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Introduced pine sawfly

White pine defoliation caused by introduced pine sawflies was reported across southern Beltrami and Hubbard Counties, especially on ornamental trees. In 1998, summer started early in northern Minnesota so three generations of sawflies were produced this summer. The first generation was active in June, feeding on previous years' growth. The second and third generation larvae fed on the current year's needles as well as the previous years' growth. Sawflies overwinter as larvae in cocoons in the litter material on the forest floor where they are protected from killing temperatures.

A venture into a pine plantation

A private landowner has a plantation of sapling size Norway and Scots pines. The tree species were not intermixed but were planted in two separate parts of the plantation. We observed no symptoms of insect infestation on the Norway pines. In stark contrast, however, over seventy percent of the Scots pines were infected with Zimmerman pine moth (ZPM), as well as, pine root collar weevil (PRCW).

Zimmerman pine moth prefers Scots pine but will also attack many other pine species. Caterpillars feed just under the bark on tree stems, especially at the branch whorl. Large accumulations of coagulated pitch are formed at the feeding site. Reddish frass may be mixed in with the pitch. Branches hang down and flag as if an overweight deer hunter had perched himself on top of the trees for a few days. The main stem may be swollen above the infested whorl or may be broken off if the tree is heavily injured.

Pine root collar weevil injury to pine is cause by larvae feeding in the inner bark and wood below ground in the root collar and between the major roots. The soil and bark near the root collar becomes blackened and soaked with pine pitch. Larvae can be found in tunnels in the bark and pitchy soil. They're easier to find if you use a large knife to pry and slice into this mess. Trees are weakened and girdled at the ground level and may fall over or die or both. The canopy fades to pale green to yellow then red. Off-color trees that are leaning or tipped over at the ground line are symptoms of pine root collar weevil attack. These trees become brood trees for continual feeding and damage by subsequent generations until they are so damaged that they die.

It is highly probable that both insects will start to damage the Norway pines once the Scots pine food source and habitat were used up. It was recommended that the landowner destroy the Scots pine component of the plantation in order to protect the Norway pines from becoming infested and damaged.