What's That Smell?

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There have been many instances when one finds an interesting looking insect, picks it up, and is instantly greeted with a foul smell. Technically, all insects have an odor, but there are several insects that are known for their toxic smell. A couple insects that commonly come to mind are stink bugs (F. Pentatomidae) and lady bugs (F. Coccinellidae). The science behind insect's chemical abilities is quite fascinating, and even though chemical usage might leave behind a foul smell, chemical usage is very important for many insects and their survival. 

There are about 200 species of stink bugs in the United States and Canada, and a species that is quite popular is Halyomorpha halys. The common name is the brown marmorated stink bug, and it produces a bad smell as both adults and juveniles to discourage predators from eating or attacking it (Shipman, 2019). This odor is produced through the stink bug’s thorax. Within the thorax are special glands that possess a chemical mixture of odors. If a stink bug feels attacked or threatened, they release the chemicals onto a part of their exoskeleton called the evapatorium. The evapatorium is a rough part of the exoskeleton and because of the texture, it helps the chemical evaporate more quickly, hence causing the predator to smell it faster. 

There are, however, many other insects that use chemical defenses and are very overlooked. For instance, the bombardier beetle (F. Carabidae) is most notable for its defense mechanism of ejecting a hot, harmful chemical spray when attacked by predators. The bombardier beetle stores hydroquinone and hydrogen peroxide in a reservoir in its abdomen. When the beetle wants to emit the spray, a chemical reaction occurs and heat from this reaction produces gas to drive the reaction (University of Wisconsin-Milwaukee, 2017).  

                                    Bombardier beetle (University of Wisconsin-Milwaukee, 2017)

Another example of a chemical defense are certain insects that release hemolymph when attacked. Some crickets and lady bugs excrete hemolymph (blood) when they feel threatened. This is known as reflex bleeding, and in lady bugs, the blood is released from their leg joints. The blood smells bad and is toxic (Hadley, 2018). Lady bugs display their chemical defenses through aposematic signals. Many lady bugs have a red shell with black spots, communicating to predators that they taste bad. 

One of the main reasons insects use chemical defenses is to fight off predators. However, not all chemical usage is bad and used as a defense mechanism. There are other instances when insects use chemicals. Insects use semiochemicals to communicate with others. Bark beetles frequently use semiochemicals when they want to communicate to others about attacking a pine tree. Through pheromone mass attacks, the bark beetles can exhaust a tree’s resistance. Without a mass attack of at least a thousand bark beetles, the tree could defend itself through chemicals in the resin (Raffa et al., 2008). Hence, bark beetles need to be able to communicate with one another, which they do through chemicals. 

Another reason insects use chemicals are for intraspecific interaction purposes. This includes releasing chemicals called sex pheromones into the air in order to attract mates. One insect that displays this behavior are moths. Most female moths release their sex pheromones into the air so male moths can find her. Another insect that using sex pheromones are bumble bees. The male bumble bee flies along plants and “rub” their scent on the leaves and wait for a female to arrive on one of his perches (Hadley, 2019). 


Moth Releasing Sex Pheromones (Deviant Art, 2019)

Insects’ abilities to use chemicals to their advantage is really intriguing and without this characteristic, insects would not be able to fend off predators, communicate, or reproduce. Even though the smell might come across as foul to humans, one truly comes to appreciate this complex mechanism after learning more about it.

Bibliography

Deviant Art. (2019). Moth releasing pheromones. Retrieved June 6, 2019, from https://www.deviantart.com/melvynyeo/art/Moth-releasing-pheromones-557435150
Hadley, D. (2018, October 01). All About the Ladybug. Retrieved June 6, 2019, from https://www.thoughtco.com/ladybugs-family-coccinellidae-1968144
Hadley, D. (2019, March 11). How Insects Attract a Mate. Retrieved June 6, 2019, from https://www.thoughtco.com/how-insects-attract-a-mate-1968474
Raffa et al. (2008). Cross-scale Drivers of Natural Disturbances Prone to Anthropogenic Amplification: The Dynamics of the Bark Beetle Eruptions. Retrieved from https://ausable.populiweb.com/router/courseofferings/8796085/info
Shipman, M. (2019, February 28). Why Do Stink Bugs Stink? Retrieved June 5, 2019, from https://news.ncsu.edu/2019/02/why-do-stink-bugs-stink/
University of Wisconsin-Milwaukee. (2017, March 11). False Bombardier Beetle Redux (Family Carabidae). Retrieved June 5, 2019, from https://uwm.edu/field-station/false-bombardier-beetle-redux/
Wikipedia, the free encyclopedia. (2019, May 10). Hydroquinone. Retrieved June 5, 2019, from https://en.wikipedia.org/wiki/Hydroquinone

Wikipedia, the free encyclopedia. (2019, May 02). Semiochemical. Retrieved June 6, 2019, from https://en.wikipedia.org/wiki/Semiochemical



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