In-Depth: The Real Reason Why Stink Bugs Smell

If you have ever found a stink bug in your house or squashed one, then you know just why people call them stink bugs. Some people say the smell reminds them of skunks but why exactly do they smell?

The real reason stink bugs smell is because they emit a distasteful odor as a defense mechanism caused by waxy liquid that contains aldehydes. This liquid is contained in two glands located on their thorax. Stink bugs may smell like cilantro, a herb that also contains aldehydes.  

Read on to find out more about what makes them stink and why and how they emit their smelly odor!

What Is a Stink Bug?

First, it is important to note that there is no such thing as one type of stink bug. the united states only, there are over 200 species of stinks bugs, and although most of them can smell nasty, they are generally harmless. Some stink bugs are predators, eating unwanted bugs like grubs and Colorado potato beetles.

All stink bugs are members of a family of insects called Pentatomoidae. The Penta comes from the five segments of antennas on their heads. Adult stink bugs are typically between one-half and one inch long. They have six legs and three body segments.  

Some people call them shield bugs because the shape of their back is similar to a shield. Another name for them is chust bugs, but the most common name is the stink bug.

They have a hardened exoskeleton shell, which acts as a defensive barrier. It is one reason stink bugs have very few natural predators, allowing them to spread globally.  

Another reason they have few predators is that they not only stink, but they also taste bad (another defense mechanism). Animals that eat insects tend to spit out stink bugs.

What Is Their Life Cycle?

In spring, adults leave the places where they spent the winter and begin to mate. A female can lay up to 250 eggs per season, usually in clusters of 20-30 eggs, on the underside of leaves. Then in fall, the adults begin to look for places where they can safely spend the winter. That is often when they enter people’s houses.

Stink bugs do not undergo complete metamorphosis, like a caterpillar that does not resemble the butterfly it will become. Instead, when stink bugs hatch, the nymphs look similar to adults, except they are more colorful. After their fifth molt, the nymphs are considered adults. 

The Brown Marmorated Stink Bug

Although there are many different types of stink bugs, the most common one is the brown marmorated stink bug, or halyomorpha halys (marmorated means “veined or streaked like marble”).    

The brown marmorated is an invasive stink bug native to Asia but has now spread worldwide. This species was introduced to the United States sometimes in the mid-1990s and was first reported in Allentown, PA, in 1998. The fact that they have become a problem this quickly is a sign of how few predators they have due to their aversion tactics.

Besides giving off a nasty odor, the brown marmorated present a serious threat to many agricultural products. They pierce the skins of plants with their mouths and feed on the inside of the fruit.  

Although many crops are at risk of being damaged by the brown marmorated, these plants have the highest risk of being ruined:

• Apple, pear, and peach trees
• Raspberries and blackberries
• Okra, sweet corn, and tomatoes 

Even though the brown marmorated bug gets most of the press, all stink bugs give off the pungent odor.

To identify a marmorated stink bug, look for two white bands on the antennae, dark bands at the tips of their legs, and a zebra-like pattern on the edge of their wings (and yes, stink bugs have wings).

Do All Stink Bugs Stink?

The Swedish call stink bugs Bärfisar, which literally translates to berry farts. This name obviously has nothing to do with their shape, but with their odor. As stated earlier, there are over 200 stink bug species in five subfamilies and even more tribes. These are some of the more interesting ones:

• Iridescent Black Pentatomid. Also known as Zicrona americana. These have a striking shade of blue and dark green.
• Anchor Stink Bug. The anchorage has black and orange colorations on their back and is slightly rounder than other stink bugs. 
• Vulsirea nigrorubra. A gorgeous red and black stink bug.
• Green Stink Bug. The color says it all—a green stink bug.

No matter which species of Pentatomoidae you are referring to, it will stink in defense!

Why Do Stink Bugs Stink?

