Forest Insect and Disease Newsletter: 6/99

Field notes: Late May

Spruce budworm in northern Itasca County and northern St. Louis County were mostly 2nd and 3rd instar larvae on May 19th. They are feeding on new needles on expanding buds and under bud caps on white spruce,

Black colored caterpillars of the Large aspen tortrix were found feeding on aspen leaves in Sections 25 & 35- T12N - R14W. These locations north of Duluth have continuing infestations that were found last year by aerial surveys.

Basswood thrips have caused very light damage to basswood on a site north of Duluth. The damaged leaves were small and cupped. This type of damage has become fairly common on basswood throughout northeastern MN. Heavy damage can kill buds and cause the tree to have very sparse foliage. After several years of heavy defoliation, branch and tree mortality has occurred in a number of locations in Carlton County.

Forest tent caterpillars are back, in a building year or two before the Big One. In northern Stearns, southern Cass, Crow Wing and Kanabec Counties, as well as, along the eastern shore of Mille Lacs Lake, FTC defoliation is quite noticeable on basswood trees. Caterpillars are ½ to 1 inch long. Where all the leaves are already consumed, they're spinning down out of the trees in search of more food.

Rosy maple moths

The rosy maple moth, Dryocampa rubicunda, will probably be seen in central Minnesota during mid to late May clustered on screens and buildings where light has attracted them. This moth has bright yellow bodies and light yellow forewings with pink fore patches and rear pink stripes, but it does not damage trees.  After mating and egg laying these moths die. Their eggs are stuck to the underside of maple leaves. The caterpillar stages are known as greenstriped mapleworms. These "worms" are greenstriped, have two recurved black horns just behind their heads and rows of black spines along their bodies. As they grow to about 1 and 1/4 inches by mid to late July, they can heavily defoliate maples, and even lightly defoliate oaks and other trees growing among their preferred maple hosts.

During 1997 this insect was first observed along the southeast side of North Long Lake near Merrifield, in Crow Wing County and, during 1998, it had spread to maples in Cass, Crow Wing, Kanabec and Isanti Counties. Few natural enemies (fungi, bacteria, insects, birds, and mammals) were observed reducing numbers of the caterpillars in the last two years, so this insect will probably expand its range and numbers this year. Since most maples and oak buds for next year are formed by mid to late July, even heavy defoliation will probably not cause much branch dieback.