Why Stink Bugs Invade in Fall
Brown marmorated stink bugs (Halyomorpha halys) are an invasive species from East Asia established across Missouri and most of the eastern US. They are agricultural pests during the growing season and become a residential nuisance in fall when cooling temperatures trigger a search for overwintering sites — a behavior called aggregation. Stink bugs seek sheltered, dry spaces to overwinter: wall voids, attic spaces, and any gap in the building envelope large enough to admit them. A structure that overwintered stink bugs in a prior year retains the aggregation pheromone in its walls, attracting new bugs to the same location each subsequent fall.
The Entry Points That Matter Most
Utility Penetrations
Gaps around pipe and conduit penetrations through exterior walls — even small gaps admit stink bugs and other fall invaders. Seal with appropriate caulk or foam.
Window and Door Frames
Gaps between frame and siding, deteriorated caulk lines, and missing or torn window screen material. Replace screens and re-caulk frames before September.
Attic and Soffit Vents
Standard vent screening can have large enough mesh openings to admit stink bugs. Fine mesh screen (less than 1/16 inch) over existing vent covers significantly reduces attic entry.
Chimney and Roof Gaps
Gaps at the chimney-roofline junction and at any roof penetration. Stink bugs enter attic spaces through these openings and overwinter in the attic insulation.
What Works and What Doesn't
Never squash stink bugs indoors. The defensive odor they release is also an aggregation pheromone — it signals other stink bugs to the location. Use a vacuum or flush them instead, and dispose of the vacuum bag outside.
Interior contact sprays kill individual bugs but have no effect on the population entering through unaddressed gaps. Exterior perimeter treatment with a residual insecticide applied to the building exterior in late August through September — before the aggregation window — provides meaningful knockdown at entry points as bugs contact treated surfaces attempting to enter. This works as a seasonal bridge but is not a substitute for physical exclusion of the primary entry points. D&D Pest Control provides fall perimeter treatment programs for Franklin County and rural Missouri — visit ddpestcontrolmo.com.