What do fungus gnats, drain flies, Boxelder bugs and stinkbugs have in common this time of year? The common thread is they are all nuisance household insects that can be found in any home during winter.
Fungus gnats and drain flies can be lumped together based on their favored conditions, cool temperatures and humidity. Fungus gnats often come in with our houseplants for the winter, as they are stowing away in the pots. Drain flies find their way inside at some point before cold weather shows up and need "open water" to breed.
Indoor humidity drops once the furnace comes online, and fungus gnats stay close to their food source and the higher humidity of the houseplants so they may not be a large problem. Initially you may find them anywhere there is cooler temperatures and higher humidity, such as the basement bathroom. Drain flies can become more numerous as they breed. Needing that open water means checking any floor drains and any unused sinks, toilets and other spots where you may have water that goes undisturbed. If you observe the water, you may even see movement of their larvae. The remedy is to pour several gallons of clean water into any floor drains, enough to flush the trap and routinely run water in sinks and flush toilets. An additional step for floor drains since they do collect debris is to scrub the drain at the water line to loosen the dirt and any organic matter.
Boxelder bugs and stinkbugs will show up every time we have mild weather. Neither one can bite or sting, as their mouthparts are for sucking plant juices. They find their winter home in the walls of our homes and should remain dormant until spring when they move back outside. On warmer winter days, the home wall will warm enough to allow them to move about, and in doing so, find their way inside our homes. They will wander around, be attracted to windows and light, but without a food source, they are running on reserves and will eventually die. The Boxelder in particular will leave behind little dark deposits. A vacuum or a bit of paper towel is all that is need to get rid of the dead ones and any live ones as well. Stinkbugs do smell if crushed, so collect them carefully.
Since all four of these nuisance insects frequent the same places we do in the home, sprays are not practical. While we cannot do much right now as they are already in our homes, there are a couple of practical things to do before late next summer. Stinkbugs and Boxelders literally crawl into the walls, so an outdoor foundation spray may be considered. Check for caulking around any opening into the home. Water outlets, A/C lines and corresponding electrical breaker boxes should be checked. Check or replace any door threshold strips. Make a trip into the basement or crawl space during the daylight hours, let your eyes adjust with the lights off and you will see every pinhole or larger that allows insects in. Larger openings can benefit from insulation and smaller holes from caulk. Put this checklist on your calendar for warmer months and take action, and you likely will have less unwanted guests next winter.
Richard Hentschel is a Horticulture Extension Educator with University of Illinois Extension, serving DuPage, Kane and Kendall counties.
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About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.