Answers from the Wild Side

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Pelidnota punctata, also called a grapevine beetle. — Photo by Matt Pelikan

The great outdoors can produce baffling mysteries. MVTimes Wild Side columnist Matt Pelikan tries his best to solve them. Got a question for the Wild Side? Send it to onisland@mvtimes.com.

Hi Matt: lived here all my life and don’t recall ever seeing one of these before. What is it? Thanks, Angie Waldron

Hi Angie.

This impressive beetle is Pelidnota punctata, sometimes known as the grape or grapevine beetle. It seems to be reasonably common and quite widely distributed around Martha’s Vineyard. But like a the vast majority of our beetles, its habits and life history mean that it isn’t often seen.

Adults, which can exceed an inch in length, are unmistakable with their yellowish color and array of six black spots. They eat grape leaves (as the beetle’s common name suggests) but are largely nocturnal; they roost for most of the day on the underside of leaves, where you’re unlikely to spot them unless you’re specifically looking. This beetle, though, is active at night, feeding and searching for a mate, and it is attracted to artificial lights. So most of the time when humans encounter it, Pelidnota is hanging from a shingle or window-screen near someone’s porch light. Despite their daunting size, these beetles are harmless, rather sluggish, and quite easy to handle.

The grub-like larvae of this beetle are even harder to find than the adults. They hatch from eggs laid on rotten logs and stumps and spend most of their youth feeding either on the decaying wood or on roots and detritus underground. As is not unusual for very large beetles, it takes a long time for a Pelidnota larva to mature: two full years may elapse between when an egg is laid and when the adult beetle finally emerges.

The grapevine beetle is a scarab beetle, belonging to the same family as the scarab that was sacred to the ancient Egyptians as well as more familiar species such as the Japanese beetle. There are many thousands of scarab species worldwide, but the members of this very large group can be recognized by their characteristic stocky shape, often glossy or colorful exterior, and short, bent antennae, tipped with a little group of finger-like tabs. These tabs are actually organs that are exquisitely sensitive to chemical signatures — in other words, scarab beetles smell with their antennae, and they do it very well. Scent helps these insects find others of their species and locate suitable food plants or sites to lay their eggs.

Pelidnota punctata has a broad geographical range, occurring across the eastern and central United States and in most of southeastern Canada. Across this vast area, this species shows a good deal of variation in features such as leg coloration and the size and placement of the six black spots. In fact, at one point entomologists have treated this beetle as multiple species — as many as 10 at one point! But current thinking is that only a single species is involved.

In theory the grapevine beetle might be undesirable because it feeds on grape leaves, and under the right circumstances this insect can get common enough to cause commercially significant damage in vineyards. But it’s rare for Pelidnota to occur in numbers large enough to amount to any harm, and this beetle plays useful roles breaking down old wood as a larva and serving as a prey item for birds as an adult (one of these beetles makes a significant snack for a bird like a blue jay or catbird). So on balance the grapevine beetle is a positive addition to our world, and given its striking looks, imposing size, and docile manner, it’s a beetle you should be happy to find visiting your porch light.