Brown Marmorated Stink Bug: An Emerging Threat to Pacific Northwest Agriculture

Nik G. Wiman
Latest revision: 
March 2023

The brown marmorated stink bug (BMSB) (Halyomorpha halys) (Hemiptera: Pentatomidae) is a highly damaging invasive crop pest that feeds on a wide range of plants (more than 170 species), has strong capacity for dispersal and population increase, and until recently had no major natural enemies in the Pacific Northwest. The brown marmorated stink bug was first observed in the United States in Allentown, Pennsylvania, in 1996, and is now found in most U.S. states, southern Canada and Europe. In the Pacific Northwest, BMSB was discovered in Portland in 2004, and in Vancouver, Washington in 2010. There have been multiple introductions of the pest from Asia into the region, as well as introductions of BMSB inadvertently brought from the eastern U.S in vehicles and freight. At this point in time, this pest has become widely distributed across the region, and it should not be surprising to encounter BMSB anywhere in the Pacific Northwest in urban environments. Its distribution tends to be more limited to cities and towns in more arid environments east of the Cascades where there are concentrations of suitable ornamental and other trees and plants that can be used as a food source. West of the Cascades, BMSB is widely naturalized in forest environments where it can subsist on wild host plant species, particularly maples and ash. The agricultural problems have been most severe in the Columbia Gorge, Willamette Valley, and in the Umpqua and Rogue Valleys. Commercial crops that have sustained damage from BMSB include hazelnut, apple, pear, peach, blueberry, and cherry. Wine grapes may also be attacked throughout the pest’s established range, but grapes are typically not as susceptible to the damage. Vegetable crops are susceptible to BMSB, but damage to commercial vegetable crops has been minimal to date. Nuisance problems can be severe anywhere BMSB is established. Nuisance problems encompass damage to noncommercial crops such as backyard orchards and vegetable gardens, aggregation on homes and other structures in the fall, and infestation of homes during winter. Nuisance problems are particularly severe in the greater Portland-Vancouver area and outlying communities, and more recently severe nuisance problems have been reported in the Eugene area.

Pest description and life cycle

Adult BMSB are approximately 17 mm long, generally a mottled (“marmorated”) brown in color on the back, but coloration on the ventral (bottom) abdomen is variable, and can be gray, yellow, green, or red. The distinct white bands on the otherwise darkly colored antennae are a key character for identification. They also have alternating dark and light bands on the dorsal (top) part of the abdomen that protrudes out beyond the folded wings and dark bands on the tips of the membranous sections of the wings. Male BMSB are smaller than females and have a small notch in the distal end (tip) of their abdomen. Brown marmorated stink bugs look most similar to two other genera of stink bugs common in the PNW, brown stink bugs (Euschistus spp.) and rough stink bugs (Brochymena spp.), but can be distinguished by the antennal bands and by having a smooth anterior (forward) margin of the thorax (shoulder), while the other species have drab antennae and/or rough or spined anterior margins on their thorax. The five immature stages of BMSB get larger and look more like the adult after each molt. The first instar is the smallest immature motile stage at about 2.4 mm in length. The fifth instar is approximately 12 mm in length. Immature BMSB have deep red eye color. Stink bugs have glands that emit a pungent aroma that resembles cilantro when they are disturbed or crushed.

Adult BMSB overwinter in protected areas such as houses, outbuildings and outdoors in sheltered locations such as logs and rock outcrops. When the weather warms up in the spring, BMSB exit their overwintering sites and disperse to vegetation to feed and reproduce. Many ornamental and naturalized plant species are important host plants, such as catalpa (Catalpa speciosa), tree-of-heaven (Ailanthus altissima), female English holly (Ilex aquifolium), Oregon grape (Mahonia aquifolium), lilac (Syringa spp.), dogwoods (Cornus spp.) mountain ash (Sorbus aucuparia), Oregon ash (Fraxinus latifolia), empress tree (Paulownia tomentosa), various maples, particularly big leaf maple (Acer macrophyllum). BMSB will feed on developing buds, fruit, and trunks of thin-barked trees, such as maple and peach. BMSB also feeds on corn, pepper, tomato, green beans, peas, and a wide range of other vegetable plants. The eggs are laid in clusters ranging in number from 25-30 eggs per egg mass (28 on average), are typically blue-green, and are attached to the underside of leaves. Eggs are most easily detected on broad-leaf hosts (e.g., Catalpa, Paulownia). After the eggs hatch, immature BMSB will molt five times as they mature into adults. Two generations per season can occur in Oregon.

The BMSB, like other plant-feeding stink bugs, damages plants during feeding. All nymphal stages and adults can cause damage except for the first instar nymphs, which feed on the egg mass. Stink bugs feed by inserting their piercing-sucking mouthparts (stylets) into plant tissue, secreting digestive saliva, and then extracting the digested plant fluids. The extraction of plant fluids following the injection of the saliva results in deformed plant parts, loss of turgor, and occasionally aborted plant ovaries (which can cause empty hazelnut shells, or blanks). It is frequently observed that BMSB feeding on fruit and vegetables results in pithy, loose cell textured tissue surrounding the feeding site (corky tissue). BMSB feeding on apples and pears close to harvest may not readily show apparent damage. However, damage worsens during storage so that apparently undamaged fruit comes out of storage full of brown spots. Damage to fruits and nuts may not be apparent without cutting away the skin of the fruit or shelling the nut. BMSB damage can sometimes be difficult to distinguish from fruit physiological problems or nutrient deficiencies such as cork spot and bitter pit. However, pithy corking damage from BMSB is typically only located within 1 cm of the fruit surface, whereas the aforementioned disorders generally have corky tissue throughout the fruit. See for a printed BMSB identification guide with images of damage.

