The Boxelder Bug, Boisea trivittata, is a common insect that often causes homeowner concern, especially when it aggregates in large numbers around structures during cooler months. This species is a true bug, and many true bugs share a distinctive warning coloration of black and red or orange. This pattern signals potential toxicity to predators and is frequently used by several unrelated species, leading to frequent misidentification. Accurately distinguishing the Boxelder Bug from its look-alikes is important for understanding their behavior and determining if intervention is necessary.
Identifying the True Boxelder Bug
The adult Boxelder Bug is typically an elongated oval shape, measuring around one-half inch (12 to 14 millimeters) in length. Its body is predominantly dark brown to black, highlighted by specific reddish-orange markings that serve as the primary identification feature. These markings include three thin, longitudinal lines running down the top of the thorax. A reddish-orange line is also clearly visible along the outer edge of each forewing when the wings are folded flat over the back.
The immature Boxelder Bugs, known as nymphs, lack fully developed wings and are much smaller than the adults. They are characterized by a striking, bright red abdomen. As they mature, black markings gradually develop on their head and thorax, and dark wing pads become apparent. All life stages are often seen congregating in large clusters, particularly on the sun-warmed, south- or west-facing sides of buildings during the fall and spring.
The Most Common Look-Alikes
Several other true bugs mimic the Boxelder Bug’s black and red coloration. One common mimic is the Small Milkweed Bug, Lygaeus kalmii, which is similar in size but displays a geometric pattern of black and red or orange. A close relative, the Large Milkweed Bug, Oncopeltus fasciatus, is typically more orange than black and has a solid, wide black band across the middle of its back. These two species are strongly associated with the milkweed plant, where they feed on the seeds.
Another frequently confused insect is the Red-Shouldered Bug, Jadera haematoloma, which shares the Boxelder Bug’s habit of feeding on tree seeds. This bug has a slender, grayish-black body and is about one-half inch long. Red-Shouldered Bugs are often found near goldenrain or chinaberry trees, which are their preferred hosts. While they share similar aggregating behavior, their specific markings are distinct from the Boxelder Bug’s characteristic linear pattern.
Key Differences in Appearance and Behavior
The easiest way to differentiate these species is by examining the specific pattern of the colored markings on the back. The Boxelder Bug is primarily black, with its red coloration appearing as narrow, linear accents, including the three stripes on the thorax. In contrast, the Small Milkweed Bug has a black body with a prominent, orange-red pattern that often forms a distinct “X” shape on its folded wings, creating a much bolder geometric design. The Large Milkweed Bug is distinctly patterned with a wide, solid black band separating two large orange or red patches on the back.
The Red-Shouldered Bug is best identified by the location of its red markings, which are limited to the edges of the “shoulders” or the margins of the pronotum. Unlike the Boxelder Bug, this species has no red lines along the outer edges of its wings. Behavioral differences also provide clues, as Boxelder Bugs are strongly attracted to boxelder, maple, and ash trees, and adults enter homes to overwinter. Milkweed Bugs, however, rarely enter homes, preferring to remain near their milkweed host plants.
Why Correct Identification Matters
Accurate identification is important because it dictates whether a species is a mere nuisance or potentially harmful to plants or structures. The Boxelder Bug is generally considered a nuisance pest due to its tendency to invade buildings in large numbers seeking winter shelter. While they do not bite, sting, or damage structures, their excrement can stain curtains and walls, and they release an unpleasant odor if crushed. Recognizing the specific pattern of the Boxelder Bug confirms the source of the household invasion.
Other look-alikes, such as the Milkweed Bugs, are almost exclusively found on their host plants and rarely pose a threat inside homes. These insects are often harmless in the landscape. Mistaking a harmless insect for a Boxelder Bug can lead to unnecessary or inappropriate actions. Understanding the subtle differences in markings and host plant preference ensures that any response is targeted only at the true nuisance species.