Flower hermit beetle grubs

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The hermit flower beetle, Osmoderma eremicola (Coleoptera: Scarabaeidae), is one of the largest beetles found in Michigan. The larvae or grubs are very impressive. The grubs are giant white grubs like the ones that feed on our lawn. But hermit flower beetle grubs live in decaying trees. They are often discovered when old rotted trees or limbs are removed or cut up. This usually leads the owner of the tree to believe the grubs are responsible for the demise of the tree or portion of the tree. Not true. These large scarab beetle grubs are secondary invaders, that is, they are in the wood because of the presence of wet and rotting wood in the tree. Adult beetles emerge in mid-summer and females soon begin to lay eggs in hollow trunks, tree holes or other wet and decayed portions of the tree. There the larvae feed for three years on the wet, decaying wood. Both standing and felled trees, and occasionally stumps, serve as larval food. Just before the larva is ready to pupate (change into the adult), it constructs an oval pupal cell by cementing together small wood particles around itself. When completed, it then transforms into the adult.

Adult beetles are large, black beetles that resemble June beetles. They are nocturnal and spend the day hiding in nooks and crannies in trees. Larvae have been found in several species of hardwood trees including beech, hickory, boxelder, elm, ash, hackberry, apple, cherry, oak, maple and cottonwood. There is no need to attempt to control the larvae with insecticides when they are discovered in a tree. They are harmless and probably make great fish bait.

To view an image of an adult flower hermit beetle, click here.

osmodera grubs
An impressive sample of Osmodera grubs
that were sent to the lab this past March.
The Petri dish is approximately 3 1/2
inches in diameter.

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