Only a few of the very large number of wasp species in California live a social life; these species are referred to as social wasps. Social wasps are predators for most or all of the year and provide a great benefit by killing large numbers of plant-feeding insects and nuisance flies. They become a problem only when they threaten to sting humans. One of the most troublesome of the social wasps is the yellowjacket. Yellowjackets, especially ground- and cavity-nesting ones such as the western yellowjacket, tend to defend their nests vigorously when disturbed. Defensive behavior increases as the season progresses and colony populations become larger while insect prey becomes scarce. The behavior of foraging yellowjackets switches from that of predator to that of scavenger and these wasps start to show up at picnics, barbecues, around garbage cans, at dishes of dog or cat food placed outside, and where ripe or overripe fruit is accessible. At certain times and places, the number of scavenger wasps can be quite large.

The term yellowjacket refers to several different species of wasps in the family Vespidae. Included in this group of ground-nesting species are the western yellowjacket, which is the most commonly encountered species and is sometimes called the "meat bee".

Nests are normally built in rodent holes, but other protected cavities, like voids in walls and ceilings of houses, sometimes are selected as nesting sites. Colonies, which are begun each spring by a single reproductive female, can reach populations of between 15 and 15,000 individuals, depending on species. The wasps build a nest of paper made from fibers scraped from wood. It is built of multiple tiers of vertical cells, similar to nests of paper wasps, but with a paper envelope around the outside that usually contains a single entrance hole. If the rodent hole is not spacious enough, yellowjackets will increase the size by moistening the soil and digging. Similar behavior inside a house sometimes leads to a wet patch that develops into a hole in a wall or ceiling.

Immature yellowjackets are white, grublike larvae or white pupae that develop adult coloring at the time that they emerge as adults. Immatures are not normally seen unless the nest is torn open or a sudden loss of adult caretakers leads to an exodus of starving larvae.

Aerial nesting yellowjackets, build paper nests that are in the open, protected from above by an eave of a building or a limb of a tree. The entrance is normally a hole at the bottom of the nest. These aerial nesters do not become scavengers at the end of the season. They do become extremely defensive when their nests are disturbed. Defending wasps sometimes bite and/or sting, simultaneously. Wasp stingers have no barbs and can be used repeatedly, especially when the wasp gets inside clothing. As with any stinging incident, it is best to leave the area of the nest site as quickly as possible if wasps start stinging.

Paper wasps are long (1 inch), slender wasps with long legs and a distinct, slender waist. Background colors vary, but most western species tend to be golden brown, or darker, with large patches of yellow or red. Preferring to live in or near orchards or vineyards, they hang their paper nests under eaves, in attics, or under tree branches or vines. Each nest like an open umbrella from a pedicel (stalk) and has open cells that can be seen from beneath the nest. White, legless, grublike larvae sometimes can be seen from below. Paper wasp nests rarely exceed the size of an outstretched hand and populations vary between 15 to 200 individuals. Most species are relatively unaggressive, but they can be a problem when they nest over doorways or in other areas of human activity, such as fruit trees.

Mud Daubers. Mud daubers are black and yellow thread-waisted wasps that build a hard mud nest, usually on ceilings and walls, attended by a single female wasp. They belong to the family Sphecidae and are not considered social wasps, but may be confused with them. They do not defend their nests and rarely sting. During winter, you can safely remove the nests without spraying.

INJURY OR DAMAGE
Concern about yellowjackets is based on their persistent, pugnacious behavior around food sources and their fervent colony defense. Usually stinging behavior is encountered at nesting sites, but scavenging yellowjackets sometimes will sting if someone tries to swat them away from a potential food source. When scavenging at picnics or other outdoor meals, wasps will crawl into soda cans and cause stings on the lips, or inside the mouth or throat.

Responses to wasp stings vary from only short-term, intense sensations to substantial swelling and tenderness, some itching, or life threatening allergic responses.

Social wasps provide an extremely beneficial service by eliminating large numbers of other harmful insects through predation and should be protected and encouraged to nest in areas of little human or animal activity. Although many animals prey on social wasps (including birds, reptiles, amphibians, skunks, bears, raccoons, spiders, preying mantids, and bald-faced hornets), none provide satisfactory biological control in home situations.

The best way to prevent unpleasant encounters with social wasps is to avoid them. If you know where they are, try not to go near their nesting places. Wasps can become very defensive when their nest is disturbed. Be on the lookout for nests when outdoors. Insects that are demonstrating directed flight (a beeline in and out of a single location) usually are flying to and from their nest.

Scavenging wasps will not become a problem if there is no food around to attract them. When nuisance wasps are present in the outdoor environment, keep foods and drinks covered or inside and keep garbage in tightly sealed garbage cans. Once food is discovered by wasps, they will continue to hunt around that location long after the source has been removed.

Wasp baiting often provides acceptable control of scavenging yellowjackets in the latter part of the season. Use fish-based canned cat food, cooked hamburger or salmon, or processed meats to formulate a poison bait for yellowjackets. The insecticide labeled for this use is microencapsulated diazinon, manufactured as Knox-Out 2FM. This material is not routinely found on store shelves, but you may be able to get a store to special order it or get it from a pest control company. Just a half teaspoon of insecticide is blended thoroughly into 6 teaspoons of the meat product to attract wasps. Place small portions of the mixture outdoors in specially designed bait dispensers and replace with fresh bait every other day (or every day if temperatures are over 90°F) as long as the bait is being consumed. In the microencapsulated formulation, diazinon is not repellent to wasps. The bait is taken back to the nest and shared with other adults and larvae. In a week or so, colonies within flight range of the bait stations should no longer pose a problem. Diazinon is toxic to animals other than insects, so follow the label exactly to prevent damaging or killing mammals. Only put the bait out in specially designed dispensers so that animals other than wasps are not able to get at it.

Aerosol formulations of insecticides on the market labeled for use on wasps and hornets can be effective against both yellowjackets and paper wasps, but they must be used with extreme caution. Wasps will attack applicators when sensing a poison applied to their nests, and even the freeze-type products are not guaranteed to stop all wasps that come flying out. It is prudent to wear protective clothing that covers the whole body, including a veil over the face and gloves. In addition, you need to wear protective eyewear and other clothing to protect yourself from pesticide hazards. Wasps are most likely to be in the nest at night. But even after dark and using formulations that shoot an insecticide stream up to 20 feet, stinging incidents are likely. Underground nests can be quite a distance from the visible entrance and the spray may not get back far enough to hit the wasps. Partially intoxicated, agitated wasps are likely to be encountered at some distance from the nest entrance, even on the day following an insecticidal treatment.

Wasp trapping is only effective towards the end of the season when prey are scarce and wasps convert to scavenging. At that time, meat- and fish-based attractants (cat food) can attract hundreds or thousands of yellowjackets into water traps to meet their demise by drowning or into cages from which they can't escape. Traps and attractants can be purchased at hardware stores or nurseries. However, despite the large number of wasps caught, trapping usually does not reduce populations of scavenging wasps to acceptable levels.

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