[Termite Research] Termite Caste Differentiation: A Social Affair

Editor’s note: Matt Tarver was a 2006 finalist for the NPMA/Bayer Young Scientist of the Year Award and received a $1,000 scholarship.

Out of the millions of organisms in the world, termites are one of the few to develop a social lifestyle, meaning they work together to increase their colony’s chances of survival. Although this might be good for termites, it isn’t good for homeowners.

In the dictionary the word “social” has a number of entries: 1) relating to human society and how it is organized, 2) relating to the way people in groups behave and interact, or 3) living or preferring to live as part of a community or colony rather than alone. While all of those definitions describe the word “social” from a human perspective, in the insect realm “social” is defined by three characteristics: 1) overlap of generations, 2) cooperative care of the young, and 3) a reproductive division of labor.

Social insect castes are physically and behaviorally distinct groups of individuals that cooperate with each other and perform colony tasks (Miura 2004). Most termite colonies are made up of three distinct castes that include workers/pseudergates, soldiers and reproductives. Similar to the manner in which an assembly line works, each caste helps the colony as a whole to be more efficient. In other words, the different castes work together to get the job done quicker and more efficiently than having each individual doing all the jobs by themselves.

HOW THEY WORK TOGETHER.Termite workers constitute the majority of individuals within the colony and perform much of the work within the colony. Workers feed other caste members, groom the queen and each other, excavate the nest, make tunnels, tend to larvae, forage for food, dispose of corpses and help keep the colony clean to prevent the spread of disease. One theory is that worker termites perform their duties altruistically, helping the colony even though they themselves only have a slim chance to reproduce.

The reproductive caste is responsible for the production of offspring. There are multiple forms of termite reproductives. Primary and secondary forms develop from the winged pathway. Primary reproductives are the alates that disperse from the colony, mate and start a new colony. Secondary reproductives are alates that never develop their wings and stay within the colony. There also are third-form reproductives that develop from the wingless line and act as supplementary reproductives for the colony.

The third caste of termites is the soldier caste. Within the termite colony the main role of the soldiers is believed to be colony defense. Soldiers are specialized to have larger, heavily sclerotized heads and well developed mandibles that aid in defense. Unlike the worker castes, which can still differentiate into other castes, the sterile soldier caste is at the end of its developmental pathway (it is terminally developed). Because of their large mandibles and reduced digestive tracts, soldiers are unable to feed themselves and rely on workers for nutrition (Henderson 1998). Worker termites differentiate into soldiers by first molting into presoldiers and then into soldiers. This a process that takes about four weeks.

WHAT IS “CASTE DIFFERENTIATION”? The general biology of the different castes has been known for years, but what is not known is what factors control the physical and behavioral differentiation of individual termites into different castes. Some might think that termite eggs are designated as either a solider egg, alate egg or reproductive egg, but that is false. All termite eggs and all young termites are considered, or are thought, to have the ability to differentiate into any of the three castes.

Termite caste differentiation can proceed along two routes, the winged (imaginal) or the wingless (apterous) route. All forms are considered immature in lower termites except soldiers, alates and the three reproductive forms. The first developmental branch is where the larvae differentiate into either workers or nymphs. Workers can 1) undergo status quo worker-to-worker molts, 2) differentiate into a presoldier (immediately followed by a molt into a soldier) or 3) differentiate into an apterous eyeless third-form reproductive. Nymphs can 1) regress into worker-like pseudergates, 2) differentiate into fully winged and eyed adult alates that disperse, mate and become primary reproductives, or 3) differentiate into wingless and eyed non-dispersive second form reproductives that serve as supplemental reproductives (Buchli 1958, Laine & Wright 2003 and Scharf et al. 2003).

Therefore, termite caste differentiation is an intricate and complicated process. The factors that determine which caste an individual becomes are based on a number of internal and external factors. Examples of internal factors include nutritional status, age, hormonal levels, genetic composition or sex of the individual. External factors might include the number of caste members already present within the colony, temperature or moisture.

The ability to regulate caste differentiation is extremely important to a social insect colony. Having the optimal numbers of each caste is necessary for the colony to be efficient in its primary job of survival. If there were increases or decreases of specific caste numbers, the colony would have an abundance or be missing an important part of its workforce. For example, if all the workers within a termite colony changed into reproductives, then there would not be any workers left to work and the colony would suffer and probably collapse.

REGULATING CASTE PROPORTIONS. Throughout the social insect world there are a number of examples of castes that influence the behavior and physiology of nestmates. Termite caste proportions are thought to be regulated mostly through chemical communication. Most of these interactions are probably mediated by chemicals called “primer pheromones.” Primer pheromones are chemicals released by one individual that causes or represses a physical change in another. A classical example of primer pheromone usage occurs when queen honeybees use chemical pheromones to suppress other individuals from becoming queens.

HOW IMPORTANT ARE SOLDIERS? Currently, our research is examining the interactions between castes and how such interactions affect caste differentiation — specifically the influence that termite soldiers have on worker termite caste differentiation. We are investigating how chemicals present in soldiers work in creating new termite soldiers. From our research, we have discovered that chemicals from soldier heads, when applied in combination with an insect hormone (juvenile hormone, JH) cause worker termites to molt into soldiers. This means that chemicals from the soldiers can influence the fate of nestmates around them.

For example, it is possible that when a soldier dies, primer pheromones present in the corpse stimulate worker nestmates to molt into soldiers, replacing the soldier that was lost. This method of chemical communication helps the termite colony maintain its fine balance between the number of individuals that are of a specific caste, keeping the colony as efficient as possible.

These discoveries are not only interesting from a scientific standpoint but they also can have a great influence on future termite control measures. As stated previously soldier termites have enlarged mandibles that interfere with the soldier’s ability to feed, making soldier termites rely on worker termites for food. If we can manipulate the chemical communication system the termites are already using to “tell” the colony to produce more soldiers, we can theoretically force all of the worker termites in the colony to become soldiers. With an overload of soldiers in the colony, the colony would rapidly die of starvation or have a reduced impact on its environment. This new technology would represent a significant step forward in the development of environmentally friendly, termite-specific control methods. With the pest control industry moving in a “green” direction, this kind of novel technology may have major implications.

The authors are with the University of Florida Entomology and Nematology Department and Toxicology Laboratory. Their research is supported by USDA-NRI-CSREES grant No. 2007-35607-17777.

Bayer Environmental Science, NPMA Announce Third ‘Young Scientist of the Year’ Program

Bayer Environmental Science and the National Pest Management Association late last year announced the third Bayer Environmental Science “Young Scientist of the Year” contest, a scholarship and grant program for graduate students, the next generation of researchers to advance the pest management industry.

This year, scholarships ranging from $1,000 to $2,500 will be granted to three individuals whose research advances knowledge on the biology, behavior, ecology or management of urban pest species. One individual selected as the “Young Scientist of the Year” will also receive a $10,000 grant to continue his/her research. In addition, a $1,000 scholarship will be donated to the general scholarship fund at the schools attended by each of the three finalists. To enter the contest, entomology students seeking a master’s or doctoral degree at an accredited university will submit their urban pest research to be judged by a panel of university and industry professionals. Prizes will be awarded to three finalists who will present their research at NPMA’s PestWorld 2008, Oct. 22-25, in Washington, D.C., with one finalist named the “Young Scientist of the Year.”

For more information, call 215/790-4342.