Boxelder Bugs and Cluster Flies in Fall: A Gillette Home Guide

Short answer

Boxelder bugs and cluster flies gather on sunny south and west walls in Gillette each fall, then squeeze into siding, attics and window frames to spend the winter. They do not bite or breed indoors, but they come out on warm days. Seal gaps before September, treat the outside walls in early fall, and vacuum the ones that get in.

Why do boxelder bugs and cluster flies show up in fall?

Boxelder bugs and cluster flies show up in fall because they need a protected place to spend the winter. As nights cool in September, they gather on the warmest walls of your home, usually the south and west sides, and look for cracks to crawl into.

Boxelder bugs feed on boxelder and maple seeds, both common shade trees in Gillette yards and parks. Cluster flies grow up in the soil feeding on earthworms, which is why homes next to lawns and fields see so many.

Are they harmful?

Boxelder bugs and cluster flies are not harmful to people. They do not bite, do not carry disease and do not breed inside your home in winter. They are a nuisance because they show up by the hundreds, stain curtains and walls with droppings, and leave a smell when crushed.

The bigger issue is numbers. Once they find a way into your wall voids or attic, they come back every fall to the same home.

How do you keep them out of your walls?

Keep them out of your walls by sealing gaps before they arrive and treating the outside walls in early fall. Timing matters. Once they are inside the walls, sprays on the outside will not reach them.

1. Seal early. Caulk gaps around windows, doors, siding, soffits and utility lines before mid-September.
2. Screen vents. Put fine screen on attic, soffit and gable vents.
3. Fix window screens. Replace torn screens and gaps at window air conditioners.
4. Exterior treatment. A perimeter treatment on south and west walls in late August or September stops many before they get in.
5. Reduce food sources. Rake boxelder and maple seeds, and consider which seed trees are close to the house.

What should you do with the ones that get inside?

Vacuum the ones that get inside and empty the bag outside. Do not crush them, since boxelder bugs stain and cluster flies smell. A shop vac with a little soapy water in the tank works well on windows.

Avoid spraying indoors. Dead bugs in wall voids can attract carpet beetles. Instead, find where they are coming in, usually around window trim, light fixtures and ceiling fans on upper floors, and seal from the outside next season.

When is the best time to treat for fall invaders in Gillette?

The best time to treat for fall invaders in Gillette is late August to mid-September, right before nights turn cold. That is when boxelder bugs and cluster flies start gathering on warm walls but have not yet crawled into wall voids. A treatment after they are inside the walls does very little.

Watch the south and west sides of your home on sunny afternoons in late summer. When you see the first clusters of red-and-black boxelder bugs or slow, large flies sunning on siding, it is time. Pair the exterior treatment with sealing work on window trim, soffits and siding gaps for the best result.

If you missed the window, focus on vacuuming and sealing interior gaps around window frames, light fixtures and outlets until spring, then plan an early treatment next year.

When should you call for pest control?

Call for pest control when you see hundreds on your walls every fall, when they keep showing up indoors through winter, or when you also have spiders, ants and wasps. A household pest control Gillette WY plan includes a fall perimeter treatment timed for these invaders. If mice are also moving in as it cools, read our guide to mice in the house this fall or ask about rodent control Gillette WY. Homes in pest control Central Gillette with mature trees see the most boxelder bugs.

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