prune meticulously. Manage equipment properly. Don't allow mower or string-trimmer attack. Remove weeds that surround trunks, and don't plant heavy shrubs like juniper against trunks.

Though this pest is serious, trees will often coexist with and survive a large infestation. However, if heavily affected for several seasons they may die of nutrient loss and general decline.

Mount Fuji cherries (Prunus serrulata) and weeping cherries (Prunus subhirtella) are quite susceptible. The pest prefers cherry but will move to alternate hosts in the rose family when it has killed out the cherries.

An encouraging note regarding edible and ornamental cherries is that naturally-occurring parasitic wasps can provide some biological control.

The parasitic wasp lays its egg on the larvae of this moth, killing it. Washington State University researcher Dr. Lynell Tanigoshi has visited areas of Europe to check on natural enemies of this moth. Cherries in Europe aren't being as heavily affected as those in Western Washington, and the investigation seems to indicate these moths are managed, though not eliminated, by natural enemies in its European, Asian and Siberian ranges where both the moth and its wasp predators have coexisted for decades.

Although there are not enough parasitic wasps to slow the spread of cherry-bark tortrix in Western Washington, that could change. Predators build their numbers following the arrival of their prey. As the pests increase, wasps will exploit them and also increase.

Gardeners can help encourage these wasps by avoiding whole-yard sprays of toxic insecticides that can kill them.

Get professional help if you suspect the problem and wish to use an insecticide on the tree. Don't have the trees sprayed if you eat the cherries: Pesticides aren't available for use on edible cherries.

Certain insecticides can provide some control on ornamentals if used properly and at the right time. Because the larvae tuck themselves inside the living tree tissue and the frass tubes, it's hard to reach them with insecticides during most seasons of the year.

Timing and pesticide choice are vital. The first two weeks in October, when adult moths have ceased flying and weather is relatively dry, are effective times to treat. Larvae are cleaning out frass tubes and are susceptible to insecticides. A well-timed pesticide application can keep trees cherry bark tortrix-free for multiple years without having to treat again.

Registered pesticides — for ornamentals only — include synthetic pyrethroids (bifenthin, cyfluthrin). A Washington state pesticide applicator license is required to purchase and apply these.

Another choice in the first two weeks of October is acephate (Orthene), which is toxic to the beneficial wasps and other beneficial insects whose populations are important in garden-pest management. Use very little.

It's not necessary to spray the whole tree canopy: you want to only spot-treat the infested areas of the trunk and scaffold limbs.

Researchers hope that biological-control agents such as wasps, cultural controls, and well-timed pesticides will, in combination, provide eventual relief for landscape trees.

The Practical Gardener column will keep you informed as further control measures are developed. (Thanks to Todd Murray, pest-management specialist at WSU Cooperative Extension, Whatcom County, for material used here.)

Mary Robson is area horticulture agent for Washington State University/King County Cooperative Extension.