Sacramento Pest Calendar: What to Watch Each Month

What does the Sacramento pest year look like?

Sacramento has about 18 inches of rain a year, nearly all of it from November to March, and a long dry summer with highs around 87 to 92 F. Pests respond to that split. Dry months push ants and rodents toward indoor water. Wet months bring termite swarms and send squirrels and voles to higher ground.

The easiest way to read this calendar is to ask where the water is. Sacramento's Mediterranean climate gives it a wet winter and a long, rainless stretch from roughly June through September. NOAA's 1991 to 2020 normals put average highs at 87 F in June, 92 F in July and 91 F in August, and weather summaries put annual precipitation at about 18 inches. The first 100 F day has usually come around June 4. Cold air drawn through the Sacramento-San Joaquin Delta, the Delta breeze, can cool evenings noticeably, so heat waves often break within a few days.

That pattern means pests behave differently by season, and in ways that are local. A dry August in Midtown or Land Park puts ants on the move toward faucets. A wet January pushes rodents into attics and wall voids and leaves ground squirrels and voles working around saturated ground near levees and canals. The table below summarizes the year, and the sections after it explain each stretch.

Treat every month boundary as approximate. Weather shifts from year to year, and swarms and nest growth follow the weather, not the date on the calendar.

What happens from January to March?

January to March is the wet season. Rain is heaviest in December and January, the clay soil is saturated, and rats stay in attics and wall voids. Ground squirrels and voles remain active on open ground and near levees. This is a good time to check roofs, attics and crawl spaces for signs from last year.

Winter is when problems you did not notice in summer show up as sounds and stains. UC IPM says roof rats are present in almost all California cities and nest in attics, wall voids, ceiling voids and cabinets. They are climbers, so mature landscaping and riverbank vegetation make good habitat, and Sacramento has plenty of both along the American River Parkway and in older neighborhoods such as East Sacramento and Curtis Park. Scratching overhead at night and droppings near insulation are the usual signs. The longer guide to roof rats in Sacramento attics goes through them.

Out in the open, California ground squirrels dig burrows that UC IPM describes as 5 to 30 feet long and 2.5 to 4 feet below the surface, and the same source calls their tunnelling the biggest threat to California levees. Sacramento sits behind a lot of levee, so that is a local concern as well as a yard one. Voles leave runways, the narrow trails between burrow openings, and chew bark and roots. Reducing dense vegetation is the main management step UC IPM names. See ground squirrel and vole control for how a technician approaches both.

Moisture inside the home is the other winter theme. Wet clay soil swells, and leaks and damp crawl spaces give subterranean termites and other pests what they need. If you have not looked under the house since the rains began, this is a reasonable month to ask a technician to do it.

What should you watch in April and May?

April and May are when queen yellowjackets start new nests, ground squirrels become active on open grass, and subterranean termites may swarm in spring in the northern valley. The rain is easing but the ground is still soft. Early action on a nest is much easier than dealing with a full colony in September.

Yellowjacket colonies do not carry over. UC IPM explains that the colony declines in late fall and queens start new nests each spring. A spring queen building a small nest in a wall void, an attic or the ground is a defined, manageable problem. By late summer the same nest can hold a large colony, which is why we have a separate guide to yellowjackets in late summer in Sacramento. If you see a single wasp repeatedly entering the same crack or hole, note the spot and keep children and pets away from it.

Ground squirrels often become more visible on lawns, parks and any open grassy areas where they can colonize residential edges. The 23-mile American River Parkway runs through open grassland and oak woodland, so homes along its edge are close to this habitat.

UC IPM also says western subterranean termites may swarm in spring in the northern valley, in addition to their main swarm after the first autumn rains. A cluster of winged insects at a window, or discarded wings on a sill, is worth a closer look and a call. Our guide on the California termite inspection report explains what a formal inspection records.

Why do ants and rats show up from June to August?

From June to August, rainless weather and highs of 87 to 92 F push Argentine ants to search homes for water, including bathroom faucets and drains. UC research summaries describe the peak in August and September. Drywood termites begin swarming in August, and yellowjacket colonies grow all summer.

