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Introduced pine sawfly

Introduced pine sawfly larvae, Diprion similis, were widely scattered in central Minnesota and had increased in August to such large numbers that moderate to heavy defoliation of pines had occurred. In early September, large numbers of introduced pine sawfly larvae were reported from the Walker area in Cass County and throughout central and southern Hubbard County. Defoliation in the fall can be severe enough to cause top-kill or entire tree mortality in one season. On the sites inspected, none of the trees had defoliation severe enough to warrant insecticidal control.

Introduced pine sawflies usually have two generations per year and up to three generations per year have been reported in Wisconsin during some outbreaks. First generation larvae are active in late May and June. Second generation larvae are active in late August and September. In June the larvae feed on the previous years foliage but the second generation feeds on current and previous years growth.

A small number of third generation needle-consuming larvae develops in September, but most second generation cocooned larvae remain dormant until the next spring when they change into pupae and then adults. Natural control of this pest is caused by very low or rapidly fluctuating temperatures or heavy rainfall during the egg and early larval stages, wasp-like and other insect parasites, predators, and low winter temperatures. Predators include insects, spiders, rodents burrowing in the leaf litter, chickadees, and other birds.