Black and orange markings on a small insect are a form of aposematism, a warning signal to potential predators. This striking pattern indicates the insect may be unpalatable or toxic, a defense mechanism that helps ensure its survival. While this description fits several species of true bugs, two are most frequently encountered across North America due to their specific host plants and behaviors. Identifying the species requires a closer look at the insect’s size, the exact pattern of its markings, and the environment in which it is found.
Identifying the Small Milkweed Bug
The small milkweed bug, Lygaeus kalmii, is a slender insect measuring approximately 3/8 to 1/2 inch long. Its body is predominantly black, but the wings are adorned with a bold, reddish-orange pattern. This pattern forms a distinct X-shape on the back when the wings are folded. It also has a reddish-orange band across the segment directly behind its head, known as the pronotum.
This species is strongly associated with milkweed plants. Both nymphs and adults feed primarily on the seeds, ingesting cardenolide glycosides from the plant. These toxins make the insect taste foul. While they may occasionally be found on other plants, their population density is highest in areas abundant with milkweed. They are not considered household pests and do not seek to establish themselves within structures, though they may occasionally wander indoors seeking warmth.
Identifying the Common Box Elder Bug
The common box elder bug, Boisea trivittata, is a slightly more robust insect, typically reaching about 1/2 inch long. Its body is dark, brownish-black with bright reddish-orange lines outlining the edges of its wings and the sides of its body. A distinguishing mark is the presence of three thin, longitudinal red lines located on the thorax. When the wings are at rest, the red margins often form a V-shape or a subtle X-pattern on the insect’s back.
This species feeds almost exclusively on the seeds, leaves, and flowers of box elder trees, though they also utilize maple and ash trees. The box elder bug is known as a nuisance pest, particularly during the late summer and fall. As temperatures drop, large masses congregate on the warm, sunny sides of buildings, searching for overwintering shelter. Once inside, they remain dormant in wall voids and attics, though they may emerge into living spaces on warm winter days. They stain light-colored fabrics and surfaces with reddish-brown excrement and emit an unpleasant odor if crushed.
Distinguishing Between the Species and Safety Concerns
Distinguishing between the two species involves observing the specific pattern on their backs and their location. The small milkweed bug has a clearer, more open X-pattern and is rarely seen away from its host milkweed plants. Conversely, the box elder bug has the three distinct red lines on its thorax and is the species that invades homes in large numbers in the fall. If the insect is congregating on the sunny side of a house or crawling inside, it is almost certainly the box elder bug.
Neither the small milkweed bug nor the common box elder bug poses any direct threat to human health. They do not sting, carry diseases, or cause structural damage to homes. Box elder bugs rarely bite, and any instance is a defensive probing action causing only minor, momentary irritation. The primary safety concern relates to the box elder bug’s nuisance behavior and the potential for staining surfaces when they are accidentally smashed.