ISSUE 15    August 17, 2006


The hot dry summer has been good for many insects including hornets and yellowjackets. The heat has been pushing insect development resulting in larger numbers of hornets and yellowjackets this year. Hornets and yellowjackets belong to the family Vespidae which includes hornets, yellowjackets, and paper wasps. Hornets are wasps that build nests, but are considered yellowjackets by entomologists. All yellowjackets sting and their stinging behavior is considered a defensive reaction when the colony is threatened. They can sting more than once because their stinger stays with the wasp. Yellowjackets are more aggressive during August into September and more likely to sting people. Although yellowjackets are actually a beneficial insect feeding on other insects, they often become a pest problem when nests are located near homes, schools, picnic areas, or playgrounds. Pest control is often warranted.

Biology: These wasp are social insects and build nests of paper-like material. Nests generally resemble a teardrop-shaped soccer ball and often seen hanging in trees. Sometimes nests are located underground in mammal burrows, cavities or inbetween house siding. In the northern temperate climates, only the mated queen wasp overwinters from the previous year’s colony. Queens are inactive during the winter, hiding in protected places like under tree barks, attics, ... In early spring, the overwintering queen builds a new nest and lays an egg in each cell. Larvae hatch from the eggs and are dependent on the queen for food. The queen forages outside the nest and brings food (caterpillars and other insects) back to the larvae until pupation. Sterile female workers emerge from pupae and take over nest building and brood rearing, while the queen stays in the nest. During late summer into early fall, adult males and newly produced queens leave their parent colony. The colony dies off, and only newly mated queens will find a protected place to overwinter.

Control: Vespid wasps are active outside the nest during the daylight hours. Nearly the entire colony is in the nest during the evening and night-time hours, so control measures should be applied to the nest then. There are many insecticide labeled for control of hornets and yellowjackets. The difficulty is making the treatment without being stung. Usually an aerosol spray of one of the many fast-acting wasp killer aerosol will quickly kill all workers present in nest. Examples are permethrin, synergized pyrethrins or pyrethroid insecticides. A slower-acting insecticidal approach is to apply a dust of carbaryl (Sevin) directly onto the exposed nest and entrance hole. After treatment, check the nest for any activity the following day and re-treat if necessary. Nests should be removed to avoid attracting dermestid beetle infestations at some later time and to keep wasp pupae from possibly reestablishing the nest. If dealing with yellowjacket nests in structures like homes, the nest entrance should never be plugged from the outside. If constrained yellowjacket workers cannot escape to the outside, they may locate a way to escape toward the inside of the home or structure, creating a possible stinging threat for people inside. Yellowjacket nests become an important source of carpet and other dermestid beetle infestations in the home, so the nest should be removed whenever possible. When outside enjoying your picnic, avoid wearing bright colors and perfumes which are attractive to hornets and yellowjackets.

Eastern yellowjacket
Eastern Yellowjacket (courtesy J. A. Payne, USDA
Agricultural Research Center,

Janet Knodel
Extension Entomologist

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