Sampling and Management

During the summer and fall months, BMSB can be sampled by visual observation of adults, nymphs, and egg masses on the crop or by use of beating trays to collect them from plants. Timed visual observations or other metrics of effort can help standardize samples. Typically, chances of successfully sampling BMSB on host plants decrease during hot weather when the insects become very active and beat sampling will be most effective early in the morning when the insects are cool. Pheromone traps are available commercially and can be used for monitoring, but captures of BMSB on traps may not correlate well with crop damage. However, particularly early in the season, any captures of BMSB on traps near the crop is reason for concern. Traps work best in the late summer and fall months in areas where BMSB are present in higher numbers and more receptive to aggregation pheromone. Traps or pheromone lures should not be placed in the crop as an aggregation of BMSB around the trap can cause more damage than would naturally occur. Rather, place monitoring traps along crop perimeters to better understand immigration of BMSB.

There are no established treatment thresholds for BMSB in the Pacific Northwest, but given the severity of damage that occurs when populations are noticeable, growers readily spray insecticides. Current insecticide programs are based upon pyrethroid, carbamate, organophosphate, and neonicotinoid insecticides. All of these insecticides are disruptive to various natural enemies and have the potential to cause secondary pest outbreaks. Gardeners and growers with small plots may be able to exclude BMSB with fine netting, but this approach is not feasible for larger farms. Alternate row or perimeter treatments can give control without being as disruptive as cover sprays. Vegetation along farm borders can provide a refuge for BMSB and may need to be managed to reduce pressure on a crop. Treatment of crop plants with particle films are an alternative to broad-spectrum insecticides to deter BMSB, and kaolin clay is available as a tool for both home and commercial use (“Surround” and “Surround at Home”).

Biological control of BMSB was initiated with the adventive arrival of the samurai wasp (Trissolcus japonicus) to the PNW. This parasitoid wasp attacks and parasitizes BMSB eggs and has a parasitism rate of approximately 60-70% of BMSB egg masses in its native range of China, with observed rates as high as 90%. Wild populations of the wasp were first recorded in Vancouver WA in 2015, in Portland OR in 2016, and in Beaverton OR and Walla Walla WA in 2017. Since 2016, samurai wasps have been widely redistributed around the region and are becoming established. Urban or riparian areas appear to be the ideal habitats for samurai wasp establishment. As the current range of samurai wasp continues to expand through both assisted and unassisted means, it is expected that this wasp will reduce BMSB populations locally. Several northeastern and mid-Atlantic states also have wild samurai wasp populations. Adults are 1.5 mm, black, prefer shaded areas, and have been spotted on BMSB egg masses laid on ornamental trees including maple, hawthorn (Crataegus spp.), and boxelder, as well as catalpa and hazelnuts trees. Although the wasp’s small size makes it difficult to detect, several signs indicate the presence of samurai wasp or other parasitoids. Wasps can be recognized by their “guarding behavior” – walking on and laying eggs in BMSB egg masses. Parasitized egg masses will turn a dark black color 3-5 days after being attacked. After adult parasitoids emerge from the eggs by using their mandibles to chew their way out, the eggs are left with uneven circular holes. For further detail on recognizing parasitized egg masses, see EM 9164 (link below). Generalist arthropod predators can also feed on BMSB egg masses and nymphs. A predatory wasp in the genus Astata has been widely observed in Willamette Valley. The female will sting and carry off late instar nymphs from the host plants to a nest in the soil where she uses the dead BMSB to feed her offspring. Nests are less commonly observed than the adults.

Brown marmorated stink bugs will overwinter in homes, sometimes in extraordinary numbers. This can be a good opportunity to reduce the local population that will disperse to nearby crops the following spring. From late August through November, BMSB aggregate on sides of houses and buildings. They then work their way into the buildings through cracks and other openings. They can be in a semi-dormant state during most of the winter, but warm spells cause them to move around and become more noticeable. They often appear in the home interior after working their way in from unfinished spaces entered from the outside. The best way to prevent them from entering homes is to seal all the openings with caulking or other material to exclude them. Once they are on or in a structure, vacuuming them is the best way to capture and remove BMSB. A rigid extension hose for a shop-vac can be an effective way to remove BMSB from eaves and walls of houses. Crushing them can cause them to release their defensive aroma, which is disagreeable and lingers for a time. Note that vacuuming BMSB can permanently impart the defensive odor to the machine.

For further information and to report BMSB or Samurai Wasp:

Report sightings and nuisance or agricultural problems from brown marmorated stink bug. Oregon State University.

Management of brown marmorated stink bug in US specialty crops and information for homeowners.

Samurai wasp: promising egg parasitoid for management of brown marmorated stink bug (BMSB). EM 9164: