Field plate · Boisea trivittata
Boxelder bug
Black-and-red flat bugs that mass on sunny walls near boxelder and maple trees each autumn.
Plate 9 of 28. Drawn in profile to the identification marks named below; the scale rule is a drawing convention — sizes are given in the text.
Identification marks
Flat, oval, about 12 mm, black with three red lengthwise stripes on the thorax and red veins and margins on the wings. The nymphs are smaller and bright red.
Look-alikes it is confused with
Eastern boxelder bug look-alikes include the imported stink bugs (shield-shaped, no red striping) and milkweed bugs (different host plants, bolder orange pattern).
Where it hides in a house
Sun-warmed walls, rock faces, and fences near seed-bearing boxelder trees, then window frames, siding, and sheds for the winter. Indoors they drift out singly and do not settle.
Health & property risk, stated plainly
No bite of consequence, no disease, no indoor breeding, and no structural damage. Their droppings can spot curtains and pale siding, and sheer numbers make a wall look alarming.
What actually works
Remove or replace a problem female boxelder tree where that is practical, rake seed litter, and seal the wall in late summer. A soap-and-water spray knocks down outdoor gatherings without residue.
What doesn't
Broad indoor spraying for a bug that is only passing the winter, and panic treatments of the whole yard when the gathering is on one warm wall.
When to call a licensed pro
Call a pro when the tree and wall combination produces unmanageable numbers yearly, or when sealing work is at height.