BUGS – The Margined Blister Beetle

By Jim Revell, Bedford Master Gardener, September 2010

This month’s article will be a bit late for many of you who have experienced an additional obstacle in growing tomatoes this year. Our featured “bug” is the Margined Blister Beetle.

I found the Epicauta pestifera is actually a very attractive bug, dressed in its “best” Sunday attire, i.e., black suit trimmed in smoky grey-to-cream margins around the edge of each wing cover.

One of our Master Gardeners, Lee Walker, was the first to alert me to this beetle when he found it on his tomatoes. Over the next couple of days after Lee encountered the uninvited visitor, there were several other inquiries about the beetle, and our Help Desk received at least one request for identification of the insect.

The Margined Blister Beetle can grow from 1⁄2 inch to 1 1⁄4 inches long. Its head is wider than its thorax and, as its name implies, it can actually cause a blister on your skin if mishandled (meaning, indi- viduals usually handle it before they know what they are dealing with!). The toxin produced is an oil known as cantharidin, and it is this oil that causes the blister when it comes in contact with the skin. These blisters may become infected if left untreated, and the toxin is highly stable even after the beetle is killed.

Due to the wide range of its appetite – including many agricultural crops such as alfalfa, clover, soybean, potato and tomato – in addition to some ornamental plants – it can have a major economic impact on both small gardeners and large-scale farmers.

For livestock, it can be damaging as well, as the toxin can cause inflamma- tion in the gastrointestinal system. As mentioned earlier, the toxin may persist even if the beetles are dead. It is known that horses are very sensitive to this poison.

The Margined Blister Beetle lays its eggs in the soil and will lay up to six egg masses of 50-300 eggs. The larvae are pale and grub-like. In their first instar stage, they are mobile, but during future instar stages, they are sedentary. One very beneficial action is the larvae feed on grasshopper eggs. It is interesting to note here that I have seen more grasshoppers in my gardens this year than normal. There has been observation recorded that heavy infestations of blister beetles often occur during or just after a grasshopper outbreak.

The beetle larvae will go through a series of seven molts before overwintering in the soil as a pseudo-pupal stage. Fortunately, there is one generation per year, BUT they emerge as an adult in late May-June and are active until about September. Only the adults feed on the plants. They can form swarms and, although they feed primarily on flowers and blossoms of plants, they will also feed on foliage - - and, as some of our folks growing tomatoes can attest to, they can do considerable damage.

The Margined Blister Beetle has a geographic range extending through most of the eastern U.S., west to Texas and South Dakota, north to Massachusetts, and south to northern Florida.

Controls include:

For small infestations – pick off or knock off beetles into a jar of kerosene. Be sure to wear gloves. Rutgers Cooperative Research & Extension.

Cover valued plants before June. Use a fine mesh net- ting to prohibit beetles.

Spray or dust with Carbaryl (Sevin). Be sure to read all labels carefully.

Spray Spinosad. Be sure to read all labels carefully.