The Western Conifer Seed Bug (Leptoglossus occidentalis) is a common insect often noticed when it seeks refuge indoors for the winter. These true bugs complete a life cycle that typically spans about one year in temperate climates, producing a single generation annually. This duration is defined by a prolonged adult phase, which includes a period of dormancy. Their existence is closely linked to conifer trees, as both immature and mature insects feed primarily on the seeds within the cones.
Identifying the Western Conifer Seed Bug
The adult Western Conifer Seed Bug is a large insect, measuring approximately three-quarters of an inch (15 to 20 mm) in length, with a reddish-brown to dark brown coloration. A distinctive characteristic is a faint white or yellowish zigzag line across the middle of its wings. It belongs to the family Coreidae, commonly called leaf-footed bugs, due to the flattened, leaf-like expansions on their hind legs.
When the insect takes flight, it produces a loud buzzing sound. The wings lift to reveal bright patches on the abdomen, typically yellow or light orange with five transverse black patches. The bug uses its piercing-sucking mouthparts to feed on the developing seeds of various conifers, such as pine, spruce, and fir trees. If disturbed, it may release a pungent odor as a defense mechanism.
The species is native to western North America, from the Pacific Coast to the Rocky Mountains. It has significantly expanded its range across the entire United States, Canada, Europe, and Asia, making it a common sight far beyond its original habitat.
Duration of Developmental Stages
The initial stages of the life cycle occur entirely outdoors on conifer host plants during the warmer months. Following mating in the spring, the female lays small, cylindrical, brown eggs, about two millimeters long. These eggs are laid in rows or clusters, usually glued along the needles of conifer trees.
The egg stage is brief, lasting approximately ten days before hatching. The immature insects, known as nymphs, emerge and begin to feed immediately. Nymphs are initially small and orange, gradually becoming reddish-brown as they mature.
The nymphal period involves an incomplete metamorphosis, progressing through five distinct growth phases, called instars, before reaching adulthood. Nymphs develop wing pads and grow in size, feeding on cone scales and developing seeds. The total time from the egg stage through all five instars to full adulthood is short, typically spanning six to eight weeks across the summer months. Adulthood is usually reached by mid-August.
Adult Lifespan and Seasonal Activity
The adult stage represents the longest portion of the bug’s life, lasting up to eight to ten months. Newly matured adults feed on ripening conifer seeds through late summer and early fall. As temperatures drop, they seek protected overwintering sites.
This search for shelter often brings the bug into contact with humans. They congregate on sunny walls before finding cracks and crevices to enter buildings. They seek unheated voids, such as attics, wall voids, and basements, where they enter a state of dormancy (diapause) to survive the winter.
The adult bugs emerge from overwintering sites in late May or early June when temperatures rise. They feed briefly on conifer flowers and new cones to regain energy before mating. Once the reproductive cycle is complete, the adults typically die, completing the approximate one-year duration of the single annual generation.