Boxelder Bugs in Boulder: Fall Entry and How to Seal It

What do boxelder bugs do on a Boulder house?

They gather on the outside of a house on warm days, usually on the south and west walls, and they look for a crack to enter through. They do not feed on the house or on stored food.

The gathering is a staging behaviour. Warm sunlight on a wall draws the insects out of the sheltered places where they spend the cold months, and they wait on the surface until a gap appears. CSU Extension describes this pattern for Colorado on warm fall and winter days, and the gathering often looks sudden because it follows a stretch of warm weather.

A homeowner may see dozens of bugs on one wall and assume the house is infested. In most cases the bugs are outside, waiting. The inside problem is the bugs that have already found a crack, and that is a smaller number that is easier to handle once the entry points are closed.

Where do they get in?

They enter through cracks and gaps around doors and windows, at siding joints and at soffit and vent openings. The gaps are usually small, and the most common ones are at the corners of window frames.

A window frame is made of several pieces that meet at corners and at the sill. Over the years the caulk shrinks or cracks, and a gap opens that is hard to see from a distance. The technician checks each corner on the south and west faces with a light from the inside to show where light passes through.

Soffit joints and vents are the second group. A vent with a torn screen is an open door, and a soffit joint can move with the seasons. The cable and pipe penetrations are the third group, and they are often overlooked because they look finished from the outside.

What is the right order of control?

Seal the entry points first, remove the bugs already inside by vacuum, and then consider a residual on the foundation if the bugs still gather. That order keeps the job focused on the lasting step.

Sealing comes first because it stops the route. A residual placed on a wall with an open gap only treats the surface, and the insects still come through the gap. Once the gaps are sealed, the number arriving drops, and the residual, if it is used, has less work to do.

Vacuuming comes next. A shop vacuum removes the bugs inside without a stain. CSU advises vacuuming rather than crushing, because the fluid can stain light fabric and leaves an odour. Seal the vacuum bag and remove it from the house so the bugs do not crawl back out.

When is a residual worth adding?

A residual is the longer-term option CSU describes. It is worth adding where the bugs keep gathering after the gaps are sealed, and it is placed according to the label, usually along the foundation or on the lower courses of the siding.

The residual works on the insects that touch or pass over the treated surface. It does not close a gap, and it does not stop a bug that enters through a crack that is not treated. That is why CSU presents it as a longer-term option rather than the first step.

The placement is recorded so the next check can see whether the residual is still working. Weather can break a residual down, and a repeat application may be needed on the schedule set by the technician.

When should the work be done?

The sealing is best finished before the first warm autumn spell, usually in late summer or early autumn. A check in the spring looks for gaps that opened over the cold months.

A sealing job done in August has not yet been tested by a warm October, so a follow-up in the autumn is useful. The follow-up looks at the same frames and joints and records any new gap. A homeowner who does the sealing themselves can use the same list.

Spring is the time to check the screens and the vents, which are often damaged by the winter and by ice. A torn screen in spring is an open route for the next autumn.

What can a homeowner do this week?

Walk the south and west faces and look for light coming through a frame, a draught at a door and a torn screen. Note the dates of any sightings and vacuum the bugs inside.

A walk with a flashlight on a cloudy day is the simplest check. Hold the light on the outside of a window frame and look for brightness on the inside. Mark each spot with a piece of painter’s tape, so the sealing can be done quickly afterward.

Keep the mulch below the siding and the downspouts clear, since moisture and debris near the wall do not change the boxelder bugs directly, but they make the wall a more hospitable place for insects generally. Those are small changes that cost nothing.

How does a seasonal sealing plan run through the year?

The plan starts with a check in late summer, sealing in early autumn, a follow-up in the warm spells of autumn and a spring look at the screens and vents. Each step has a purpose and each one prepares the next.

In late summer, the technician or the homeowner walks the outside, marks the frames and joints, and notes any gap that opened over the hot months. That list becomes the sealing job for early autumn, when the sealing material can be applied in mild weather.

A follow-up in the warm spells of autumn checks whether any bugs are still getting in and whether the sealing is holding. Spring is the time to replace torn screens and look at the vents, because winter ice and wind often damage them. The plan records each step and its date.

What does a homeowner need to avoid?

Avoid crushing the bugs on light surfaces, avoid spraying indoors without a reason and avoid sealing a gap while the bugs are inside the wall without first checking the route.

Crushing a boxelder bug releases a fluid that can stain light fabric and paint. Use a vacuum, and empty it outdoors into a sealed bag. Spraying a living space is not the first step CSU describes, and it can leave a residue on surfaces where children and pets spend time.

Sealing a gap that a bug is using at that moment can trap it inside the wall, where it will come out on another warm day. Seal the gap once the room is clear, and check the route first.

Which parts of the house should a homeowner check first?

Start with the south and west faces, then the north and east, then the foundation and the soffit. The sun drives the gathering, so the sunny walls are where the bugs are most often seen and where the sealing matters most.

On the south face, check every window and door frame, the siding joints at each corner and the vents in the soffit. Use a flashlight from inside on a bright day to see where light passes through the frame, and mark each spot with a piece of tape.

On the west face, check the same items and also the cable and pipe penetrations, which are often hidden by a meter box or a bracket. The north and east faces get less sun, so the gathering is less common there, but a gap is still a gap, and it is worth checking.

The foundation is the last check. A crack where the siding meets the foundation is a route for insects into the sill space, and the sealing material should match the surface.

How does a homeowner know the sealing has worked?

The sealing has worked when the bugs stop getting inside through the same route over a full warm season. The homeowner can check with a light, a note of sightings and a follow-up in autumn.

A simple test is to walk the house in the evening, after the sun has left the walls, and note any bug seen inside. A bug inside a room that was sealed points to a gap that was missed. A bug in a room with no gap may have come in through a door that was opened on a warm day.

A follow-up visit in the next autumn compares the sightings with the previous year. If the number has fallen, the plan is working. If the number has stayed the same, the technician should look at the gaps again and consider whether a residual adds anything.

What about a house with a maple or box elder nearby?

A tree close to the house can increase the number of bugs that gather on the walls, because the trees are part of the insects’ life cycle. That is an observation about the setting, and the sealing work is the same.

The sources in this file do not state how the trees affect the number of insects, so the guide does not give a figure. It notes that a house near a box elder or a maple is more likely to see the bugs on its walls, and it recommends that the sealing be checked earlier in the year for that house.

A homeowner who does not want to remove a tree can still reduce the entry points, which is the step that matters most. The tree does not decide the result on its own.