Mountain Ash Sawfly Makes a Surprise Entry to the Pacific Northwest

Bill Granger
Learn more about Bill Granger.

After a hard day of pruning, mulching, and watering at my place on Bowen Island, BC in mid-June, I finally sat down to admire the airy look of the plantings along the front walkway. With the bright sun going down in the background, one tree looked a little too airy, so I went out to take a closer look. This was a volunteer Sorbus aucuparia that had sprung up in the informal border several years ago. Remembering the Wiccan mythology that Rowan trees are supposed to keep witches away, I left the tree, which will eventually guard the front door to my house, thinking to crown raise and shape it in later years.

I found that the lower branches (about half-way up from the base) were being defoliated by an army of small yellow caterpillars, moving in groups on each compound leaf, leaving only the leave stem and ribs intact. I crushed all the larvae that I could reach by hand, and tried to recall if I had ever seen this pest before.

When I was a very young arboriculture student, I spent a summer working at the Royal Botanical Gardens in Hamilton, Ontario, where the scientists would sometimes come out to the Arboretum and allow us in on what they were looking at. I cross-referenced the memory of Mountain Ash Sawfly with the old texts of the day. The first was a colour soft-cover entitled Pests of Ontario published in 1975 by the Forest Management Branch, Province of Ontario, and the second was Johnson and Lyon’s excellent hardcover, full colour Insects that Feed on Trees and Shrubs. Both texts stressed that the insect pest is an introduced species of European origin first identified in Ontario in 1938, Pristiphora geniculata, a member of Order Hymenoptera, specific to 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 larvae 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 18mm 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 is some information that this pest, which has no known natural predators but can be somewhat controlled by the introduced parasitic wasp parasitoid Olesicampe geniculatae, (Ichneumon Flies), was identified in Washington State as recently as 2008.

But, in my experience, I hadn’t seen this for over forty years. Two of my near neighbours had damage on their Sorbus aucuparia, and one, like me, had found the larvae eating and pruned them off and embargoed the insects and affected branches in a tight plastic bag. She had no idea what the insects were, just that they were eating her ornamental trees. I checked with arborist friends in West Vancouver, North Vancouver, Port Coquitlam, and looked myself in Stanley Park, Vancouver, with no damage or larvae in evidence. So, how did it get here?

I am only guessing, but about eight years ago I bought a Callicarpa bodinieri for its beautiful purple autumn berries (which persist through the winter) and planted it in the border. The Mountain Ash is growing near to the root ball, and both seed and pupae may have come with the Beautyberry.

As a cultural control, I intend to dig vigorously around the base of both plants when the fall rains return, and then mulch with clean fresh compost. I will also watch carefully next June. With climate change swirling around us, species shift across the continent is happening along with extreme weather pattern changes. This even jolted my memory back over forty years, and I’m glad to report that I can still remember some of what we learned in our early years as young arborist students. The damage to our Mountain Ash trees will not likely be fatal, as the insects attack the foliage well into the growing season. We may not be so lucky with other transported pest species, and should all be on the lookout for the unexpected.