If you happen across a stink bug, you might not notice their smell right away. The first thing most people notice is their appearance. Because of their mottled look and the hard shell, many people find them unattractive, especially the brown marmorated species. You might notice a slight musty smell, but it will not stink right away.

A stink bug begins to really smell when they think it will be advantageous to their survival. The odor they secrete does is not intended to be an offensive way of survival. Instead, their smell is a defense mechanism or anti-predator behavior.  

Most animals have a defense mechanism of some sort. Turtles’ hard shells keep them from being eaten. A lizard can shed its tail while it runs away. Other animals can blend into their environment. Stink bugs stink.

When left alone, a stink bug does not give off that strong odor because it does not perceive any danger. That is why it is a surprise to find one. Stink bugs that gave off that odor would not survive as it would call attention to their location. The same is true of a skunk—you usually only know one is around when it sprays!

Essentially, this really is how they earn their place in the wild. In order to avoid being eaten, stink bugs also taste foul to most of their potential predators. That, along with their stench, makes them a meal that may not be worth eating.

What Is That Smell?

A stink bug’s smell comes from a waxy liquid that contains compounds called aldehydes. These compounds are created due to “oxidative stress.”

A common type of aldehyde that most people have encountered is formaldehyde, which is used to preserve animals. Anyone who has ever dissected a frog in biology class has encountered formaldehyde.

How Do People Describe a Stink Bug’s Smell?

Although few people would compare the smell of a stink bug to something of car exhaust, a common comparison is cilantro. That is not surprising since the herb also contains an aldehyde compound. It may even be considered that stink bugs smell like rancid almonds. That is also not surprising since almond skin also contains minute amounts of aldehydes.  

People often compare the smell of a stink bug to that of a skunk. However, a skunk’s spray contains thiols, not aldehydes. This compound is also found in onions and garlic, and it gives skunk spray its earthy odor. Many folks describe the stink bug’s smell as “earthy.” Perhaps that is why the bug’s smell is often compared to a skunk’s spray.

Of course, smelling a stink bug might be slightly better than getting a whiff from a skunk (or even sprayed.)

Interesting, the aldehyde compounds that stink bugs have in the liquid also contain antifungal and antibacterial properties which could help a stink bug fight disease.

Where Does the Smell Come From?

The liquid that contains the nasty smell they are known for is located in two scent glands. These glands are located on the underside of the bug’s thorax, one on either side. If you look closely, you can see that the glands are located behind the middle legs which hold the liquid containing aldehydes.

When a stink bug releases the liquid from those glands, it travels onto a structure called an evapatorium. Located on the exoskeleton, the evapatorium’s rough texture and shape help to release the odor quickly.

Once a stink bug feels threatened and gets defensive, it will release the liquid from its glands and omit its odor into the air.

Not All Odors Stink Bugs Release Smell Badly

We can smell the odor that stink bugs release as a defense mechanism. We cannot, however, smell every odor they give off. They also release other scents that only other stink bugs can smell.  

These scents, or pheromones, are a subtle way that stink bugs communicate with one another. Other animals give off scents to mark their territories or to attract possible mates. Since there are several different things stink bugs need to communicate, they have different pheromones for each purpose.

The Aggregation Pheromone

One pheromone they release is an aggregation pheromone.

Essentially, aggregation is the process of bringing things together. In nature, an example of that is fish that aggregate together into schools to avoid being eaten.

Since stink bugs have few predators, their aggregation pheromone, murgantiol, serves a different purpose. It signals that they have located food. In addition, the pheromone alerts nearby stink bugs that they have found a safe place to hibernate.

Attraction Pheromones

Most animals have pheromones to attract mates, and stink bugs are no different. However, their attraction pheromones have two unique properties. According to entomologists at the University of Maryland, male stink bug pheromones attract not only females but other males and stink bug nymphs as well.

Another odd finding for researchers is that the attraction pheromones also attract different stink bug species. For example, a brown marmorated stink bug may be attracted to an Asian brown-winged green stink bug due to their pheromone.

