San Francisco Pest Calendar: What Happens When

Why is this calendar built on triggers and not months?

Because most monthly pest calendars are written for other places and then reused. We could source only two San Francisco timings, so for everything else this page names the event that sets a pest off, such as the first rain, a cold snap, uncovered garbage or a branch touching the roof, and leaves the month out.

A pest calendar looks authoritative the moment it has twelve rows. That is exactly why they are dangerous. A table that says a particular insect is active in a particular month can be copied from a Sacramento page, a Los Angeles page or a national one, and nobody reading it can tell. San Francisco has its own climate, with a cool, foggy summer that behaves nothing like the inland valleys just an hour or two away, so a month borrowed from elsewhere may simply be wrong here.

So we built this page the other way round. We started from what could be sourced for San Francisco and the Bay Area and put that in the calendar with its source. Where we could not source a month, the page says so in plain words and gives you the trigger instead. A trigger is something you can observe at your own building. You can see that the first rain has come. You can see that a tree now touches your roof. You can see that the garbage cart lid does not close. Those are better prompts than a date on a page.

Read the page as a set of things to watch for, in the order a Bay Area year tends to bring them. If a later source gives us a San Francisco month for something, it can be added. Until then, an absence in this calendar means unsourced, not unimportant.

What does the San Francisco year look like?

Winter rain runs from November to March and accounts for over 80 percent of the year's total. Summer is essentially dry, with sea fog at night, overcast mornings and sunny afternoons. In short: dry summers and wet winters, with the first autumn rain marking the change.

The shape of the year matters for pests because moisture and temperature change what insects and rodents do, and what a building offers them. The climate description we can source for the city says that winter rain falls from November to March and makes up over 80 percent of the annual total, while summers bring sea fog, overcast mornings and sunny days. We deliberately print no rainfall or temperature numbers here, because the secondary summaries we saw were not confirmed on the publishers' own pages. The pattern is the useful part: dry summers, wet winters.

That pattern means the first rain of the season is a real event in the city. After months of dry weather, the change to damp ground and wet wood is sudden, and some insects respond to it directly. Termites are the clearest example, and the next section covers them. Rain also pushes some animals toward shelter, which is one reason people notice rodents and insects at the edges of a wet spell.

The fog matters in a different way. Damp coastal air keeps exterior wood, such as trim, window frames, decks and fascia, in a state that dry inland air would not. Wood that stays damp is wood that wood-destroying organisms like. That is a slower, quieter issue than a swarm, and it is one reason an inspection looks at more than the termites themselves.

When do termites swarm in San Francisco?

Western subterranean termites swarm in calm, sunny periods right after the first autumn rains, according to UC IPM, and the effect is especially pronounced in the San Francisco Bay Area. That is the only sourced termite timing here. No month is given for drywood termites because none could be sourced.

The termite swarm is the one pest event in this calendar that has a sourced trigger and a sourced region. A regional news report quotes Andrew Sutherland of UC IPM describing western subterranean termites swarming in calm, sunny periods right after the first autumn rains, and notes that the Bay Area sees this especially strongly. The practical reading is simple. When the first real rain of the autumn has passed and the weather clears and calms, watch for winged insects near windows, light fixtures and the base of walls.

A swarm is a reproductive flight. It shows that a colony exists somewhere nearby, though it does not by itself show that your building is infested. Cast wings left in a pile on a sill are a more useful clue than the flying insects themselves, because they mean swarmers entered or emerged inside. The full detail is in our guide on when termites swarm in the Bay Area.

Drywood termites are a different animal. They live inside dry wood, such as attic framing and trim, and they do not depend on soil contact the way subterranean termites do. We have no sourced San Francisco month for their swarms, so this calendar gives none. If you find small, pellet-like droppings or hear nothing at all but see damage, the trigger is the evidence in the wood, not the calendar. The termite inspection page describes what a technician examines.

What are the triggers for rats and mice, and when do they apply?

For rodents the triggers are garbage left out, open gaps around the building, food and shelter, and vegetation touching a roof. The San Francisco Department of Public Health names garbage as a major attractant. No month is given for roof rat or Norway rat activity because none could be sourced for the city.

Rodents do not follow a calendar the way a swarm does. They follow opportunity. The San Francisco Department of Public Health names Norway rats, roof rats and house mice as the important urban rodents, and it treats garbage as a major attractant. If garbage is left in open bags, if a cart lid will not close or if food waste sits out, that is the trigger, in any week of the year.

Roof rats are a good example of why a trigger beats a date. UC IPM says roof rats climb and nest above ground in shrubs, trees and dense vegetation, and indoors in attics, walls, false ceilings and cabinets. A tree limb that touches the roof, or ivy that climbs a wall to the eaves, is a bridge. Trimming it back is a seasonal-feeling job that you can do any time. Norway rats, by contrast, burrow along foundations and under rubbish or woodpiles, so a stack of lumber against a fence is the trigger for them.

Weather can act as a nudge. A cold snap or a long wet spell can make a warm attic or a dry garage more attractive than the outdoors, and that is when people first hear scratching. We will not name which weeks, because we cannot source them. What we can say is that the right moment to seal a gap is before you hear anything. Details on species and who handles what are in rats and mice in San Francisco and on the rat control and mouse control pages.

What can I use as a seasonal checklist instead of a date table?

Use the table below. Each row pairs a pest with the trigger to watch for and the only timing that is sourced, if any. Where the timing column says not sourced, no month is given and the trigger is the guide. It is a prompt to inspect your building, not a schedule.

The table is short on purpose. Rows that would need a made-up month are written as triggers. Use it as a prompt to walk around your building and look, and call if you see something.

• Walk the outside of your building after the first big autumn rain and look at the base of the walls and window sills.
• Check that garbage and recycling carts close fully and that nothing is piled against a fence or foundation.
• Look at where branches, vines and shrubs meet the roof or siding and cut back what touches.
• Look in the attic and crawl space when you can safely do so, or ask a technician to look.

What should I do after the first big rain of the season?

Look for winged insects and shed wings near windows, lights and thresholds, check for new gaps or damp spots, and make sure gutters and downspouts drain away from the building. The first rain is the sourced trigger for the termite swarm, so it is the right moment for a quick walk-around inspection.

Because the first autumn rain is the sourced cue for the western subterranean termite swarm, it is also a sensible cue for a short routine. After the rain passes and a calm, sunny spell arrives, spend ten minutes looking. Check window sills and light fixtures for piles of shed wings. Look at the base of exterior walls and the wood near the ground. Note any soft or damp-looking wood on trim and decks, which is the early sign of the slower moisture problems that fog and winter rain can bring.

Next, look at how water moves around the building. Gutters that overflow, downspouts that empty against a foundation and soil that slopes toward a wall all keep wood damp. Dry rot and termites are separate problems, but a damp building tends to invite both, and an inspection often covers them together. The dry rot and wood damage page explains what that looks like.

If you do see swarmers or wings, collect a few in a jar or tape, and take a photo. That helps a technician tell termites from flying ants. Then call (855) 974-4733 or book through the contact page. A technician can inspect the building and tell you what is going on and what, if anything, needs to be done.