Overwintering Pests – Cluster Flies, Lady Beetles, Boxelder Bugs,Western Conifer Seed Bugs

Boxelder bugs aggregating on the windows.

Boxelder bugs aggregating on the south side of the house and trying to get inside around the windows. Photo by Jody Green, Nebraska Extension in Lancaster County.

Some pests are considered seasonal or occasional pests and their grand entrance into your house in the fall may go unnoticed, but they are not accidental. Overwintering pests require a protective place to spend the adult stage of their life without freezing to death. They normally require a winter habitat between 40-50°F for hibernation. In the fall, large populations congregate on the warm, sunny side of the structure, usually the southwestern facing exterior wall and begin moving upward to find a gaps that leads inside.

With the temperatures finally dropping, I’m guessing most of our unwelcomed guests have entered the structures for the winter. They do not lay eggs or multiply indoors, but will overwinter in wall voids, attics and unheated garages. Most homeowners become aware of the infestation in the early spring when the sun warms up the structure, stimulating the exit from their winter harborage. They often come out from the ceiling through vents, lights, fans and other gaps, accumulating by the windows and leaving excrement and stains on walls. They are considered nuisance pests because of the sheer number of individuals that emerge and fly around trying to escape.

Multicolored Asian Lady Beetles are overwintering beetles that are very similar to the native lady beetle but with a distinct black M-shape on the thorax before the wing covers They can be many variations of color from yellow to orange to dark red, and can have as many as 19 black spots on their wing covers or no spots at all. They are sometimes referred to as the Halloween beetle because they are seen congregating in late October. Outdoors they feed on aphids, mealy bugs, mites, and other soft-bodied plant pests, making them beneficial insects. Indoors when they emerge, they stain walls (reflex bleeding) when handled, and have been known to bite, making them a nuisance pest.


Characteristic “M” or “W” behind the head of the multi-colored Asian lady beetle. Photo by Jim Kalisch, UNL Department of Entomology.

Cluster flies are slightly larger than house flies, with yellow hairs on the thorax. They are attracted to the light-colored surface of structures and walk up the walls looking for a gap to get inside to overwinter. They do not damage anything, but can leave dark fecal spots on windows and walls when they try to escape in the spring. When the adult cluster fly leaves in the spring, females lay eggs in soil and their maggots grow to be parasites of earthworms.

Cluster flies have yellow hairs and will aggregate on the sunny side of the structure in late fall.

Cluster flies have yellow hairs and will aggregate on the sunny side of the structure in late fall. Photo by Jody Green, Nebraska Extension.

Boxelder bugs are very common outdoors, active in all stages, feeding on juices from the boxelder tree and sometimes maples. The adults are ½ inch long, bright red or black, with narrow reddish lines on the back. They are a nuisance pest in the fall when they gather on the building, but they do little damage indoors, until the spring when they emerge inside the house. They seek to get outside to lay their eggs on boxelder trees. They have been known to produce an unpleasant odor when crushed and may puncture skin with their mouthparts if handled.

Boxelder bug

Boxelder bug basking in the warmth of the sunshine. Photo by Jim Kalisch, UNL Department of Entomology.

Western conifer/pine seed bug is a dull reddish-brown bug with sucking mouthparts, some patterns on the back and antennae almost as long as its body. It can be distinguished from the boxelder bug and other true bugs (like stink bugs) by the wide section of the lower hind leg.  Outdoors they are found near evergreen trees feeding on the sap from green cones, twigs, seed pulp and sometimes pine needles. Indoors, they may release a terrible odor as defensive mechanism, but are not known to damage property, hurt people or pets.

Western conifer seed bug or pine seed bug

Western conifer seed bug or pine seed bug. Photo by Jim Kalisch, UNL Department of Entomology.

Pest Management for Overwintering Pests

  • Secure points of entry, especially under exterior doors and windows; caulk and seal around utility entrances, cracks in foundation, gaps around fascia boards and install screens over pipes and enclosed chimney caps.
  • Install weather-stripping under and around doors and windows, replace rubber seals along bottom of garage doors, and repair or replace damaged screens. Keep doors closed when not in use.
  • Physically remove individual pests: trap, sweep and vacuum up bodies.
  • Practice proper vacuum management: empty contents into outdoor trash receptacle, discard the bag or wash and dry the canister after each use.
  • Insecticide application in the form of an outdoor residual treatment may be warranted for major infestations boxelder bugs and cluster flies in the fall, but timing is key and we recommend hiring a professional.
  • If you are present during the time insects are aggregating and crawling up the exterior walls, spraying them with soapy water may knock them down and prevent them from entering the building.
  • If you know that you have overwintering populations in wall voids or attic, do not attempt to treat and kill them while they are hibernating. Dead bodies will attract secondary pests such as dermestid beetles.

Confessions of an Extension Entomologist: Our new house seemed to be a fantastic overwintering site for all the above pests. I watched many a lady beetle, cluster fly and boxelder bug come to rest on the warmth of our blue siding on the south exterior wall and then quickly sneak inside a crack before I could get a specimen or photo. I didn’t have the time or effort to try and fight them this year, so I’ll let them stay for the winter and practice great patience in the spring when they want out. I’ll have my Shop Vac ready!

Keep calm and respect the critters,







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