Big spotted beetle on grape leaves

Bob Dluzen
Special to The Detroit News

One morning this week while tying some unruly grapevines to their trellis wire, I came across an interesting insect that was hard to miss.

It was a grapevine beetle just resting there on a grape leaf. I thought it must have just emerged from the soil and flew over to the grapevines and hadn’t started to feed yet since there were no damaged leaves nearby.

A few vines over I spotted another one that was actively feeding. The beetle was sitting on a freshly chewed leaf that was eaten all the way around  leaving a leaf piece just big enough for it to perch.

The second beetle was a different shade of color, more orange,  than the first one I saw but was otherwise identical.

Grapevine beetles range in color from tan to orange and have distinctive spots.

Grapevine beetles are remarkable because of their size. At an inch long, they are quite big for a native beetle. This was the first time I found grapevine beetles in this particular vineyard.

Grapevine beetles need rotting wood nearby in order to complete their life cycle. The adult females lay their eggs in rotting wood that is in contact with the soil. They particularly like hardwood tree stumps because the roots reach  deeper into the soil.

Rotting wood, along with the fungus responsible for the breakdown of wood cellulose, provides all of the nutrition a growing beetle grub needs. Later, they pupate and after a period of time, eventually emerge as full sized adult beetles.

In this case the nearest rotting tree stumps are about fifty yards away. A female beetle sometime in the past sensed  that it was close enough for her progeny and went ahead and laid her eggs. Turns out she was right since they were able to hatch into larvae, grow and fly over to these grapevines.

The big beetles are harmless to people.

The beetles look scary if you’ve never encountered one before, but after a while you realize they are just a big, harmless bug. They are not poisonous and cannot bite humans. The worst thing that can happen is you will frighten them enough that they will leave a green stain on your hand.

Grapevine beetles behave like many other related beetles and according to some sources, will sometimes be found around exterior lights at night. They are very agile flyers for such a large, hard-shelled insect.

Even though they are large, they don’t cause any real damage to grapevines other than a leaf eaten here and there. You would do much more damage by picking some grape leaves for a batch of dolmades. I suppose if you had an extreme outbreak of them you would have to control them. Plus, the grub stage contributes its part to the environment by helping to recycle dead wood releasing nutrients to the soil from their digestion.

So there’s no need to panic and drag out the insecticide sprayer. Consider them an interesting part of the grapevine ecosystem.