Author Archives: lrb6

“Stink, stank, stunk!” A closer look at the chemical defense of stink bugs

On Thanksgiving Day, I was playing outside with my cousins when one of them noticed a cool looking bug crawling on the side of the house. I rushed on over to examine it, and noticed that it was in fact a true bug (order Hemiptera, suborder Heteroptera), and recognized it as a stink bug. I did not agitate this bug to test this hypothesis, but from my experience I believe it to be the case (it is always possible that it was a mimic). It is generally safe to assume that if you live in a wooded area, you have run across stink bugs at some point. It is just as safe to assume that if you have run across a stink bug and have made the mistake of picking it up (since it is pretty cool looking after all), you can imagine the horrible smell as you are reading this. The potent odor is something not quickly forgotten, so I decided to do a little research about it.

I was mainly curious about the odor’s components, when it’s released by the bugs, and its effectiveness in warding off predators. Luckily, it did not take me long to find a paper that addressed exactly those three topics (although it discussed a different species of stink bug than the one I found in my cousins’ backyard). A study published in the Journal of Chemical Ecology tested a variety of aspects of the chemical defenses of the stink bug Cosmopepla bimaculata through a range of methods, some of which were rather…unorthodox (like sticking the bugs on their tongues, for instance).

Chemical Composition

Using the paper mentioned above as well as a section from the Encyclopeda of Entomology, I found that the components of the chemical odor consisted primarily of long chain alkanes, aldehydes, and esters (which is to be expected as esters especially are the primary components of many odors).

Some of the organic compounds found in stink bug secretions. Source: Encyclopedia of Entomology

Since what I remembered of stink bug odor reminded me of the classic smell of a skunk, I did some research into the chemical composition of skunk odor to see if there were any common components. I was surprised to find that, at least according to this paper that studied the components of spotted skunk spray, stink bug secretions and skunk spray do not share any chemical components. Rather, the skunk spray contains primarily sulfurous compounds as opposed to esters. The evolutionary significance of this difference would be an interesting topic for further study.

When is the odor released?

The original paper I cited from the Journal of Chemical Ecology had some interesting findings relative to when the stink bugs actually use their chemical defense. Firstly, it seems that they are more reluctant to use it than one might think. Generally, when provoked by prodding, the bugs will simply walk in the other direction or try other evasive techniques rather than secrete their foul odor. The experimenters found that it was not until actually picked up that the bugs secreted a significant amount of the odor. Perhaps this is due to the energetic costs of secreting the odor, or even the detrimental effect of secreting the odor too often and actually attracting predators. It would be interesting to research the various pressures affecting this behavior.

Additionally, the researchers found that while undisturbed females generally did not secrete any pheromones, 3 of the 4 undisturbed males did secrete one of the compounds. It is possible that this is evidence of the use of sex pheromones similar to the ones studied in an experiment conducted by Ho and Millar. In their experiment, Ho and Millar found male stink bugs to be the sole sex pheromone producers and suggest that this may be due to the benefit in having males signal to females once they have found a good habitat for mating or  because the shorter average lifespan of males makes the risk of actually attracting predators amount to a smaller cost than it would be for the females.

Success in Deterring Predators

As I alluded to earlier, the experimenters in the first experiment  tested the effects of the chemical secretions on potential predators both by examining the behaviors of various birds and lizards as well as by tasting the bugs themselves. All of the birds and lizards that were tested by being given the choice of stink bugs or control prey (either crickets or houseflies) exhibited an aversion to eating the stink bugs which demonstrated the success of the chemical defense method of preventing predation. Not only would they spit out or run away from the bug that was currently emitting the odor, but they oftentimes would also avoid any subsequent stink bugs that were placed in the cage. In either an act of dedication to their field or sheer madness, the experimenters actually placed the stink bugs on their tongue and even chewed them in order to ascertain the exact sensation that would be experienced by a predator. Doing so caused an “instantaneous burning sensation and chemical taste that lingered for up to 20 minutes…followed by a slight localized numbness of the tongue, which lasted 1-2 hours.” Not only does the stink bug produce an stench and unpleasant taste, but it also causes prolonged pain in the mouth of its predators.

Given the effectiveness of this defense, it is interesting that it is not employed by more prey. Determining the costs of producing chemical defenses would be an interesting future study.

