Overview
Boxelder bugs (Boisea trivittata) are one of the most recognizable fall invaders in Montreal and across Quebec. Every autumn, as temperatures drop, enormous aggregations of these black-and-red insects gather on the sunny sides of buildings and squeeze inside through any available gap to overwinter. They do not bite, sting, breed indoors, or cause structural damage — but their sheer numbers, the staining their excrement leaves on walls and fabrics, and the foul odour they release when crushed make them a significant nuisance. A single south-facing wall can host thousands of bugs on a warm October afternoon.
How to Identify Boxelder Bugs
- Size: 11–14 mm long — roughly the size of a large apple seed
- Colour: Black with distinctive bright red or orange markings along the edges of the wings and thorax
- Wings: Flat, overlapping wings that form an X pattern on the back when at rest
- Nymphs: Bright red with black markings — very vivid and easily noticed
- Antennae: Long, thread-like, dark
- Odour: Releases a pungent, unpleasant odour when crushed or disturbed
- Aggregation behaviour: Clusters in large groups on warm, south-facing surfaces in fall
- Host trees: Associated with boxelder (Manitoba maple), silver maple, and ash trees
Biology & Seasonal Behaviour
- Overwintering: Adults overwinter in protected locations — inside wall voids, attics, and under siding
- Spring emergence: Adults emerge in April–May and fly to host trees to feed and breed
- Eggs: Females lay eggs in crevices of host tree bark in spring
- Nymphs: Hatch in May–June; bright red, develop through 5 instars over 6–8 weeks
- Adults: New generation of adults appears in late July–August
- Fall aggregation: Adults aggregate on warm surfaces in September–October before seeking overwintering sites
- Diet: Feed on seeds of boxelder, silver maple, and ash trees — do not feed indoors
- Indoor survival: Do not reproduce indoors; gradually die off over winter if they cannot exit
Risks & Nuisance
- No health risk: Boxelder bugs do not bite, sting, or transmit disease
- Staining: Their excrement leaves reddish-brown stains on walls, curtains, and upholstery that can be difficult to remove
- Odour: Crushed bugs release a strong, unpleasant smell
- Plant damage: Minor feeding damage to host trees — not significant enough to harm healthy trees
- Nuisance: Large numbers indoors in winter as they emerge on warm days seeking warmth
- No structural damage: Do not chew wood, fabric, or food
Signs of Infestation
- Large aggregations on south-facing walls, fences, and tree trunks in September–October
- Bugs clustering around window frames, door frames, and utility penetrations
- Individual bugs found indoors throughout winter, especially on warm sunny days
- Reddish-brown excrement stains on walls, window sills, and curtains
- Presence of boxelder (Manitoba maple), silver maple, or ash trees on or near the property
- Bugs emerging from wall voids and baseboards in spring
Prevention
- Seal all gaps around windows, doors, utility penetrations, and soffits before September
- Install door sweeps on all exterior doors
- Repair damaged window screens
- Caulk gaps around window and door frames
- Seal gaps where utility lines enter the building
- Remove or reduce boxelder (Manitoba maple) trees near the structure — the primary host
- Vacuum aggregations on exterior walls before they enter — do not crush them
- Apply exterior perimeter insecticide in late September before bugs begin entering
Professional Treatment
Professional boxelder bug treatment focuses on exclusion and exterior perimeter treatment. A residual insecticide is applied to the exterior walls, foundation, and entry points in late September before the fall aggregation peaks. Interior treatment involves vacuuming aggregations and applying crack-and-crevice insecticide to wall voids where bugs are overwintering. Do NOT crush boxelder bugs — the odour attracts more bugs and the staining is difficult to remove. Call 514-809-1999 for fall perimeter treatment.