I noticed some small holes between the bricks in my driveway. There are little piles of sand near the holes, and I was wondering what insect might be making them. There have been some unusually large wasps flying around this area, but I thought they made paper nests.
— Rick Jefferson, Niles
It sounds from your description like cicada killer wasps are nesting in your driveway. Too many people initially respond to seeing wasps or hornets in their garden by calling a pest control company. These wasps are actually beneficial insects, as they help control cicadas and katydids. I was happy to see them return to my brick driveway last week when I saw the first characteristic hole appear. Since then, I have routinely seen them buzzing around the driveway, paying me no attention as I move about the area. Cicada killers are large — about 2 inches long — and black to red, with yellow banded markings on the abdomen. The head and transparent wings are reddish brown. Their large size makes them appear very intimidating in the garden. A male cicada killer wasp may fly up to you to investigate as you enter his territory, but he is unable to sting. The females can sting, but lack the instinct to guard their nest like a honeybee. You may get stung if you accidentally step on one with bare feet or grab one with your bare hands. This insect is no cause for concern, thus, there is nothing to worry about for your garden. I had the good fortune a couple of years ago to see one bring in an insect to place in the burrow for egg-laying.
Cicada killers are solitary wasps. This wasp hunts cicadas to feed its young, hence the common name. The female digs a 6- to 10-inch-deep burrow in the ground that is ½-inch wide. You are seeing the entrance holes and the typical pile of soil that surrounds it in your driveway. The female locates and stings a large insect such as a cicada or katydid and then brings it back to the burrow. She places the insect into a chamber and lays an egg on it. Sometimes, she puts two in a burrow but lays an egg on only one. She then covers the burrow, digs another and repeats the process. The egg hatches into a legless larva that consumes the paralyzed insect. Full-grown larvae overwinter in the burrow, pupate in the spring and emerge as an adult during the summer, usually in July and August. They return to the same area in my driveway each year.
You will often find cicada killers in areas with bare soil or sand like in a sandbox or underneath playground equipment. If this insect truly is a nuisance for you, use mulch, plant ground covers or install sod to cover bare soil areas. Cover sandboxes with a tarp when not in use and rake sand under playground equipment to help deter wasps. Consider using mulch instead of sand in play areas. Irrigation can also be used to deter nesting. The best course of action is always to just leave them alone.
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For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at [email protected]. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.