This is a big deal because it makes it difficult for scientists to develop a lure that will keep the brown marmorated bugs away from crops. Researchers have developed a more effective lure by combining pheromones from both species.

The Death Pheromone

When a stink bug is killed, it releases another pheromone, the death pheromone. This one serves as a warning to other bugs that danger is nearby. Stink bugs are not the only insect to have a warning pheromone. Bees guarding a hive emit an alert pheromone when the hive is disturbed. This pheromone encourages the bees to behave aggressively and leave the hive.  

A stink bug’s death pheromone signals other stink bugs that danger is lurking nearby and that they should stay away.

Can Stink Bugs Bite?

There is a lot of confusion about this when you search the internet. Some sites claim they do not, while others discuss how dangerous a stink bug’s bite can be. Where does this confusion come from?

Some stink bugs are predators. Predatory stink bugs are useful in controlling the insect population. They eat harmful insects, like the Colorado potato beetle and the Mexican bean beetle. They also feed on caterpillars as well as other stink bugs.

Predatory stink bugs are capable of biting, and if they are disturbed, biting you would be a form of defense. If you get bitten by a predatory stink bug, the bite might be painful and result in a welt. People who claim to have been bitten by a stink bug are telling the truth. They were bitten by a predatory stink bug species.

The brown marmorated stink bug is not a predatory bug. It is an herbivore that feasts on the juices in leaves, fruits, and other plant matter. Even if it wanted to bite you, it could not because of the shape of its mouth.

Its mouth consists of a long beak that is about 1.5 millimeters long and something called a stylet, which is approximately .1 mm long.  

When the brown marmorated bug feeds, the style comes out of the beak and pierces the plant. A brown marmorated bug could try to prick your skin, but the style is so short that it is difficult to imagine you would feel it.

So if someone claims they were bitten by a stink bug, they are probably telling the truth. They just were not bitten by the brown marmorated. 

Can a Person Be Allergic to Stink Bugs?  

Some people have reported allergic reactions to stink bugs. Although the most common allergen is some type of food, some people are allergic to compounds in the air, such as pollen. Another allergen is insects, and two of the most common ones are dust mites and cockroaches.  

Since brown marmorated stink bugs have only been in the United States since the mid-1990s, it is not surprising that people are just now reporting allergic reactions to them. In a study reported in the Journal of Allergy and Clinical Immunology, researchers discovered that people who had known allergies to insects also had reactions to stink bugs.  

Bottom Line

Stink bugs smell because of a defensive mechanism. They hope that their potential prey will leave them alone long enough for the stink bug to fly away. Even if an insect takes a bite, it will not enjoy the disgusting taste. Ironically, when a stink bug is inside people’s homes, the defense mechanism backfires on them if they get squashed.

Besides being stinky and a nuisance, stink bugs are nondestructive. Only the invasive brown marmorated stink bugs are harmful but only to certain crops. They cannot bite, but they can sure take the juice out of an apple or peach.

References

McQueen, C. (2017). Comprehensive toxicology. Elsevier.

Peiffer, M., & Felton, G. W. (2014). Insights into the saliva of the brown marmorated stink bug Halyomorpha halys (Hemiptera: Pentatomidae). PloS one, 9(2), e88483.

Mertz, T. L., Jacobs, S. B., Craig, T. J., & Ishmael, F. T. (2012). The brown marmorated stinkbug as a new aeroallergen. Journal of allergy and clinical immunology, 130(4), 999-1001.

Sagun, S., Collins, E., Martin, C., Nolan, E. J., & Horzempa, J. (2016). Alarm odor compounds of the brown marmorated stink bug exhibit antibacterial activity. Journal of pharmacognosy & natural products, 2(3).

Bolling, B. W. (2017). Almond polyphenols: methods of analysis, contribution to food quality, and health promotion. Comprehensive Reviews in Food Science and Food Safety, 16(3), 346-368.