The brown marmorated stink bug (BMSB), Halyomorpha halys, is a non-native insect introduced to North America from Asia. Since its arrival, it has become recognized both as a serious agricultural pest and a significant nuisance to homeowners across the United States. This pest’s status as a household problem stems from its behavior of aggregating in large numbers on and inside human dwellings. These insects seek out sheltered locations in the autumn, often entering homes and buildings by the hundreds or even thousands to survive the colder months. This mass invasion causes concern for residents across the regions where the bug has established a presence.
The Seasonal Timeline of Overwintering
The brown marmorated stink bug is present inside structures starting in late September or early October. As the days shorten and temperatures drop, adult bugs begin a dispersal phase, migrating from feeding sites to find suitable places to spend the winter. This initial movement involves large numbers of insects congregating on the sunny exterior walls of homes before finding entry points. They remain sequestered inside walls, attics, or crawl spaces throughout the winter season.
The bugs complete their emergence from these overwintering sites from late March through mid-May. The exact timing is dependent on local climate, specifically the sustained increase in ambient temperatures. While they are dormant for the coldest part of the winter, a sudden warm spell can cause a temporary, false emergence. These brief periods of activity are misleading, as the bugs quickly return to their sheltered locations when cold weather resumes. The full departure is only triggered by the persistent warmth of spring.
Why Stink Bugs Seek Shelter in Structures
The impulse to invade homes is driven by a biological requirement to enter a state of metabolic suppression known as diapause. This pre-programmed, dormant phase is necessary for adults to survive the harsh conditions of winter. Structures like houses, sheds, and garages provide the ideal environment for diapause, offering a cool, dry, and protected harbor that insulates the insects from lethal freezing temperatures. The bugs are not seeking to feed or reproduce while they are inside a home.
Their primary goal is to wait out the cold by slowing down their bodily functions. This behavior mimics their natural overwintering sites, which typically include rock crevices, the undersides of large stones, or the loose bark of dead standing trees. The insects often squeeze into very tight, dark spaces, such as behind baseboards or within wall voids, where they aggregate in large clusters. The modern dwelling acts as a highly effective, artificial substitute for these natural, thermally-buffered sites, significantly increasing their survival rate.
What Happens After They Leave
The adult bugs exit the structures in the spring, prompted by rising temperatures, often sustained above 50 to 60 degrees Fahrenheit. The emerging adults are in a reproductive state and immediately disperse into the surrounding landscape to find food and mates. Their first priority is to locate vegetation to begin feeding and replenish the energy reserves they depleted during the long winter diapause.
Following a period of feeding on early spring foliage, mating begins, and female adults start laying their characteristic barrel-shaped egg masses. These eggs are deposited on the undersides of leaves of various host plants, including ornamental trees and agricultural crops. Egg laying typically starts in May and continues through August. This spring migration ensures that the pests are truly “gone” from the house until the process begins again with the new adults in the following autumn.