Boxelder Bugs

What Are Boxelder Bugs?

Boxelder bugs (Boisea trivittata) are common North American bugs known for their seasonal appearance, often becoming a nuisance pest when they invade homes and buildings in the fall.

Are Boxelder Bugs Harmful?

Boxelder bugs are not harmful in a traditional sense—they don’t bite, sting, transmit disease, or damage structures. However, they become a significant nuisance when they invade buildings en masse, stain surfaces, and create an unpleasant living environment. Here's a breakdown of their potential impact:

• Indoor Nuisance: They are problematic primarily because they invade buildings in large numbers to overwinter. Once inside, they may stain walls, curtains, upholstery, and clothing with their excrement, which leaves behind reddish-brown spots. When crushed, they emit a foul odor from their defensive scent glands. They can reappear indoors during warm winter days or early spring, creating a repeated annoyance.
• Biting or Stinging: Boxelder bugs do not bite or sting. They have piercing-sucking mouthparts used to feed on plant material, not to bite humans or animals.
• Toxicity: They are not venomous or toxic to humans or pets. However, if pets (particularly dogs or cats) eat them in large numbers, it may cause mild gastrointestinal upset due to the bugs’ bitter-tasting defensive compounds.
• Allergic Reactions: Very rarely, some sensitive individuals might experience mild skin irritation upon contact, but this is uncommon and not serious.
• No Structural Damage: Boxelder bugs do not chew wood, wiring, insulation, or building materials, so they do not cause structural damage to homes or buildings.

Boxelder bugs are classified as nuisance pests—meaning they’re more of a comfort and cleanliness issue than a threat. Despite not being dangerous, they can generate considerable frustration for homeowners because they cluster visibly in large numbers on sunny exterior walls in the fall. Their presence indoors is unsettling, especially in winter when homeowners don't expect insect activity. DIY control is often ineffective without professional exclusion and perimeter treatments.

Learn more: Do Boxelder Bugs Bite?

Boxelder Bug Appearance

Boxelder bugs are distinctive in appearance and fairly easy to identify, especially once you know what to look for:

Adult Boxelder Bugs

• Size: Approximately ½ inch (12–14 mm) long.
• Shape: Elongated, oval-shaped body with a somewhat flattened appearance.
• Color: Matte black or dark brown.
• Markings: Prominent reddish-orange or bright red lines outlining the thorax and running along the edges of the wings. When at rest, their folded wings form an “X” pattern on their back, highlighted by the red markings.
• Wings: Adults are fully winged and capable of flying, though they are slow and not particularly agile in flight.
• Antennae: Long, thin, and black, typically held out in front when moving.

Nymphs (Immature Boxelder Bugs)

• Size: Much smaller than adults, growing gradually through several stages (instars).
• Color: Bright red or crimson body in early stages. As they mature, their black legs and wing pads begin to develop.
• Shape: Oval and wingless in early stages; wings begin to appear and darken with each molt.

Behavioral Traits That Help Identify Them

• Tend to cluster in large groups on warm, sunlit surfaces—especially on the south and west sides of buildings, trees, and rocks.
• Commonly seen in late summer and fall as they prepare to overwinter.
• Inside homes, they often appear around windows, door frames, or warm indoor surfaces.

Boxelder Bug Habitat

Boxelder bugs are found throughout North America, particularly in areas where their preferred host trees—boxelder trees, along with maples and ash trees—are common. Their range, however, and their behavior can vary slightly based on climate, urban development, and seasonal patterns.

Geographic Distribution

• United States: Boxelder bugs are widespread across the eastern, central, and western U.S., with particularly heavy populations in the Midwest (Minnesota, Iowa, Illinois, Wisconsin), the Great Plains, parts of the Northeast, the Pacific Northwest, and some parts of the Southwest, especially where irrigation supports the growth of boxelder and maple trees.
• Canada: Common in southern Canada, especially in Ontario, Manitoba, and parts of British Columbia.
• Other Areas: While not native to all regions, they have been introduced in some non-native areas where boxelder trees have been planted ornamentally.

Outdoors (During Spring to Early Fall)

• Host Trees: Boxelder trees (Acer negundo) are their primary breeding and feeding site. They also feed on silver maples, sugar maples, and ash trees.
• Leaf Litter & Ground Cover: Nymphs and adults may be seen in mulched beds, garden debris, or beneath trees where fallen seeds accumulate.
• Sunny Surfaces: In late summer and fall, adults cluster on sun-warmed exterior surfaces, especially south- and west-facing walls, siding, rocks, tree trunks, patios, and decks.

Indoors (Fall through Early Spring)

• Boxelder bugs become a nuisance indoors as temperatures drop in autumn. They seek protected, warm places to overwinter.
• Entry Points may be gaps in siding, around windows, doors, and vents, or attics, crawl spaces, wall voids, and basements. Once inside, they may remain hidden for weeks or months, occasionally emerging on sunny winter days.

Preferred Conditions

• Warmth: Boxelder bugs are drawn to sun-exposed surfaces. This thermophilic behavior is why they often gather on the sunny side of buildings.
• Dry Environments: They prefer dry conditions for overwintering, avoiding damp or humid areas.
• Light-Colored Structures: Light-colored homes and buildings often attract more boxelder bugs because they retain heat from sunlight.