The Complex Caste System of Leafcutter Ants

A couple weeks ago, during an Insect Biology Lab fieldtrip to the Spring Creek Greenway, I picked up a leafcutter ant with the specific purpose of having it bite me. Some may use this as evidence that I am crazy and others may cite it as an example of the power of peer pressure, but really it was interesting to get to see the ant up close and personal. It didn’t really hurt that much anyway and, in a weird way, it was kind of cute to watch the ant trying so hard to pinch my skin with its little mandibles. Ok, maybe I am crazy, but that’s not what this blog post is about. Actually, what I found the most interesting about this experience was picking which ant to let bite me. When we look at an ant mound, normally we just see essentially hundreds of copies of the same ant, as though they all popped out of the same mold from an ant making machine hidden underground. However, if you take a closer look, it is really quite a bit more complicated than that. As I scanned the ground, trying to decide which ant to pick up, I heard Dr. Solomon say something about how I should pick up a soldier one since they’re bigger and interesting to look at. That’s when I noticed that within this same species (Atta texana), within this same colony even, there were ants that looked widely different from each other. I had a vague idea about the division of labor in an ant colony before, but I hadn’t really realized the degree of specialization until after more research.

Soldier on my finger. Sorry for the fuzziness. Taking a picture of an ant on your finger is harder than it looks.

According to the University of Michigan Museum of ZoologyAtta texana highly specialize tasks using a caste system. Individuals are either reproductive or workers, and these workers are distinguished by twelve distinct worker morphs. These morphs can be grouped by size which reflects their function. The largest of the morphs are the soldiers, the caste that I personally picked up. The medium sized morphs are primarily foragers, but also function as excavators. Finally, the smallest morphs generally remain in the nest, functioning to break down leaves, care for the colony’s fungus gardens (the true source of food for the colony), and care for the queen and larvae.

Of course, learning about the complexity of the ant caste system made me wonder, “How is the caste of an individual determined?” In my research I found this paper in The American Naturalist about the developmental pathways that lead to caste determination in ants. In the paper, the author defines caste as “a set of workers that develops under the same developmental program”” rather than looking at it from a purely morphological perspective. The three variables that she lists as regulating these castes are critical size ( the size of a larvae once it begins the first step towards metamorphosis), growth parameters (growth rate vs dampening rate), and reprogramming (changes in critical size and growth parameters). The paper goes into quite a bit of detail about all of the factors that affect critical size. The one main means of determining critical size is by the amount of food the larvae receives. This can be driven both by environmental factors (such as the availability of food) or by the regulation of food flow by the “nurse” worker ants. In fact, nursing workers carry quite a bit of the responsibility of caste determination for the larvae as they manipulate the pheromone concentrations and temperature that also contribute to reprogramming. Something that this paper doesn’t address, but would be an interesting thing to find in further research would be the factors that affect the various behaviors of the nursing ants that cause them to drive the caste determination.

Some examples of leafcutter ants and the polymorphism in their castes. Source:http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1002007

While the division of labor through the caste system has its clear advantages in developing specialized workers that improve efficiency, the paper mentioned above also discusses the possible issues with specializing too much and too early in the developmental process. Obviously, a limit has been placed on the number of specialized functions. For the hundreds of thousands of ants in a colony, there are twelve castes. If more specialization equals more efficiency, why not give every individual its own specialized role? Well, as discussed in the paper, with the morphological specialization to one class comes a decrease in “individual flexibility.”

Source: http://trentflix.com/wp-content/uploads/2013/01/2839-Antz_1.jpg

Remember that 1998 Dreamworks movie, Antz? Of course you remember. It’s the one about the neurotic worker ant (not surprisingly voiced by Woody Allen) who rebels against the injustice of the social structure in his colony and ultimately marries the princess ant. Well, it wasn’t until doing more research into this caste system that I realized that despite its many many inaccuracies, it actually represented a true component of ant colonies and reflected (although with different reasons) the detriments of hyperspecialization. Throughout the movie, the main character is battling against a caste system that restricts his individual freedom. While his objections were more from a human perspective, the concern is still valid to a certain extent.

