Yellowjackets in Late Summer in Sacramento: Why They Turn Aggressive

Why are yellowjackets worse in late summer in Sacramento?

UC IPM explains that colonies grow all summer, so by late season they are at their largest while natural food is harder to find. Defensiveness rises, and workers turn to picnics, garbage cans, pet food and ripe fruit. In Sacramento, that lines up with the driest, hottest stretch of the year.

A yellowjacket colony begins in spring with a single queen. She builds a small nest, raises the first workers, and then the colony takes off. By late summer, the nest can hold a large number of workers, and the colony's demands for food rise with it. UC IPM ties the increased defensiveness to exactly this: the colony is as big as it will ever be, and food is scarcer.

Sacramento adds its own backdrop. June to September bring little or no rain, with July highs averaging about 92 F, so lawns, gardens and ripe fruit trees stay in the picture as irrigated food and water sources while the surrounding grass dries. The result is that yellowjackets show up where you eat outside, where the trash sits and where windfall fruit lies.

None of that makes every wasp you see a yellowjacket. Paper wasps and other species also appear in the same months. If you can see only one or two wasps and no sign of a nest, the situation may be different from a ground nest with a steady stream of traffic. A technician can identify the species and the nest on the first visit.

What does the yellowjacket season look like month by month?

Queens start nests in spring, colonies grow through June, July and August, defensiveness peaks in late summer and early fall, and the colony declines in late fall. The Sacramento-Yolo Mosquito and Vector Control District runs yellow-jacket control from July to September. Exact timing varies from year to year.

The calendar below combines the UC IPM life cycle with the district's service window. Hedge the edges: warm or cool years shift things, and the month boundaries are general guides.

Where do yellowjackets nest around a Sacramento house?

UC IPM says yellowjackets nest in cavities, mostly in the ground but also in attics, wall voids and hollow trees. Around a Sacramento home, that means rodent burrows in lawns and planters, gaps in siding and soffits, and old trees. Steady wasp traffic at a single small hole is the usual clue.

A ground nest is the case most people do not expect. The entrance may be a small hole in a lawn, a flower bed, a slope or under a shrub, and the nest chamber below it can be much larger than the opening suggests. The danger is that a lawn mower, a weed trimmer or a child's play can vibrate the ground above it.

In wall voids and attics, the entrance can be a gap at the soffit, a loose vent cover or a crack in stucco. You may hear a faint buzzing inside a wall, and wasps may appear indoors near a window or light fixture. In older neighborhoods with hollow trees and in areas near the American River Parkway, tree cavities are also a possibility.

The best way to find a nest is to watch from a distance in the late afternoon, when traffic is steady. Note where the wasps go in and out, then keep people and pets away from that spot and call for help.

• Lawns, beds and slopes: a small hole with wasps flying straight in and out
• Walls and soffits: wasps entering a crack or vent; buzzing behind drywall
• Attics: wasps near attic vents, or in the house near light fixtures
• Trees and stumps: wasps going in and out of a hollow
• Outbuildings and sheds: gaps near rooflines and under siding

What should I never do around a yellowjacket nest?

Do not block the entrance, flood it, hit it or spray it from close range. A blocked nest sends wasps looking for another way out, which may be into your house. Stay away, keep pets and children inside, and call for help. Anyone with a severe reaction to stings should call 911.

Yellowjackets defend the nest, and a disturbed colony can sting many times and in numbers. That is the main reason this job is not a good one for a ladder and a can of spray. Spraying at night is a common suggestion online, and it is also how many people get stung when a flashlight disturbs the nest.

Blocking an entrance with a rag or a rock does not seal a nest. If the nest is in a wall, the wasps may chew through drywall and come into the room. Pouring liquids into a ground nest can have the same effect.

If someone has been stung and has trouble breathing, swelling of the face or throat, dizziness or a spreading rash, call 911. For ordinary stings, wash the area and watch for changes.

• Do not plug or flood the entrance
• Do not knock a nest with a pole or a hose
• Do not mow or trim near a suspected ground nest
• Keep pets away from the area
• Call 911 for signs of a severe allergic reaction

Should I call the mosquito district or a pest company about yellowjackets?

The Sacramento-Yolo Mosquito and Vector Control District performs yellow-jacket control from July to September, per Sacramento LAFCo. Call the district at (800) 429-1022 to ask whether your nest is covered. For a nest outside that window, or when you want a technician scheduled, call (855) 974-4733 or another licensed local company.

The district is a public agency, and it has handled yellow jackets since it was renamed in 1990 to reflect expanded services for ticks, yellow jackets and other pests. It began in 1946 as the Sacramento County-Yolo County Mosquito Abatement District. Its focus today is mosquito and vector work, with West Nile virus as the main public-health concern.

Sacramento Pest Control is not the district and is not affiliated with it. A call to this number connects you with an independent, licensed local pest-control company whose technician does the work. The practical difference is scope and timing. The district's yellow-jacket service runs July to September. A private technician can be scheduled outside that period and can also handle other pests on the same visit.

For the full comparison, see the guide to what the mosquito district does and what a pest company does.

What does a technician do about a nest?

A technician finds the nest entrance, decides whether it is in the ground, a wall or an attic, and treats it according to the product label and the location. Wall and attic nests may need an entry point sealed afterward. You see the price first, and a written service report follows the visit.

The first step is identification. Yellowjackets, paper wasps and hornets look similar from a distance but nest differently and call for different handling. The technician looks at the traffic pattern, the entrance and the setting, then picks a method that fits.

Nests in walls and attics take more care because the treatment product must reach the nest without driving wasps into the house. Afterward, the technician may recommend sealing the entry gap so another queen does not use the same cavity next spring. In UC IPM's life cycle, queens begin new nests each spring, so the abandoned cavity is more of a concern than the old nest itself.

If wasps are the immediate emergency, see yellowjacket and wasp removal. For a nest at night or on a weekend, the phone is answered around the clock, and most calls are scheduled the same day when a technician is available. That is not a promise of an arrival time.

How can I keep yellowjackets away from my yard this season?

Cover garbage cans, rinse recycling, pick up fallen fruit, bring pet food in after meals, and keep drinks covered outdoors. UC IPM says late-season workers scavenge at picnics, garbage cans, pet food and ripe fruit, so removing those draws reduces visits, though it will not remove a nest already in place.

These habits are most useful from August to October, but they cost nothing in any month. In a Sacramento backyard with a fruit tree or a vegetable bed, windfall fruit is often the largest attractant, and it gets less attention than the trash can.

If you eat outside, check a drink before sipping, especially from a can. Keep food covered, and clean up soon after. If you see a stream of wasps crossing your yard in a straight line, they may be flying to a nest that is not on your property. Note the direction, and mention it when you call.

• Use garbage cans with tight lids and rinse bottles and cans
• Pick up windfall fruit every day in late summer
• Feed pets indoors or remove bowls after meals
• Keep drinks and food covered outside
• Seal wall and soffit gaps over the winter
• Fill abandoned ground holes after the colony dies back