These sucking insects (family Pentatomidae) are shield shaped. Their common name is because when disturbed they can excrete fluid with an unpleasant odor. Over 50 stink bug species occur in California. Some are beneficial predators of pest insects, including Perillus and Podisus species, such as spined soldier bug (Podisus maculiventris) and twospotted stink bug (Perillus bioculatus). Most stink bugs are plant feeders and most any plant can at least occasionally be fed upon by these sporadic pests.
Adult stink bugs are distinguished from other insects by the large triangle shape with a rounded rear end (scutellum) that occurs on top of the back, covering the rear of the thorax and front portion of the abdomen. In comparison with most insects, adults are wider relative to their length.
Stink bug eggs are barrel shaped with circular caps and are generally laid in a group commonly on leaves. The hatching nymphs are nearly round and their coloration may or may not resemble that of adults of the species. Nymph coloration and markings commonly vary among instars. Coloration of adults and nymphs varies by the species from a mix of bright or contrasting colors (e.g., black and orange) to a relatively uniform color, such as mostly brown, gray, or green. For more photos of common species, see CalPhotos from UC Berkeley and Hemiptera (Bugs) from Orange County, California from UC Irvine.
Bagrada hilaris is an introduced pest that feeds on and damages crucifers (Brassicaceae). It is particularly damaging to crucifer seedlings and leafy mustard greens. It can also be a nuisance in landscapes containing mustards or sweet alyssum. Bagrada bugs are mostly black and orange and the adults and older nymphs have white markings. Eggs are oval, and creamy white when first laid and turn orange as they age. Females lay eggs in the soil beneath host plants but may also oviposit on leaves. First instars have a bright red abdomen and thorax. Later instars are black and orangish red.
In comparison with adults of the similar-looking harlequin bug (see below), Bagrada bug adults are smaller (about 1/4 inch long). Adults and late instars have white markings while harlequin bug has no white. Bagrada bug nymphs superficially resemble those of several other stink bug species including the consperse stink bug (see below) and harlequin bug. See Bagrada Bug, Bagrada hilaris and Bagrada hilaris (Hemiptera: Pentatomidae), An Invasive Stink Bug Attacking Cole Crops in the Southwestern United States for more information.
Brown marmorated stink bug. Halyomorpha halys is an invasive species and pest of various tree fruits and fruiting vegetables. Adults are mottled brown or gray and about 5/8 inch long. Alternating dark and pale markings along the abdominal margins help to distinguish adults from those of other stink bug species.
Brown marmorated stink bug most resembles the rough stink bug, Brochymena sulcata, which is a beneficial predator. However adults of rough stink bug have two white bands on their otherwise blackish, hind legs and lack bands on the antennae. Brown marmorated stink bug adults have one white band on the hind legs and white bands on their dark antennae. Consperse stink bug resembles both of these species but has no white bands on its legs. For more on how to discriminate these three species see Brown Marmorated Stink Bug Provisional Guidelines for Apple.
Consperse stink bug. Euschistus conspersus adults are about 1/2 inch long and grayish brown to green. They resemble adults of brown marmorated stink bug and rough stink bug. However, unlike the white leg bands described above for these species, consperse stink bug has grayish legs with black speckles.
Harlequin bug. Murgantia histrionica adults have mostly black bodies with a variable pattern of orange, red, and yellow. Eggs are grayish yellow with two black bands. First instars have a light orange abdomen with black markings and a brown head and thorax. Later instars develop more black on their bodies with contrasting bright red and yellow. This species resembles Bagrada bug, but it lacks Bagrada bug's characteristic white markings. Adult harlequin bugs are 1/3 inch or longer, while adult Bagrada bugs are 1/4 inch or shorter.
Say stink bug. Chlorochroa sayi adults and nymphs are bluish green with a paler green or whitish margin on their abdomen and thorax. Adults are 2/5 to 2/3 inches long.
Southern green stink bug. Nezara viridula adults are bright green and about 1/2 inch long. Early instars are pale yellow and late instars are green. Eggs are white to pale yellow.
Stink bugs develop through three life stages: egg, nymph, and adult. Adult females lay barrel-shaped eggs in clusters on foliage or litter on the ground. The nymphs commonly remain close together at first but scatter as they grow. They develop through five increasingly larger instars gradually developing wing pads before the adult emerges from the skin of the last instar.
Overwintering is on the ground in liter. Stink bugs commonly become active beginning in late winter or spring. Stink bug infestations commonly originate when adults fly in from weedy areas, such as along creeks and sloughs where blackberries grow. In years with a lot of spring rain and late weed growth stink bugs can become especially numerous and cause more widespread damage.
Adults suck and feed on plants with their strawlike mouthparts. Stink bugs attack a variety of fruits, nuts, and vegetables from beans to tomatoes and apples to stone fruits. Their sucking feeding can cause blemishes in fleshy plant parts and mar plant appearance with drops of dark excrement. Their feeding causes blemishes, dark pinpricks, and other discoloration on fruit and fleshy vegetables. Damaged tissues become pithy and white and remain firm instead of developing their normal color and tasty texture. Where stink bugs earlier fed, fruit can become distorted as it grows.
On green tomatoes, damage appears as dark pinpricks surrounded by a light discolored area that remains green or turns yellow when fruit ripen. The surface of feeding spots can become discolored and tissue beneath feeding spots on fruit (e.g., apples, pears, and tomatoes) becomes pithy and brown or white and remains firm as the fruit ripens. On peaches, fruit turns brown and corky. Fed upon seedlings and leafy greens may develop a tattered appearance. Except for the fruit and nuts established woody plants are not seriously harmed by the feeding of stink bugs in most situations.
Because other true bugs such as lygus bugs can cause virtually the same damage, the particular cause of plant injury may not be reliably determined unless the bugs themselves were observed feeding on the plants.
Handpick the bugs and their eggs from small plants. Eliminate groundcovers and other herbaceous vegetation such as weeds in early spring before stink bugs become abundant there and then move to alternative hosts.
Insecticides are generally not recommended in gardens and landscapes for stink bugs. Commonly stink bug feeding damage does not become apparent until after plant tissues grow, by which time the stink bugs may no longer be present. Parasites and general predators can contribute to the control of some stink bug species if natural enemies are not destroyed, such as by the application of persistent, broad-spectrum insecticides. See Protecting Natural Enemies and Pollinators for tips on preserving parasites and predators.
If bugs are coming indoors, see Integrated Pest Management Checklist for Early Care and Education Programs from UC San Francisco. This lists steps to take in and around buildings to eliminate or greatly reduce insects coming inside.
Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).