Wasps, sometimes called hornets if they build their nests above ground and yellowjackets if they live below ground, are thinner than bees and fold their wings along their bodies. They eat pollen, nectar, sap, insects and sometimes meat. Their stings are painful and can be dangerous to people who are allergic to their venom. Wisconsin has several kinds of wasps that gardeners may encounter, and its solitary wasps are less apt to sting than its social wasps, which build nests.

Paper Wasp
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Paper wasps sting aggressively.

Solitary Wasps

Solitary wasps in Wisconsin include the great black wasp (Sphecidae family) and the cicada killer wasp (Sphecius speciosus).

The great black wasp is almost 1 1/2 inches long with spiny legs and a stinger at its abdomen's end. This solitary wasp may dig tunnels in lawn. It paralyzes insects and buries them with its eggs so the larvae that hatch from the eggs will have food. The great black wasp may sting, but it is not aggressive and should not bother you if you leave it alone.

The cicada killer wasp is so named because it kills cicadas. It is yellow and black and measures up to 1 1/2 inch long. Adult cicada killers appear from mid- to late summer, digging burrows up to 10 inches deep and 1/2 inch wide in the ground. The males don't have stingers, and the non-aggressive females usually sting only when handled.

Social Wasps

The Common wasps ( Vespula vulgaris ) .
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Social wasps tend their hive.

The state's social wasps include paper wasps (Polistes spp.), the bald-faced hornet (Dolichovespula maculata) and the German yellowjacket (Vespula germanica).

Paper wasps are 3/4 to 1 inch long and slender with narrow waists, have smoky black wings and their bodies may be brown with yellow on their heads and yellow bands on their abdomens. They chew on paper, bark or wood to obtain cellulose that they mix with saliva to yield a kind of paper to make hives. The female workers have stingers. A paper wasp does not die after it stings. It can sting repeatedly and will do so to protect its nest. It releases a pheromone that causes other wasps to join the attack.

Black with ivory-white markings on its face, a bald-faced hornet is 1/2 to 3/4 inch long. It builds gray, football-shaped papery nests in trees and bushes and on building exteriors. A single nest, which may be up to 24 inches high and 18 inches wide, may have 100 to 400 worker hornets. Bald-faced hornets attack in a group if their nest is disturbed. They are beneficial insects because they kill other wasps and help pollinate flowers.

The stings of the aggressive German yellowjacket can be painful and dangerous to individuals allergic or sensitive to the species' venom. The sterile female German yellowjacket workers are black and yellow, and about 1/2 inch long. Each one typically has a black, spade-shaped mark on the first section of its abdomen. The female workers aggressively defend their nests, which are large and may contain up to 15,000 yellowjackets by late summer. Pennsylvania State University entomologists suggest hiring a professional exterminator to eliminate German yellowjackets that nested in a building wall instead of attempting to do that job yourself, and a University of Wisconsin-Milwaukee article states that amateurs should not remove a large German yellowjacket nest that is above or below ground.

Prevention and Precaution

Several tactics can help prevent problems with wasps. For example, keep garbage cans clean and covered. If wasp nests are in your lawn, then wear long-legged trousers, socks and closed-toe shoes when you mow grass. Jerky, sharp movements and trying to hit a wasp can provoke it to sting. A smashed wasp gives off a scent that encourages other wasps to attack. If a wasp lands on your drink, wait patiently for it to leave.

A wasp sting can cause its victim to feel flushed and dizzy and to have difficulty breathing. Keep a wasp sting below the level of the victim's heart. Wash the sting, and put ice on it. A swelling sting on the mouth or throat can suffocate the victim. Give the victim an ice cube to suck, and call a physician or 800-222-1222, which is the 24/7 emergency hotline of the American Association of Poison Control Centers. The hotline operator will transfer your call to the poison control center nearest you.

Traps and Removal

Many garden supply centers sell cone-shaped wasp traps containing a bait or yellow sticky traps that can be attached to a fence near wasp nests. Plans are available for homemade cone-shaped traps that are baited with grenadine, fruit punch or jam. Move traps only in the cool hours of early morning or late evening. Kill the wasps in a cone trap by putting it in a plastic garbage bag, sealing the bag and putting it in a freezer overnight or in direct sunlight for several hours.

When removing a wasp nest, wear protective clothing designed for beekeepers, including a bee hood or veil under a pith helmet to keep the veil away from your face. Wear gloves with extra-long arm coverings, and tape them to your sleeves. Remove the nest at night, using a flashlight covered with red acetate film.

Removal with Pesticide

When using a pesticide on an outdoor wasp nest, wear protective clothing that covers your skin and hair, goggles and a dust mask, and blast the nest's entrance hole for one-half second with an aerosol pesticide that is 1/3 to 1/2 percent of the active ingredient pyrethrin. It will kill the guard wasps. Cover the nest with a heavy-duty, black, plastic garbage bag, remove the covered nest from its location and seal it in the bag with a tie-twist. Leave the bag in hot sunlight for two to three hours to kill the nest's wasps.

A ground wasp nest has a main entrance plus secondary entrances. Apply a one-half-second blast of an aerosol pesticide containing pyrethrin at the secondary entrances to kill the guard wasps. Give the main entrance a one-half-second blast to kill the guard wasps. Then stuff all the entrance holes with paper towels or newspaper, and pile soil over the nest.

Aerosol pyrethrins are typically sold with a four-way tip or nozzle. Insert this tip into the nest's entrance hole, spray the product into the hole for five to 10 seconds and then block the hole with paper towels or newspaper. Remove that block material after a few minutes, insert into the hole a bulb duster containing silica aerogel mixed with pyrethrin and give the bulb duster a few pumps. If necessary, use a length of pipe to extend the reach of the bulb duster. Many garden supply centers sell silica aerogel that is 1/3 to 1/2 percent pyrethrin. The silica absorbs the waxy or oily surface of wasps, drying them out.

Finally, dust steel wool or copper mesh with silica aerogel containing pyrethrin, and push the steel wool or copper mesh into the hole. Any remaining wasps trying to escape will chew on the wool or mesh, poisoning themselves.