Issue 2, May 5, 2021

Boxwood Leafminer

Boxwood leafminer (Monarthropalpus flavus) is a tiny midge that can harm boxwoods in its larval stage. The adults emerge in the spring around the time weigela is in bloom. Females lay eggs under the surface of a leaf and larvae feed on the tissue within the leaf. Larval feeding causes a raised green blister that can be seen on the leaf’s surface. The blister may become discolored or flake off later in the season. The leafminers overwinter within the leaves as larvae. They have one generation per year in Illinois.

Left: Boxwood leafminer (Monarthropalpus flavus) leaf discoloration, Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.org.
Right: Boxwood leafminer (Monarthropalpus flavus) larvae in open leaf cells, Jim Baker, North Carolina State University, Bugwood.org.

Boxwood leafminers can cause leaf discoloration, blistering and early leaf drop. In heavy infestations, twigs may begin to dieback. Most varieties of boxwood are susceptible to boxwood leafminers.

Contact insecticide can be applied when adults are actively laying eggs. Adult emergence and egg laying coincides with weigela blooming (GDD base 50: 400-600). Contact insecticides targeting adults include, carbaryl (Sevin), abamectin (Avid), acephate (Orthene) and pyrethroids labeled for use on boxwood leafminer. Imidicloprid (Merit or others) can be applied to target feeding larvae but must be applied after blooming is complete.

Author:
Sarah Hughson

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