Poplar Blackmine Beetle

Poplar Blackmine Beetle - Affecting cottonwood, poplar, and aspen trees.

                                       
                                   
                                                           Cottonwood leaves with large blackened areas.

The culprit, a leaf miner suspected to be Poplar blackmine beetle (Zeugophora scutellaris).  The shadow of the larvae is indicated by the white arrow above.  Hold an affected cottonwood leaf up to a light.  You may be able to see the leaf miner inside the leaf.

The culprit exposed.  The outer layer of dead leaf material is torn away exposing the larvae of the beetle.  The larvae feeds inside the leaf, well protected from insecticides, predator insects, and birds.
All photos taken by City of Cheyenne Urban Forestry Division

The adults emerge as beetles from May through July. The adults feed on the under surface of the leaves producing small skeletonizing injuries. The female beetle lays eggs in the skeletonized areas of the leaves. The eggs hatch and the larvae burrow inside the leaves and feed until late summer. The larvae then fall to the ground, burrow into the soil, and pupate several inches below the surface. The injury caused by this beetle is more unsightly than it is damaging to the tree.  Chemical control is usually not necessary.  If chemical control is desired, insecticides containing carbaryl or permethrin can be sprayed on the tree in early to mid-June to control the adult beetle prior to egg laying.  A systemic insecticide containing Imidacloprid (Merit is one brand name) can control the larvae inside the leaves.  There is one generation of this insect per year.

Links:

Colorado State University Cooperative Extension

University of Wyoming - Cooperative Extension Service, publication B-1035 is particularly helpful for tree care. This online publication contains information on: Aphids, Borers, Cottonwood blotch leaf miners, Cytospora canker, Fireblight, Gall makers, Aspen leaf spots, Oystershell scale, Pear slugs, Powdery mildew, and Spider mites.