Larch casebearer, an exotic (introduced) needleminer of tamarack, continues to cause defoliation in the north central part of the state, noticeably in southern Aitkin County. Casebearer adults are moths that fly from late May to August and lay eggs on needles. Larvae hatch from the eggs and bore into needles and mine during the summer. The larvae use a hollowed out needle as a portable shelter or case. They overwinter in the case fastened to a twig at the base of a bud. In the spring they resume needle mining before pupating and changing to a moth to complete the life cycle. Each larva needs to feed on 24 to 76 needles to complete its development. The most severe damage is done, by the larvae, in the spring of the year. Damaged trees and stands look off color, tan or brown, very similar to flooding damage. Needles have to be examined carefully to see the entrance hole in the mined out needle or to find the cases containing the larva.
The current outbreak of larch casebearer in Minnesota started in about 2000. A number of parasites imported from Europe, as well as a few native insects, reduce outbreak severity, but larch casebearer can still have serious consequences.