Yellowjacket Season in Iowa: When the Wasps Peak

A yellowjacket colony in Iowa is founded by a single overwintered queen around April 30, grows all summer, and reaches maximum size roughly August 8 to October 10. The first hard freeze, typically around October 10 here, kills every worker. Those weeks before the freeze are when almost every sting complaint arrives.

Two things change at once. The colony is at its largest of the year, so there are simply more wasps. And the queen stops laying, which means the larvae the workers were feeding protein to are gone, and the sugary secretion those larvae gave back in exchange is gone with them. Thousands of workers with nothing to do and no sugar source switch to scavenging, which is why they suddenly appear at your drink and your plate. They are not angrier. They are unemployed and hungry.

Look at the nest, not the wasp. Enclosed paper nest, almost always underground in an old rodent burrow, or hidden in a wall void or compost heap. The only sign is a steady stream of wasps in and out of one small hole. The grey, layered paper football hanging in a tree, shrub, or under an eave, with one entrance near the bottom. An open umbrella of bare hexagonal cells with no paper envelope, hung under an eave, a rail, a mailbox, or a grill lid. Yellowjackets are the ones that show up at food; the other two almost never do.

You went over a ground nest. Most yellowjacket colonies are underground in an old rodent burrow with a single entrance hole the size of a coin, and mower vibration is read as an attack on the nest. That is why mowing stings come in bursts of several rather than one. Walk the lawn first in late summer and watch for a steady traffic of wasps in and out of one spot in the grass.

Only if it is somewhere people cannot avoid: a doorway, a playground, a lawn that has to be mown, or the wall of a house. A nest in a far corner of the yard dies at the first hard freeze and is never reused, so leaving it alone costs you one season. If it must go, treat it after dark when all the foragers are home, from as far away as the product allows, with an exit route planned. A nest in a wall void or above head height is a job for a professional, and anyone with a known sting allergy should not do it at all.

Move away first, because a yellowjacket can sting repeatedly and its alarm pheromone recruits others. Then wash the site, apply a cold pack for ten to fifteen minutes, and take an antihistamine if it itches. Swelling around the sting itself, even a lot of it, is normal and not an allergy. Call emergency services immediately for trouble breathing, throat or tongue swelling, dizziness, widespread hives, or vomiting: those are anaphylaxis, and they are the reason wasps send more people to hospital each year than snakes do.

Anyone who has previously had a whole-body reaction to a sting, and anyone whose doctor or allergist has prescribed one. That is a medical decision and not one to make from a web page. If you have had a bad reaction, an allergist can test for venom allergy and venom immunotherapy is highly effective. Carrying two injectors is standard advice, because one dose is sometimes not enough.

Yes, and it is the argument against destroying every nest you find. A single colony removes an enormous quantity of caterpillars, flies, and other insects over a summer, which is free pest control for a garden. Adults also visit flowers for nectar and do a share of pollination. The difficult behaviour is confined to a few weeks at the end of the colony cycle, and it ends on its own.