Stink bugs, such as the Brown Marmorated Stink Bug, do excrete waste. Like nearly all living organisms, these insects must eliminate metabolic waste and undigested food. This biological process is often confused with their well-known defensive mechanism, which is an entirely separate chemical discharge used for defense, not a byproduct of digestion.
Differentiating Excretion from Defensive Spray
The foul odor released by a stink bug is a specialized defensive secretion, not feces. This chemical defense is produced and stored in glands located on the underside of the thorax in adults, or on the dorsal abdomen in nymphs. When disturbed, the insect expels this pungent, oily liquid through small openings as a deterrent to predators like birds and lizards.
The primary compounds responsible for the characteristic odor are aldehydes, specifically trans-2-octenal and trans-2-decenal. These chemicals are highly volatile, meaning they easily evaporate into the air, making the smell readily noticeable and potent. This chemical cocktail is distinct from the digestive waste, which is generally dry and has no strong odor.
The Internal Biology of Waste Elimination
The process of waste elimination involves a system analogous to kidneys in vertebrates, known as the Malpighian tubules. These slender, branching structures extend from the digestive tract, filtering metabolic waste, salts, and water from the insect’s circulatory fluid, called hemolymph.
The tubules convert nitrogenous wastes into uric acid. This uric acid and other filtered waste travel to the hindgut, where they combine with undigested food material. The hindgut reabsorbs necessary water and salts before the final, concentrated waste product is excreted.
Identifying Stink Bug Droppings
Stink bug droppings, or frass, are a physical sign of their presence and look nothing like the defensive spray. The waste appears as tiny, dark specks, typically dark brown or black. These droppings are usually dry and have a granular texture.
These spots are most commonly found in areas where the insects congregate or rest, such as on window sills, along curtain edges, or on walls near entry points where they seek to overwinter. While the droppings are not structurally damaging, they can leave noticeable stains on surfaces. Their presence is a clear indication that stink bugs have been occupying the space.