Mountain Ash Sawfly Larvae Eat Their Way Into Year Three

Bill Granger
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This is the third, and likely the final, article on Pristophora geniculata, Order Hymenoptera, identified consuming up to 90% of the foliage of certain Mountain Ash trees in Metro Vancouver, BC in 2015, with subsequent defoliation in summers of 2016, 2017. This chewing insect pest was previously reported in Ontario after 1938, of European origin, and never reported west of Michigan. This pest has a complete life cycle and, once established, does not have to move far from the host plant, exclusively in the genus Sorbus. There are 100 to 200 species within this genus worldwide. Of particular interest to arborists are ornamental Mountain Ash trees in gardens and public landscapes.

Bill Granger checking mature Sorbus aucuparia, Museum of Anthropology, University of British Columbia, July, 2017. Photo credit: Melissa McClave-Brule. Click image for full size.

I observed the progress of the pest locally around Vancouver, British Columbia and nearby regional parks, universities, and public grounds over the past three years. This was a new pest to the Pacific Northwest, first reported in Washington State in 2008.

The insect has a complete life cycle, emerging as moths in late May to early June from silken capsule-shaped cocoons that overwinter under the host trees in leaf litter or in the top layer of soil. The moths can fly quickly and for some distance to find a mate, with the female laying eggs in notches cut along the margins of each leaflet. The mass of chewing larvae consume entire sections of the trees, eating the compound leaf tissue, with only the leaf rachis, veins, and petiole remaining. The trees do not re-foliate during the current growing season, and bud development for the subsequent year continues. New leaf, flower, and twig growth the following spring appears normal and is a testament to the vigor of this genus.

The geographic spread observed of chewing larval damage from initial trees where I saw defoliation in 2015 to 2017, to new infestation, is in excess of two kilometres. It is possible that the moths could also be distributed by winds to new locations where Sorbus is growing, or that overwintering pupae are transported to new locations in soil being moved for landscaping purposes.

The larvae emerge in June and immediately begin to eat, being hatched right at the food source. Mature pupae are bright yellow with four longitudinal rows of black spots on each side and two broken rows down the middle of the back. Larvae measure approximately 18 mm in length in the final instar, and usually hatch in one generation per year, feeding between June and August. A second generation is possible, feeding between late July to August, and that may be the case this year. All larvae had gone to pupation by mid-July this summer in my observations, with only leaf skeletons remaining on the trees.

I have seen almost entire defoliation of younger trees now for three consecutive growing seasons. The fruit is left untouched and matures normally, to be eaten by birds late in fall. A mature specimen at the entrance to the Museum of Anthropology, University of British Columbia, has been defoliated for the third year, predominately in the lower 2/3 of the crown, is setting fruit for this season, and also has several mosses and lichens now populating the lower twigs and branches. Shade and senescence are also likely factors in this tree’s overall decline (see photo).

Mountain Ash berries, particularly varieties such as “Pink Pagoda,” are attractive as a landscape feature, an important food source for migrating and resident songbirds; while the immature fruit is not affected by the chewing insect pest, overall habitat value of the host trees may be affected by continued years of defoliation. Trees with uneven or patches of missing vegetation may be removed for aesthetic reasons. Sanitation (removing fallen leaf litter, cleaning the site and replacing with clean compost or mulch) does not seem to eliminate the overwintering cocoons sufficiently. The winter of 2016-17 was cold with unusually heavy snowfall, and seems to have favoured survival of the buried cocoons.

If you notice a Mountain Ash tree in a landscape that you care for looking unusually open, especially in the lower canopy, Mountain Ash Sawfly is the likely culprit.

As an end note, I would like to express sincere thanks to Susan Munro for her outstanding work as Editor of the PNW-ISA newsletter for over thirty years. Susan has welcomed several of my articles, on Lyme Disease and now a new chewing insect pest in the Pacific Northwest, and I appreciate her continued interest and ongoing dedication to education and the Pacific Northwest Chapter, ISA. Thank you, Susan!