Fall boxelder bugs and cluster flies on Cicero brick

Every fall the black and orange boxelder bugs appear on the sunny brick of Cicero bungalows, and by winter cluster flies are waking at the upstairs windows. Both are overwintering insects, and both key on the same thing: the warm south-facing brick that this neighborhood has in abundance.

Why the brick draws them

Boxelder bugs feed on box elder and maple trees through the summer, and Cicero has plenty along the parkways and in the yards. As nights cool, the bugs look for a warm surface to gather on before winter, and the wide south and west brick faces of bungalows and two-flats hold the afternoon heat perfectly. That is why one wall can be covered while the shaded side stays clear.

Cluster flies do the same thing more quietly. They gather on the warm walls, slip into the gaps around windows and soffits, and settle in wall voids and attics for winter. On a mild winter day they wake and drift toward the light at upstairs windows, which is when most people first notice them.

Sealing before the cold

The most useful thing a homeowner can do is close the gaps the insects use to get from the wall into the house. Caulk around window and door frames, repair torn screens, and seal the openings around utility lines and where the soffit meets the wall. A bug that cannot get into the wall void cannot overwinter there.

Do the sealing in late summer or early fall, before the cluster begins in earnest. Once the insects are already in the walls, sealing traps some inside, so timing matters.

What treatment adds

An exterior treatment timed to the fall cluster, focused on the warm south and west walls, the eaves, and the window surrounds, knocks down the insects as they gather and reduces how many work inside. Paired with sealing, it is what keeps the winter window flies to a minimum. Inside, the safest response to the few that appear is simply to vacuum them up.

When to call

If the brick is covered every fall or the flies return at the windows each winter, an experienced local exterminator can time an exterior treatment to the cluster and point out the entry points worth sealing. Call 708-391-3858 and describe which walls they gather on.

References

• University of Minnesota Extension: Boxelder bugs
• University of Illinois Extension

Have this problem now? Call 708-391-3858 and describe it.