Detroit Pest Calendar: What to Watch Each Month

How does Detroit's weather shape the pest year?

Long, cold winters push mice, rats and overwintering insects indoors in autumn, and warm, wet summers bring ants, mosquitoes and wasps back outside. Detroit averages about 40.7 inches of snow a year, so the move indoors and the spring re-emergence are the two turning points of the calendar.

Detroit's winters are the key difference from southern pest guides. In the National Weather Service monthly normals, the average January high is about 32 degrees and the low about 19, while July runs about 84 degrees for the high and about 64 for the low. Snowfall averages about 40.7 inches a year, with January, February and December the heaviest months. Precipitation is spread through the year at roughly two to just under four inches a month, and April through July tend to be the wettest.

What that does to pests is simple. Outdoors, most insects go dormant, die back or retreat. Indoors, the heated house becomes a refuge. Mice and rats look for warmth and food, overwintering insects find cracks to hide in, and pests already living inside, such as German cockroaches and bed bugs, continue as they were. In spring the thaw sends the outdoors pests back to work and the winter guests try to leave.

The calendar below is built on that pattern. It is a guide for Detroit and its Wayne, Oakland and Macomb County suburbs, not a forecast. Where a source supports a statement we say so, and where it is general experience we hedge it with words like typically and often.

What does the full year look like?

Winter is the indoor season for mice, rats, cockroaches and bed bugs. Spring brings termite swarms, ants and the exit of overwintering insects. Summer brings mosquitoes, wasps and the bat nursery season. Autumn is when rodents come inside and fall invaders gather on south and west walls.

Use the table as a monthly checklist. The right column says what to do, and none of it requires a technician for every item. Some, such as sealing gaps in September, are within reach of a careful homeowner. Others, such as termite checks and bat exclusion, are worth a technician's time.

What happens in winter, from December through February?

Winter pest activity in Detroit is mostly indoors. Mice and rats stay inside basements and walls, German cockroaches and bed bugs continue in heated multifamily buildings, and overwintering insects can wake on warm days in a heated attic. Outdoor pests are largely out of the picture.

This is the season when a house's weak points are tested. Mice and rats that came in during autumn live in wall voids, basements and attics, and any gap that let them in lets more follow. A full basement in an older house is warm, dry and full of places to nest. You may notice scratching at night, droppings behind appliances or chewed food packaging.

In apartment buildings and flats, cockroaches and bed bugs do not take winter off. They follow heat and people. Because shared walls and pipes connect units, a problem in one apartment is often a building-wide problem, and treating a single unit tends not to hold. If you rent, report it early and in writing.

A warm afternoon in January or February can also bring out boxelder bugs, cluster flies and lady beetles that settled in a wall or attic. They are not a sign of an infestation of the living space. They are insects that wake up in the heat and wander. Vacuum them rather than spraying. Our service for mouse control and cockroach control covers the winter pests that do need treatment.

What happens in spring, from March through May?

Spring brings thaw, breeding and re-emergence. Mice that entered in winter breed, termite swarms are possible from about March on warm, rainy days, ants begin foraging, and overwintering insects try to leave. Carpenter ants become active in damp wood by May, and bat maternity season begins around the third week.

March and April are the transition. Snow melts, the ground thaws, and water finds its way along foundations. Moisture draws ants and termites to the wood nearest the soil. Commercial sources put termite swarms from about March into early summer, so a cluster of winged insects on a warm, wet day is worth examining. Our guide, are termites a real problem in Detroit, explains how to tell termite swarmers from winged ants.

Spring is also when insects that overwintered in a building become noticeable as they try to leave. MSU Extension describes boxelder bugs, Asian lady beetles and cluster flies waking up and heading toward light, which can bring a surge indoors around windows. They are nuisance pests rather than structural ones.

By May, carpenter ants are active in damp wood, such as old porch posts, window sills and sill plates, and our carpenter ant control service handles that. Bats begin forming maternity colonies around the third week of the month, which is the reason exterior bat work waits for late summer.

What happens in summer, from June through August?

Summer is the outdoor season. Mosquitoes build through June and peak in August, wasp and yellowjacket nests grow through the summer, and bat pups are raised in attics until late July. Wayne County traps and tests mosquitoes for West Nile virus through the season.

Mosquito pressure follows heat and standing water. Gutters, buckets, plant saucers, old tires and low spots in yards all collect water. Wayne County Environmental Health traps and tests mosquitoes through the summer, and a local news report said the county's first positive of the season was a mosquito pool in Redford Township. Most infected people have no symptoms, though older adults and people with weak immune systems face the serious cases. Asian tiger mosquitoes have been found in Wayne County, but the state says they are not established, so they are not a Detroit problem. Our mosquito control service covers yard treatment.

Wasp and yellowjacket nests start small in June and grow, so the late-summer nest in a soffit, porch or wall void is often big and defensive. If you see a lot of traffic in and out of a small gap, keep away and call. Wasp and yellowjacket removal is safer than disturbing a nest yourself.

In the attic, bats may be raising young. The right approach is to wait, plan and exclude in late summer, as explained in our guide on bats in a Detroit attic.

What happens in autumn, from September through November?

Autumn is the sealing season. Rodents begin moving in as nights cool in September, fall invaders gather on south and west walls and peak in October, and mouse and rat entry continues through November at basements and coal chutes. Work done before the first frosts matters more than work done in January.

MSU Extension describes boxelder bugs, Asian lady beetles and cluster flies as fall invaders that gather on the south and west sides of buildings, the warmest walls, and crawl into cracks and crevices. Entry points include gaps around windows and doors, loose siding, soffit vents and louvers. Sealing exterior cracks with caulk reduces entry, and removing nearby female boxelder trees helps. Our guide on boxelder bugs and cluster flies on the south wall goes into detail.

Rodents work on the same schedule. As nights cool, mice test foundations and utility gaps, and a Norway rat that spent summer near a vacant lot may try your basement. Older homes with coal chutes, block foundations and loose pipe collars are the usual entry. You can read about the timeline in our guide on when mice come in, and the rodent exclusion service seals the entries.

By November the first snow falls, with about 2.8 inches on average, and the window for easy outdoor work shrinks. If you plan one thing for the year, plan an October walk around the house with a flashlight and a caulk gun.

Which pests stay active all year, and which do not?

Pests that live inside heated buildings stay active through winter: mice, rats at basements and sewers, German cockroaches and bed bugs. Outdoor pests such as ants, mosquitoes, wasps and termite swarmers are seasonal, so the useful question each month is what is moving in, what is waking up and what is outside.

It helps to separate pests by where they spend the cold months. The table groups the main Detroit pests by that pattern. It reflects general experience and the sourced items in this guide, not statistics.

The practical point is that a once-a-year spray does not match this calendar. A plan that includes autumn sealing, a spring check and a summer visit follows the seasons better, which is how our general pest control visits are scheduled. A technician confirms the price before any treatment starts, and you can reach us at (855) 974-4733 at any hour.