Yellow jackets are by far the most troublesome group, especially ground- and cavity-nesting ones such as the western yellow jacket, which tend to defend their nests vigorously when disturbed. Defensive behavior increases as the season progresses and colony populations become larger while food becomes scarcer. In fall, foraging yellow jackets are primarily scavengers, and they start to show up at picnics and barbecues, around garbage cans, at dishes of dog or cat food placed outside, and where ripe or overripe fruit are accessible. At certain times and places, the number of scavenger wasps can be quite large.
Paper wasps are much less defensive and rarely sting humans. They tend to shy away from human activity except when their nests are located near doors, windows, or other high-traffic areas.
In spring and early summer, yellow jackets are carnivores, feeding mostly on insects to provide protein to developing larvae in their colony. In doing so, they help keep garden pests, such as caterpillars, in check. As the season progresses, their population grows and their diet changes to include more sugars. As natural food sources become scarce, they turn to scavenging, and that’s when you’ll find them lurking around garbage cans and pestering picnickers. A few yellow jackets here and there are a nuisance, but a nest of them in your yard can pose a real hazard.
Yellow jackets often nest underground in rodent burrows, so if you see lots of flying insects emerging from a hole in the ground, they’re probably yellow jackets. By late summer, a colony may contain thousands of individuals that will aggressively defend their nests from intruders. They’re easily provoked and will attack in force, chasing the perceived threat for large distances. What’s worse, each yellow jacket can sting multiple times. Sounds and vibrations, such as those from a mower or trimmer, can trigger an attack, even from a distance.