Form follows function, so if the form stays more general, it has the ability to adapt to new needs within the colony. If for some reason there’s a loss in the number of foragers due to an attack from phorid flies, individuals with individual flexibility could fill in until more forager caste workers develop. (As a quick aside, another interesting division of labor is discussed in this article that deals more with foraging than caste determination. Apparently, phorid flies tend to attack individuals from the larger castes, so sometimes members of the smaller castes will ride on the leaves of foragers to protect them from the phorid flies. It didn’t really explain whether this was a self sacrifice situation or more of a distraction from the fact that there’s a larger ant under the leaf, but either way it is still pretty cool!). Without this flexibility, the colony would be left crippled by the lack of foragers to collect leaves to feed their fungus gardens. The same would be true of the loss of any other caste. Additionally, the ability to replace them with new specialized adult ants relies on the fact that the developmental pathways stay relatively similar with the ability to change direction (reprogram) easily if need be. Without the flexibility to temporarily replace specialized workers with other workers or to increase the rate at which new specialized workers mature, the colony would lose its ability to perform that function if there were ever a sudden loss in many of a specific caste. For this reason, it is important that a colony strikes a balance between optimizing efficiency through the specialization of castes while maintaining the flexibility to adapt to times of crisis.

Luck Be a “Ladybug”

One day, I was just chillin’ in Duncan commons, chatting with a friend and sort of studying at the same time when out of nowhere an insect commonly known as a “ladybug” came and landed on my arm. Of course, my instant reaction was “Insect Blog Moment!! Must. Get. Camera.” So of course I scrambled to get my phone out of my bag, but I was certain to not scare the lady bug off of my arm. Why? Well, there’s the obvious reason of needing it to take a picture, but then there was also a secondary reason, a more instinctual reason, something that has become ingrained into me over the course of my life: Having a ladybug land on you is good luck, but scaring it away will bring you bad luck. Being the superstitious person that I am, my subconscious would follow this rule just as religiously as knocking on wood and throwing salt over my shoulder. However, after thinking about it, I realized that this one is a bit more strange. Generally, people try to avoid insects, or at least avoid having them land on their arms. Even your body reflexively has you swat at things that land on your skin and move the hairs ever so slightly. There generally are only a few exceptions to this, one being butterflies and the other, ladybugs.

But why? Butterflies have the advantage of being beautiful as well as having a fragile and harmless appearance. However, I wasn’t buying the idea that simply the red coloration and polka dot pattern was enough to make ladybugs lucky, so I did some research.

The first thing I found was that, as you may have guessed, ladybugs are not actually bugs. Instead, they belong to the order Coleoptera (aka beetles) and more specifically, the family Coccinellidae. It’s estimated that in this family there are about 6,000 species with a diverse range of colorations and patterns. I also found that in the UK, they are commonly known as “ladybirds” and “ladycows,” which both confused me and made me proud that even if it is still incorrect, at least the American common name is within the right phylum.
Source:http://entnemdept.ufl.edu/creatures/beneficial/lady_beetles.htm

But still, even all of this information does not answer my question of why Coccinellidae are considered lucky. To my surprise, I found the reason for their luck as well as the origin of their common name in the same article. As we have discussed a bit in class, one of the main niches that Coccinellidae fill is as a predator for aphids. In the act of preying on aphids and other insects, they aid in crop protection. This act has been appreciated by farmers for centuries and it is speculated that it is exactly this role that has earned the Coccinellidae the universal reputation for being lucky. And, in fact, unlike other superstitions, this one may hold some validity since the scaring away or killing of these insects might actually bring you “bad luck” as you watch the aphid population spiral out of control and destroy your crops. Now, the name “lady bug” stems from a related superstition started by Catholics that states that it was the Virgin Mary (also known as Our Lady) who sent these insects to help protect the crops, resulting in their iconic name as well as providing the inspiration for yet another depressing British nursery rhyme.

There has actually been a bit of research done into using Coccinellidae as biological controls for pests as was seen in the presentation on Citrus Greening. However, there is still much research to be done to be able to effectively use and quantify the benefits of using these insects for that purpose. Specifically, more research into the effects of pesticides, proper conservation techniques, and effective sampling techniques is crucial for gaining a better understanding of the effects of the use of Coccinellidae as a biological control. To learn more about this, read this paper from the Annual Review of Entomology.

And after explaining the origins and potential validity of this superstition, I have to tell you that to get a good enough picture I actually had to take this little member of the Coccinellidae family off of my arm and put it onto a plant. Although it pained me to do so, I reasoned that since I was actually bringing it into a habitat that it is more likely to enjoy, it should spare me from any bad luck curse…hopefully.