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Mountain Ash Pristiphora geniculata (Hartig) Hanson, T., and E. B. Walker. [n.d.] Field guide to common pests of urban trees in the Northeast. Waterbury, VT: Department of Forests, Parks and Recreation. http://www.state.vt.us/anr/fpr/forestry/pubs/pest. html

Hymenoptera: Hosts: American and European mountain ashes

Evidence: Look for defoliation of all but the midribs of leaves (a). Larvae, which are yellowish-green with prominent black spots (a,b), feed gregariously and tend to defoliate an entire branch before moving on to another.

Life Cycle: There are two generations per year, except in the northernmost part of a. Larvae of the Mountain the sawfly's range. Winter is spent in the ground as a cocoon, and pupation occurs in ash sawfly consume all the spring. First generation adults emerge and lay eggs throughout June and into but the midveins of leaves. July. Eggs are laid in slits along the toothed margin of leaflets, producing a blistered appearance. Larvae of the first generation mature and drop to the soil to pupate by August. A portion of these reach adulthood and lay eggs, producing a second genera- tion which feeds from late August through September.

Management: Defoliation fluctuates with weather conditions and the abundance of natural control agents such as predators and parasites. Branches that contain leaflets with eggs or larvae can be pruned out and destroyed. Bacillus thuringiensis is not effective against sawfly larvae. Larvae can be treated with residual insecticides (Sevin) early to mid June. If you had damage this past year, make a note on your calendar to check tree next year around the same time.

Photo Credits: Figure a: E. Bradford Walker, Vermont Department of Forests, Parks and Recrea- tion, Waterbury, VT b. When Mountain ash Figure b: E. Bradford Walker, Vermont Department of Forests, Parks and Recrea- sawfly larvae are dis- tion, Waterbury, VT turbed, they tend to rear up into an S-shaped posi- References: tion. Drooz, A.T. 1985. of Eastern Forests. USDA Forest Service Miscellaneous Publication 1426. p 404-405; Johnson, W.T. and Lyon, H.H. 1991. Insects That Feed on Trees and Shrubs . 2nd edition. Cornell University Press. p 128-129; Rose, A.H. and Lindquist, O.H. 1982. Insects of Eastern Hardwood Trees. Canadian Forest Service Publication, Forestry Technical Report 29. p 278-279.

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Source: http://www.forestpests.org/vermont/mountainashsawfly.html