Every September and October, the sunny side of a lot of Treasure Valley houses turns red and black. Boxelder bugs line up on siding, window frames and fence posts, soaking up the afternoon heat. A few weeks later some of them are on the inside of the windows. They’re the best-known fall invader around Boise, but not the only one.
Here’s what fall invaders are, which ones show up in Idaho, and how to keep them on the outside of the house.
What Is a Fall Invader?
“Fall invaders” (also called occasional invaders or overwintering pests) are insects that spend summer outdoors and then look for a protected place to wait out winter. As nights cool, they gather on warm, sunny walls and work their way into gaps around windows, siding, soffits and attic vents. Once inside a wall void or attic, they go mostly dormant until spring.
They don’t breed indoors, don’t damage the structure and aren’t known to spread disease. The problem is numbers. A house that attracts them can host hundreds, and on warm winter days they wander into living spaces and onto windows.
Common Fall Invaders in Idaho
Boxelder bugs
About half an inch long, black with red-orange markings. They feed on the seeds of boxelder trees and some maples, so homes near those trees see the most. They can leave small reddish stains on fabric and curtains if crushed. Our post on getting rid of boxelder bugs in Star covers them in more detail.
Stink bugs
Shield-shaped brown or green bugs that release an unpleasant smell when disturbed. They feed on garden plants and fruit trees in summer and move toward houses in fall.
Lady beetles
Asian lady beetles look like the familiar ladybug but vary from pale orange to red and can gather in large groups on light-colored, sun-facing walls. They can leave a yellowish, smelly fluid when handled.
Other overwintering insects
Elm seed bugs, western conifer seed bugs and some leafhoppers also overwinter in valley homes. Spiders often follow, since a house full of insects is a good food source. Our post on keeping spiders out of Boise attics covers that side of it.
Why Your House Attracts Them
- Sun exposure. South- and west-facing walls warm up in the afternoon and draw insects in fall.
- Light-colored siding. Some species, especially lady beetles, are drawn to light, high-contrast surfaces.
- Nearby host trees. Boxelder, maple, elm and fruit trees support the populations that end up on your house.
- Gaps. Openings around window frames, siding seams, soffits, vents and utility lines let them inside.
How to Keep Fall Invaders Out
Seal entry points before September
Exclusion works best before insects start gathering. Caulk gaps around windows, doors and trim. Replace worn weatherstripping and door sweeps. Screen attic, soffit and foundation vents with fine mesh, and seal around pipes, cables and dryer vents.
Exterior treatment in late summer
A perimeter treatment applied to sunny walls, eaves, window frames and other spots where insects gather, timed for late August or September, is one of the most effective ways to reduce how many get inside. Once they’re in the walls, treatment options are more limited.
Manage the yard
- Rake up fallen seeds and leaves under boxelder and maple trees.
- Keep leaf piles, mulch and debris away from the foundation.
- Talk to an arborist if one tree is the main source year after year.
What to Do if They’re Already Inside
- Vacuum them up and empty the bag or canister outside. Avoid crushing them, which can cause stains and odor.
- Don’t spray inside wall voids, which can leave dead insects that attract other pests.
- Seal gaps from the inside around window trim, light switches and outlets on exterior walls.
- Write down where they’re showing up so the outside gaps can be found and sealed in spring or late summer.
The EPA’s overview of integrated pest management explains why sealing, sanitation and targeted treatment together work better than spraying alone.
When to Call for Help
A handful of boxelder bugs on the siding isn’t cause for worry. If they’re coming inside by the dozen, returning to the same windows every year or showing up in upstairs rooms through the winter, the house likely has gaps along the roofline or around windows that are worth a professional inspection and a well-timed exterior treatment next season.
Fall Invaders and Mice Arrive Together
The same gaps fall invaders use in October are the ones mice use in November. A fall check that covers both makes sense. Read more about why mice move inside in winter.
Fall Invader Control With ProGuard
Fall invaders are a core pest in our residential pest control, along with spiders, ants, wasps, mice and rats. We offer quarterly, bi-annual and one-time service, and quarterly service plan customers get free return visits. Rodent service is included at no extra cost with quarterly service plans; traps and materials are billed separately.
Serving the Treasure Valley
ProGuard is based in Eagle and serves homes and businesses in Boise, Meridian, Nampa, Caldwell, Star, Middleton, Emmett and Garden City. To get ahead of fall invaders this year, call (208) 352-2062 or contact us.