Hermit flower beetle (Osmoderma)
A number of people have discovered large grubs associated with old, rotting wood recently. This insect is a type of scarab beetle known as Osmoderma. When examining this insect, you can see an obvious reddish brown head along with six legs. Its has a dirty whitish colored body that is permanently curled into the shape of a C. It reaches up to two inches in length. There are three species of Osmoderma present in North America of which two are found in Minnesota. Of these two, by far the most common is O. eremicola, commonly called the hermit flower beetle.
Osmoderma spp. are associated with rotten wood from dead or dying trees, especially oak, maple, elm, apple, cottonwood, cherry, and hickory, and are particularly found in tree cavities filled with debris. Adults, dark brown beetles measuring one inch in length, are typically present from July to September, while the grubs are found from August through the following spring.
This timeframe mirrors what Minnesotans have noticed. One resident described cutting up some oak trees that were blown down during August in Ramsey County (St. Paul). One of the oaks was rotten in the center. Upon closer examination he found several dozen Osmoderma larvae, most between 1 1/2 and 2 inches in length.
In another case, a gentleman in Wabasha County (southeastern Minnesota) found these larvae during October in wood (oak or maple) that was being cut for firewood. Also in October, a resident in Rock County (southwestern Minnesota) found 2 inch long Osmderma grubs in a maple tree he was cutting down. The larvae were found in an site where the wood had rotted.
Osmoderma larvae are not pests, they do not cause trees to become rotted and die. Rather, some other circumstance has caused the tree to decline and die and this insect takes advantage of it. They are actually beneficial by helping to decompose wood. If you find Osmoderma, they are just a curiosity and should just be ignored.
Published in Yard and Garden News, November 1, 2008