Nature's Technology - What Chemicals Make Stink Bugs Stink?

An Asian escape, the Brown Marmorated Stink Bug (just "stink bug" to friends) was first detected in 1998 in Allentown, Pennsylvania, but quickly thereafter spread to many states, making a nuisance of itself to one small segment of the population-farmers-by damaging crops, and to a far larger segment as well, to householders. The bugs not only invade houses in large quantity, causing irritation by their presence, but if disturbed, they give off a terrible smelling liquid. Most means of killing the insect, rather than preventing secretion of the odor, assure it will occur. Even vacuuming the creatures guarantees the vacuum cleaner will smell of stink bugs.

The Stink Bug's Arsenal (Nature's Technology)

Even a slight disturbance can set the stink bug off. The brown pest emits its malodorous defensive secretion through pores along the sides of its body. This action makes it undesirable for food to lizards, birds, and other would-be predators. Unfortunately, it can be set off by even little actions such as attempting to brush it off one's clothes, picking it up to release it outdoors, etc. What is the foul-smelling component of the stink bug's secretion?

Aldehydes

Plain and simply, the odoriferous components of the brown marmorated stink bug's arsenal are aldehydes. Specifically, they are trans-2-decenal and trans-2-octenal. For visualization of these two structures, see Figure 1, associated with this article. In that figure, "C" stands for a carbon atom, "H" is for hydrogen, and "O" is for oxygen. The straight lines are bonds, which join the atoms together. The portion of a molecule that makes a compound an aldehyde is the (-CHO) linkage.

The Good, the Bad, and the Ugly

Aldehydes are generally known for the odors they produce. Some may be pleasant. For instance, some aldehydes impart the aroma of sage, citrus, roses, lilacs, and jasmine. On the other hand, some aldehydes aren't merely unpleasant, they can be unbearable. Consider malodorous formaldehyde, the nose-burning aldehyde used to preserve dead animal specimens in biology class. Or, how about butyraldehyde, which has the aroma of rancid butter. This raises the question: Is there some way to predict if a particular aldehyde will smell good and pleasant or if it will smell bad? No, not totally-however-there are rules of thumb.

Rules of Thumb for Determining Pleasance of Aroma

First, lower molecular weight aldehydes (having carbon chains less than about seven carbon atoms) tend to smell unpleasant. Higher molecular weight and longer carbon chain aldehydes tend to be pleasant. Second, Some well-known pleasant aldehydes also contain aromatic-ring structures-for instance, licorice. Unfortunately, these rules of thumb, although useful to the perfume industry, are not accurate in all cases. The two compounds constituting the aroma of the stink bug, according to the rule of molecular weight and carbon chain length, should be among the pleasant aromas.

The New York Times - Move Over, Bedbugs: Stink Bugs Have Landed

References and Resources:

Clemson University - "Detecting Stink Bugs/Damage in Cotton Utilizing a Portable Electronic Nose"

National Pest Management Association, Inc. - Stink Bugs

Perfume Shrine - "Debunking the Myth"