Species: Boxelder bug (Boisea trivittata); Large milkweed bug (Oncopeltus fasciatus)

What to look for

  1. Color and pattern. Adults are black with red lines running down the center and sides of the pronotum, the plate covering the thorax. Penn State describes brownish-black adults with three long stripes on the thorax and reddish orange wing margins and abdomen.
  2. Length. Kentucky says adults grow up to half an inch (12.7 mm). Penn State gives the adult as about 12 mm (about 1/2 inch) and somewhat flattened on top.
  3. Nymphs. Bright red, with dark red eyes on both nymphs and adults. Penn State gives first instar nymphs as approximately 1.3 mm (about 0.05 inch), wingless with black wing pads and bright red abdomens.
  4. The lookalike Kentucky names. A milkweed bug is roughly the same size and shares the colors, but boxelder bugs are mostly black with thin red stripes, while milkweed bugs are mostly orangish red broken by large patches or belt-like stripes of black.
  5. What tree it was on. Boxelder bugs rely on the seeds of female, fruit-producing boxelder trees, and are rarely found on male trees. They have also been reported on silver maples, sycamore maples, apple, pear and plum.
  6. Where it is sitting. Both Kentucky and Penn State describe large numbers congregating on the south side of trees, buildings and rocks exposed to the sun.

Where and when

Kentucky extension says boxelder bugs begin hatching in late May and stay active on host trees until early winter, and that they typically overwinter in leaf litter, though some cling to siding or find their way indoors. Penn State says they emerge from hibernation in late April to early May as boxelder buds open, and that Pennsylvania sees one to two generations a year depending on weather.

Does it matter?

Kentucky extension says boxelder bugs do not bite or sting humans or animals, that there is no evidence they transmit disease, and that because females need boxelder trees to lay eggs, they cannot form a long-term indoor infestation. Indoors they are an annoyance whose fecal matter may stain fabrics, and like stink bugs they release a pungent odor when crushed. Kentucky suggests sealing cracks around windows and doors.

Related guides

  • Asian lady beetle vs ladybug: Both are lady beetles.
  • Stink bug: The brown marmorated stink bug is a shield-shaped brown bug about 17 mm, or two thirds of an inch, long and almost as wide as it is long.
  • Earwig: An earwig is the flat, dark reddish brown insect with a pair of claw-like pincers at the rear.

Sources