Boxelder bugs

About

Black bugs about 1/2 inch long with red stripes, whose bright red nymphs crowd onto boxelder trees. They mostly feed on the seeds of female boxelders and are best known for gathering on sunny walls and slipping into homes in fall (CSU, UC IPM).

Spotting it

Damage

They do little harm to landscape plants. Western boxelder bugs sometimes pierce ripening apples, pears, cherries, peaches, plums and grapes, leaving dimpled spots with corky white flesh beneath, a lot like stink bug damage (WSU, UC IPM). Indoors they don't bite or breed, but they stain curtains and smell when crushed (UC IPM, Wisconsin).

How to find it early

Look for clusters of red nymphs and black adults on boxelder trunks and seed pods in summer, and on south- and west-facing walls in fall (CSU, UC IPM). Check fruit near boxelder trees as it ripens.

When

Spring through fall; they gather on houses in fall and on warm winter days.

Life cycle

Adults spend winter in wall cracks, buildings and debris and come out in spring to lay red eggs on host trees; nymphs hatch in 10 to 14 days and grow up over summer. There are one or two generations a year, depending on the region (Wisconsin, WSU, UC IPM).

What to do, gentlest first

Start at the top. Sprays are a last step; always read and follow the product label.

1. Seal the house and rake up seeds Prevent

   Before fall, seal cracks and gaps around windows, doors, siding, pipes and vents. Rake up boxelder, maple, ash and elm seeds, and clear a 6- to 10-foot strip of debris next to the foundation, especially on the south and west sides (UC IPM, Wisconsin).

2. Skip female boxelders Prevent

   Removing seed-bearing female boxelder trees is the most lasting fix, though bugs can fly in from several blocks away. When you plant a shade tree, choose something other than boxelder (CSU, UC IPM, Wisconsin).

3. Vacuum and hose By hand

   Vacuum bugs indoors, and wash them off walls and tree trunks with a strong stream of water (UC IPM).

4. Soapy water Organic spray

   Insecticidal soap, or a mix of laundry detergent and water, kills the bugs it hits directly (CSU, UC IPM).

   Soap works only on contact, and detergent mixes may injure plants (CSU). Don't use insecticide sprays inside the house; UC IPM says outdoor sprays are rarely better than vacuuming and hosing, and any spray should be kept out of storm drains.

Plants it goes after

Apple (backyard tree), Cherry (sweet and tart), Grape, Peach, Pear, Plum, Red maple, Strawberry, Sugar maple

Pressure by region

Two species share the name: Wisconsin and Colorado State describe the eastern boxelder bug, and UC IPM and WSU describe the western one, which WSU calls a sporadic, usually minor fruit pest across western North America. Numbers depend on nearby female boxelder trees. Ratings are estimates.

Ratings come from extension sources where available; some are estimates, as noted above.