Adults are approximately 17 mm long (25 mm = one inch) and are shades of brown on both the upper and lower body surfaces (Fig. 1). They are the typical “shield” shape of other stink bugs, almost as wide as they are long. To distinguish them from other stink bugs, look for lighter bands on the antennae and darker bands on the membranous, overlapping part at the rear of the front pair of wings. They have patches of coppery or bluish-metallic colored puntures (small rounded depressions) on the head and pronotum. The name “stink bug” refers to the scent glands located on the dorsal surface of the abdomen and the underside of the thorax.
The eggs are elliptical (1.6 x 1.3 mm), light yellow t0 yellow-red with minute spines forming fine lines. They are attached, side-by-side, to the underside of leaves in masses of 20 to 30 eggs.
There are five nymphal instars (immature stages). They range in size from the first instar at 2.4 mm to the fifth instar that is 12 mm in length. The eyes are a deep red. The abdomen is a yellowish red in the first instar and progresses to off-white with reddish spots in the fifth instar. Protuberances are found before each of the abdominal scent glands on the dorsal surface. The legs, head and thorax are black. Spines are located on the femur, before each eye, and several on the lateral margins of the thorax (Fig. 2).
source: Brown Marmorated Stink Bug Factsheet, Penn State University Cooperative Extension