UC IPM says the Argentine ant is the most common outdoor ant in California. It is about 3 millimeters long, dark, and does not sting. Nests are normally in moist soil, under debris or along sidewalks and driveways, and workers travel up to 100 feet to find food, water and shelter indoors. In hot, dry weather they often look for water in the house. A UC San Diego research summary says that in August and September, when California was too dry for them, the ants' quest for water led them into bathrooms and kitchens. Our longer piece, why Argentine ants invade during a Sacramento summer, covers what to do.

Rodents also respond to water, though this is a point to state carefully. UC IPM lists leaking faucets and sprinkler heads, birdbaths, fountains, irrigation, air-conditioner condensate lines, plant saucers and pet bowls as the water sources roof rats use. It does not say rats come indoors in summer for water, so we will not claim it. What you can do is remove those sources, which helps year-round.

Drywood termites begin swarming in August, and UC IPM describes swarms as usually late summer or fall. Finding discarded wings near a window or door frame is a reason to book an inspection. The termite fumigation page covers the one method UC IPM says reaches an entire structure.

• Fix dripping faucets, sprinkler heads and condensate lines that give ants and rodents a water source
• Trim shrubs, ivy and tree limbs back from the walls and roofline
• Store pet food and ripe fruit so it is not left out overnight
• Look at window and door frames for wings after August

What changes in September and October?

September and October feel like the driest stretch of the year. Yellowjackets scavenge and sting more around picnics, garbage cans, pet food and ripe fruit, because their colonies are largest and natural food is scarcer. Drywood termite swarms continue into November. Ants remain active indoors until the rain returns.

UC IPM explains why yellowjackets feel worse in autumn. Colonies grow through summer, so by September they are at their largest, and food gets harder to find. Workers turn to human food, and defensiveness rises. That is the moment people get stung at an outdoor table or while taking out the trash. The Sacramento-Yolo Mosquito and Vector Control District performs yellow-jacket control from July to September. It is a public agency, and Sacramento Pest Control is not affiliated with it. The difference is covered in mosquito district or pest company in Sacramento.

Because it has barely rained since spring, soil and mulch are dry, and any reliable moisture near the house draws pests. Leaking hose bibs, overwatered planter beds and damp spots under sinks stand out. If ants are still trailing indoors in October, the usual cause is that the nest is close and the water source is inside.

For homeowners, this is also the point to think about what you want in place before the weather turns. A technician can check entry points before rats look for winter shelter and can inspect crawl spaces before the soil gets wet. See pest control for how a general visit works.

What happens in November and December?

November brings the first autumn rains, and western subterranean termites often swarm on calm, sunny days right after the first rain. The wet season begins and runs to March. In December, some vendor pages report rodents moving indoors for shelter as the weather cools. Wet ground is also when crawl-space moisture shows.

UC IPM describes the subterranean termite swarm as tied to the first autumn rains, especially in the Bay Area and parts of the Sacramento Valley. The detail that surprises people is the weather: the swarm tends to happen on a calm, sunny day just after the rain, not during the storm. Subterranean colonies need contact with soil moisture at all times, which is why inspections focus on parts of a structure in or near the ground. Older raised-floor homes in Curtis Park, Land Park, East Sacramento and Oak Park are a good example, and the guide to older Sacramento homes and crawl-space pests explains what is under them.

Hedge the dates. Some years the first meaningful rain arrives in October, some years in December. Do not wait for a calendar day: watch for the first soaking rain and the clear day that follows.

Rodent behavior in December is described by pest company pages, not by UC IPM, so treat it as common experience rather than a rule. Cooling weather and wet ground make an attic attractive, and any gap you did not seal in summer is still open.

How do you use this calendar?

Use the calendar as a reminder of what to check, not as a schedule that every home follows. Walk the outside of the house before summer, look for swarms and pellets from August, check for nests in spring, and inspect attics and crawl spaces before and after the wet season. Call a technician when you see signs.

A reasonable yearly routine is short. In late winter, inspect the attic and the roofline. In spring, look for early nests and clear shrubs away from the walls. In June, fix leaks and irrigation drips before the heat arrives. In August, watch window and door frames for wings. In November, check the crawl space or foundation line after the first rain.

When you do see something, phone (855) 974-4733. The line is answered 24/7, and most calls are scheduled the same day when a technician is available. Sacramento Pest Control connects callers with an independent licensed local pest-control company, and the technician explains the price and you approve it before treatment starts. A written service report follows each visit, and products are applied per the label.

If you are not sure what you are seeing, say where and when: which room, which time of day and which month.