The brown marmorated stink bug, an invasive species, seeks shelter indoors during cooler months and carries a potent chemical weapon. When this insect feels threatened, it releases a volatile odor that many people find intensely unpleasant. However, the experience is not universal, and some individuals genuinely cannot perceive the scent at all, a phenomenon rooted in human genetic variation. The defensive odor, designed to deter predators, highlights the subtle complexities of human biology.
The Chemical Components of the Stink
The distinctive, offensive odor released by the brown marmorated stink bug is composed primarily of two volatile organic compounds: trans-2-decenal and trans-2-octenal. These molecules belong to a class of organic compounds known as aldehydes. Aldehydes are highly reactive and easily vaporize into the air, which contributes to the rapid and far-reaching effect of the bug’s defense mechanism.
The specific scent profile of these compounds is often described in widely varied terms, demonstrating the range of human olfactory perception. Many people report the odor as pungent, sharp, or bitter, while others compare it to the smell of strong herbs such as coriander or cilantro. Other descriptions include a waxy, fatty aroma reminiscent of fried chicken or pork tallow, or even a metallic or earthy note. This variation in description is an initial clue to how different people process the same chemical signal.
The Biological Basis of Specific Odor Blindness
The inability to smell the stink bug’s defensive chemicals is a form of specific anosmia, a condition where a person lacks the capacity to detect a single, particular odorant, even with otherwise normal smell function. Olfactory perception begins with the binding of airborne odor molecules to specialized olfactory receptor (OR) proteins located on nerve cells high inside the nose. Humans possess approximately 400 functional OR genes, each responsible for producing a receptor protein designed to recognize one or a small group of related odor molecules.
Genetic Polymorphism
The genetic code for these receptor proteins varies significantly from person to person, a concept known as genetic polymorphism. If an individual inherits a non-functional or poorly functioning variant of the specific OR gene meant to detect trans-2-decenal or trans-2-octenal, the odor molecule cannot bind effectively. This failure to bind means the chemical signal is never translated into an electrical signal sent to the brain, resulting in a complete lack of odor perception.
This genetic variability is particularly common with aldehydes, the chemical family to which the stink bug compounds belong, leading to a wide range of detection thresholds among the general population. For those who cannot smell the compounds, the receptor site is essentially closed or misshapen, preventing the odor molecule from fitting into the receptor protein. Therefore, the lack of smell is not due to a weak nose, but rather a specific, inherited difference in the structure of a single receptor protein.
The Defensive Purpose of the Odor
The release of trans-2-decenal and trans-2-octenal is primarily an evolved survival mechanism for the stink bug. The pungent, noxious scent serves as a highly effective deterrent to discourage potential predators, such as birds and lizards, from attempting to consume the insect. The chemicals are stored in specialized scent glands located on the underside of the thorax and are only released when the bug is physically stressed, handled, or crushed.
Communicative Role
While the primary function is alarm and defense, the secretion can also serve a communicative role among the insects themselves. The compounds can function as aggregation pheromones, signaling to other stink bugs that a safe overwintering location has been found. This dual-purpose chemical signal is why disturbing one bug indoors can sometimes lead to the appearance of more.