Stinkbugs – the smell is a form of defense

They don’t bite, and they won’t sting. But if you get too close, they might do something worse. Why do stinkbugs stink?

Like a bee sting or a spider bite, the noxious smell of a stinkbug is a form of self-defense. A threatened stinkbug releases a mix of chemicals from special glands. Depending on the species, these chemicals can smell or taste foul. They keep predators like birds, mice and lizards from eating the bug.

In some species, like the Southern Green stinkbug, the chemicals might also serve as a warning signal or alarm pheromone. When one bug releases its stink, others of the same species are alerted by the signal — and they scatter. Since only a few species have been studied, scientists aren’t sure how many stinkbug species release defensive secretions that also raise alarms.

Many insects use a form of chemical defense. If you live in the southwestern U.S., the big black insect you might call a stinkbug is actually a clown beetle. These beetles warn off predators by standing on their heads, exposing their defensive glands, and eventually spraying an oily, bad-smelling secretion.

And still other insects, like the monarch butterfly, feed on poisonous plants that make their body fluids distasteful to predators.

Worldwide, there are thousands of species of stinkbugs. Most stinkbugs suck juices from plants with their sharp beaks. Just a few, including the southern green stinkbug, are serious agricultural pests, damaging a broad range of crops from peaches to pistachios.

A stinkbug’s defensive glands can carry a sizeable amount of chemical: five to ten micro- liters, which is close to 5% of the bug’s body weight. That’s almost equivalent to a human carrying around a gallon bucket. So it’s no surprise that scientists believe these chemicals are a significant energy investment for the bug. This likely explains why they don’t give off the chemicals unless they are severely harassed.

The chemicals that make up a stinkbug’s stink are called aldehydes, and they are different in different species. In low concentrations, the aldehydes have a pleasant odor. Some of the most common aldehydes in the “stink” of a stinkbug are described by chemists like fine wine: “green, pungent, spicy vegetable odor”, “diffusive orange odor with floral topknots” and “green, citrusy, orange”. But stinkbugs concentrate these chemicals so much that they become wholly unpleasant, even irritating. The smell can even kill the stinkbug itself. If the bugs are collected in stoppered vials, or kept in cages without adequate ventilation, the chemicals can get into their respiratory system and asphyxiate them.

While these aldehydes are distasteful and even toxic to birds and other predators, according to one particularly dedicated scientist, Dr. Bryan Krall, stinkbugs taste to humans like red-hots or cinnamon gum. In some parts of Mexico, India and Africa, stinkbugs are eaten as snacks. According to Peter Menzel, the author of Man Eating Bugs, the jumil stinkbugs of Mexico have “a strong taste, like aspirin saturated in cod liver oil with dangerous sub currents of rubbing alcohol and iodine.” Yum!

The clown beetles of the American Southwest have a different chemical defense system: instead of using aldehydes, they use chemicals called quinones. Before they spray, they advertise with a headstand that exposes the glands. If that doesn’t work, they spray the harasser with the foul-smelling substance. But a common predator, the grasshopper mouse, has figured out a way to circumvent this defense system. When the beetle stands on its head, hiking its rear in the air and exposing the stink glands, the mouse grabs it and plants it rear-end first into the sand. The stink is released into the sand, and the mouse can eat the beetle head first.

And finally, a taxonomic aside: Although the word “bug” is commonly used for all different types of insects, there is only one true group of bugs, those in the Order Hemiptera. These include bed bugs, stink bugs, damsel bugs, lace bugs, and chinch bugs, to name a few. When the word “bug” is used for an insect that is not a true bug, such as a lighteningbug or a ladybug (both beetles of the Order Coleoptera), it should be written together as one word. Stink bugs are true bugs, so they’re spelled with two words. Sowbugs are not even insects at all, so they’re written as one word. In the world of entomology, it’s a small point, but an important one.

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