Yards throughout southern Michigan are becoming littered with black walnut leaves

Fallen black walnut leaves are the work of the butternut curculio.

My yard is littered with dozens of fallen black walnut leaves. I didn’t give them much thought until I got a call from a forester in southwest Michigan who asked me if I knew what was going on with all the fallen walnut leaves he was seeing in his area. I told him I’d check out some of the ones that had fallen in my yard and let him know what I found.

I brought three or four leaves in with me this morning and dissected the petioles. They had clearly been chewed off and one was completely hollowed out and filled with frass. I found the culprit inside the hollow stem: a small, legless weevil larva. A quick consult with my trusty Johnson and Lyons (Insects that Feed on Trees and Shrubs), pointed to the butternut curculio, Conotrachelus juglandis (Coleoptera: Curculionidae). I have received several more calls about fallen walnut leaves from other areas in southern Michigan, so this appears to be an area-wide event.

Besides butternut, the butternut curculio feeds on Siebold, English and black walnut. It occurs throughout the eastern United States from Texas and Wisconsin eastward to the Atlantic Coast. Both adults and larvae feed on or in nuts, young shoots and leaf petioles. The most serious damage is caused by larvae boring in new shoots, stems and branches. Winter is passed in the adult stage. The weevils become active in the spring as walnut trees begin to produce new shoots where the first eggs are laid. Later, the weevil chews crescent shaped holes in the newly formed nuts and lays eggs in these excavations. The larva eats the developing nut and completes its development in approximately five to six weeks. The larva then exits the nut and drops to the ground where it pupates. The new adult emerges from the soil in late summer to feed on terminals, shoots and leaf petioles until cold weather causes it to seek shelter in leaf litter.

It is probably not practical or necessary to control this weevil in landscape trees.

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