Yellow jackets (Vespula and Dolichovespula spp.) are among the most aggressive stinging insects in Montana. Often confused with bees, yellow jackets are actually wasps — slender, smooth-bodied, and with a distinctive bright yellow and black banding pattern. Unlike honeybees, they can sting repeatedly and will do so vigorously when they perceive a threat to their nest.
In Montana, yellow jacket colonies begin in spring when an overwintering queen starts a new nest. By late summer, a single nest can house 1,000–4,000 workers, and the colony reaches peak aggression in August and September as food sources decline. At this time, yellow jackets become opportunistic scavengers, showing up at picnics, trash cans, outdoor restaurants, and anywhere sugary drinks or food is present.
Nests are built in a wide variety of locations: underground burrows (the most dangerous and hardest to detect), wall voids, attics, decks, eaves, and hollow trees. Underground nests are especially hazardous because they can be disturbed accidentally by lawn mowers, pets, or children playing in the yard.
Size
½ – ¾ inch
Color
Bright yellow and black banding
Active Season
April–October; peak aggression August–September
Risk Level
High (aggressive stingers; anaphylaxis risk)
Habitat
Underground, wall voids, eaves, decks
Colony Size
Up to 4,000 workers
SIGNS OF INFESTATION
How to Tell If You Have Yellow Jackets
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High wasp traffic in one area — Consistent flight paths leading to a single point (a wall gap, a ground hole, or under a deck) indicate a nest.
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Visible nest paper — Open-faced gray, papery nest material visible under eaves or in shrubs (though most nests are hidden).
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Ground holes with activity — Small holes in the lawn with wasps flying in and out are a telltale sign of an underground nest.
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Wasps inside your home — Yellow jackets entering through wall voids can emerge inside your home, often near windows or light fixtures.
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Increased aggression near outdoor dining — If wasps are consistently hovering around food or drinks, a nearby nest is likely.
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Chewing sounds in walls — Yellow jackets will chew through drywall to expand indoor nests — this is an emergency situation.
