The insect commonly referred to as the “elder bug” is scientifically known as the boxelder bug. These small insects are a familiar sight throughout much of North America, particularly where their host trees are abundant. While they often gather in large numbers around residential structures, their presence is primarily a nuisance issue. Understanding their behavior clarifies why they are frequently encountered near human dwellings.
Identification and Classification
The boxelder bug (Boisea trivittata) belongs to the order Hemiptera, which includes all “true bugs.” Adults typically measure about one-half inch long with a narrow, flattened body shape. Their primary coloration is dark gray or black.
A defining feature is the bright reddish-orange or scarlet markings present on the body. These markings include three longitudinal stripes on the pronotum, the shield-like segment behind the head, and fine outlining on the edges of the wings. When the wings are folded, an inverted V-shape of red is often visible on the insect’s back.
They possess piercing-sucking mouthparts, which they use to feed on plant material. The common name “boxelder bug” is derived directly from their primary host plant, the boxelder tree (Acer negundo), a type of maple tree. This species is often confused with other true bugs that share similar red and black coloration.
Life Cycle and Seasonal Behavior
The life cycle of the boxelder bug involves three main stages: egg, nymph, and adult. Females lay small, reddish-brown eggs in the spring, typically in the crevices of bark on host trees or in ground litter. These eggs hatch into nymphs, which look like smaller, rounder versions of the adults but display a more prominent reddish abdomen.
Nymphs develop through five instars, or growth stages, over the summer months, gradually darkening and developing their characteristic wing pads. Throughout this time, both nymphs and adults primarily feed on the seeds, leaves, and sometimes the flowers of boxelder, maple, and ash trees. Their preference for boxelder seeds is strong, as this provides a rich source of nutrients.
The most noticeable behavior occurs in the autumn as temperatures begin to cool. Adults seek protected locations to enter a state of dormancy, known as overwintering. They aggregate in large, dense masses on sunny surfaces, especially the south-facing walls of rocks, fences, and homes, to maximize solar heat absorption.
This mass migration is the main reason people encounter them, as they search for sheltered cracks and crevices to survive the winter. Once a suitable entry point is found, they move inside structures, seeking dry, protected voids like wall spaces or attics. They do not feed during this dormant period, relying on stored energy reserves until spring when they emerge to reproduce.
Interaction and Nuisance Management
Despite their alarming appearance when gathered in large numbers, boxelder bugs pose no threat to human health or property structure. They do not bite people, though their piercing mouthparts may probe skin if they land on a person. They are not known to transmit any diseases to humans or pets.
Their classification as a nuisance pest stems from two main issues that occur when they enter homes. They release a foul-smelling chemical if crushed, which serves as a defense mechanism. The second issue is the staining caused by their bodily fluids, which can discolor curtains, walls, and fabrics if the insects are squashed indoors.
Management strategies focus primarily on exclusion to prevent them from entering structures, particularly during the late summer and early fall. Homeowners should inspect and seal all potential entry points before the cooling season begins. This includes repairing damaged window and door screens and sealing cracks around utility pipes and vents with caulk.
Weather stripping around doors and windows should be checked for integrity, as even small gaps allow entry when the bugs are searching for shelter. If insects have already entered the home, the simplest and most effective removal method is mechanical. A vacuum cleaner with a hose attachment can safely remove large numbers without crushing them, avoiding stains and odors.
Another simple technique involves sweeping them up or using a mixture of soap and water in a spray bottle to kill them upon contact. This approach addresses the immediate nuisance without introducing persistent chemical treatments into the living space. Removing boxelder tree seeds near the home can also reduce the local population over time, as it eliminates their primary food source.