Mountain Ash Sawfly Persists in the Pacific Northwest

Bill Granger
Learn more about Bill Granger.

This article is offered as a follow-up to last year’s article on Mountain Ash Sawfly, a new pest in the Pacific Northwest that had previously only been a problem to ornamental Mountain Ash specimens east of Michigan.

The emerging population is persisting and appears to have increased range in 2016. The larval stage of this defoliator consume all the leaves of the lower branches (about halfway up from the base) by an army of small yellow caterpillars, moving in groups on each compound leaf, leaving only the leaf stem and ribs intact. There is no apparent impact on fruit set or quantity.

For review, Pests of Ontario, published in 1975 by the Forest Management Branch, Province of Ontario, and Johnson and Lyon’s excellent, full-colour Insects that Feed on Trees and Shrubs stressed that the insect pest is an introduced species of European origin first identified in Ontario in 1938, but at the time never identified west of Michigan – Pristiphora geniculata, a member of Order Hymenoptera, specific to only hosts in the genus Sorbus. The reference material indicated that the pest has a complete life cycle, with the larvae falling to the ground once gorged and creating silken capsule-shaped cocoons in the soil or leaf litter, where the pupae can overwinter and last up to five years in diapause. The adult moths emerge in late May to late early July, can fly quickly and for some distance, live for less than a week and mate, with the female laying eggs through an ovipositor in notches cut along the margins of each leaflet. The larvae soon emerge and begin eating voraciously. When mature, the pupae appear yellow with numerous black spots on all body segments except the last. Spots are arranged in four longitudinal rows on each side and two broken rows down the middle of the back. Larvae measure approximately 18 mm in length in the final instar, with one generation per year (very seldom two). Feeding takes place between June and August and, occasionally with a partial second generation, late August into September.

There are no known natural predators but it can be somewhat controlled by the introduced parasitic wasp parasitoid Olesicampe geniculatae (Ichneumon flies).

There is some information that this pest was identified in Washington State as recently as 2008. A discussion webpage in Outdoor Gardening in the Pacific Northwest in August, 2014 identified the Mountain Ash Sawfly larvae and damage to specimen trees in Port Coquitlam and Vancouver, British Columbia, with the University of British Columbia (UBC) Botanical Garden Forums Administrator, Daniel Mosquin, noting “from Snohomish County in 2009 to here in 5 years, so they get around, I would suspect there being some sort of chemical signal they are picking up on.”

As a cultural control, I squashed all the larvae found on my young tree, and then I dug vigorously around the base of the Sorbus at fall leaf clean-up, and mulched with clean fresh compost.

In spite of these precautions, the pupae obviously overwintered, moths emerged and mated, and the eggs were laid along the margins of each leaflet. The damage has doubled from last year and the young trees I have observed are approximately 50 percent defoliated by late June. I have now noted this happening to all varieties of Sorbus across the Lower Mainland, and this will become a significant concern for property owners with specimen Mountain Ash trees in their landscapes.

With climate change swirling around us, species drift across the continent is happening along with extreme weather pattern changes. The damage to our Mountain Ash trees may not be fatal, but the trees are not able to re-foliate the eaten areas in the current growing season and will be significantly weakened. Again, as I concluded last year, we may not be so lucky with other transported pest species, and should all be on the lookout for the unexpected.