The Formosan subterranean termite has global economic impact as an urban pest. The nesting and feeding habits of this invasive pest leave many factors of its biology literally hidden in the dark. This lack of knowledge has limited scientists’ ability to evaluate the success of termite management practices and develop novel strategies for termite control. Recently, however, molecular genetic techniques are providing powerful methods to unravel many aspects of termite biology.
LSU AgCenter researchers employ microsatellite genotyping to obtain information about the organization of termite colonies, such as breeding systems, colony affiliation, movement of foraging sites and swarming behavior. Microsatellite genotyping uses polymerase chain reactions to make copies of non-coding genomic regions containing repetitive sequences with high mutation rates. This procedure results in specific banding patterns, or genotypic profiles. Genotypes are inherited according to Mendelian rules, which mean that half of each termite’s bands are inherited from each of its parents. Therefore, genotypes contain information about genetic diversity and relationships of individual termites and colonies.
The object of termite control treatments is to eliminate the termite colony, which consists of kings, queens and their offspring. To evaluate a treatment, colonies first have to be clearly identified and key factors of their organization described. Genetic analyses have revealed two different breeding systems in subterranean termite colonies. One is the simple family headed by a single pair of reproductives, one king and one queen. The other is the extended family headed by multiple reproductives.
Colonies of the Formosan subterranean termite are always founded by a pair of winged adults, called alates, formed each year during the swarming season between late April and June. AgCenter scientists have found that partner selection during colony foundation favors large body size in males and females and high genetic diversity in females. How related two potential partners are does not play a role in partner selection.
Once the partners begin to reproduce, the king with the queen and their offspring become a simple family. When this original founder pair dies, it is replaced by multiple reproductives developed from their own offspring. This colony then forms an extended family. In most investigated populations of the Formosan subterranean termite, extended families are headed by fewer than 10 reproductives. Colonies are distinct genetic units because foragers from different colonies rarely mix.
To evaluate treatment success, researchers must first determine the number, spatial extent and movement of colonies in an area before and after treatment. Colonies of the Formosan subterranean termite can have millions of workers and can occupy large foraging areas that span 100 yards and more. Thus, termites found at different infestation sites have to be assigned to foraging areas of colonies to identify the number and extent of colonies.
Molecular methods make this possible by way of diagnostic genotypes (certain bands in the genetic profile), which distinguish the genetic profile of each colony from the profiles of other colonies. Termites that possess these bands can be assigned to a particular colony. Genetic profiles can be used to tag colonies before treatment. In cases where termites appear in the same location after treatment, comparisons of genetic profiles aid in determining if these termites are remnants from the same colony, invaders from neighboring colonies or – should their profiles not match known colonies – entirely new infestations.
Assigning termites found at different foraging sites to colonies via their genetic profile provides vital information about colony number, spatial extent and movement. High colony numbers were found in every investigated area in New Orleans. AgCenter scientists identified 14 colonies in the 31.5 acres of Louis Armstrong Park. Along a 1-mile stretch of the Riverfront Railroad, 33 different colonies were found. Sixteen colonies surrounded the Supreme Court Building over the course of four years. While the colonies in relatively undisturbed, park-like locations typically had large foraging areas that were stable over time, the colonies located in areas disturbed by landscaping, construction or termite treatment moved frequently. The high colony density and frequent movement of termite colonies in disturbed urban landscapes may explain why areawide colony elimination in a city like New Orleans is difficult to achieve.
In a recently completed study to evaluate treatment success, AgCenter scientists compared colony numbers and breeding systems of Formosan termites infesting in-ground stations along the Riverfront Railroad in New Orleans before and after treatment. Both breeding systems, simple and extended families, were present.
Four simple family and 11 extended families were found before treatment. A year after treatment began, all treated colonies had vanished and did not reappear. The breeding system of the colonies had no effect on the treatment success because both simple and extended families were successfully eliminated.
Despite the successful elimination of baited colonies, the termite pressure in the area led to rapid reinfestation in a considerable proportion of in-ground stations. This underscores the importance of continued and long-term monitoring and treatment programs. After treatment 12 simple families and six extended families were found. The prevalent breeding system became simple family colonies. It is likely that elimination of the extended family colonies through treatment opened up large tracts of underground tunnels and prompted the expansion of young simple family colonies. Most of these young colonies were still headed by the original founder pair as shown by the unrelated parental genotypes and low degrees of inbreeding within the colony. Understanding these patterns and processes of reinfestation of a previously treated area by termite colonies will help to measure the impact of populationwide treatment and devise future areawide management strategies.
The AgCenter researchers also used genotypic profiles of swarming alates to attempt to find their colonies of origin in the French Quarter. The vast majority of the alates, however, could not be assigned to colonies infesting nearby in-ground stations. Some of the alates that could be assigned had flown from colonies occupying in-ground stations as much as 370 yards and even 670 yards away. Apparently, alates can fly farther than commonly assumed. The dispersal distance of alates was sufficient to guarantee mixing of an average of 13 colonies within swarm aggregations that were collected around light sources.
Genotypic frequencies differed significantly between male and female alates, suggesting that males and females originated from different colonies. Both of these mechanisms decrease the likelihood of pairing with a nest mate and limit inbreeding in newly founded colonies, even though termites do not actively avoid pairing with related partners. These observations confirm previous studies demonstrating that the majority of simple family colonies in Louisiana populations are headed by unrelated pairs of reproductives.
In addition to using molecular methods to describe important aspects of termite biology, LSU AgCenter scientists are developing an alternative to conventional chemical and bait treatments for termite control. The system involves using genetically engineered microbes as “Trojan horses” to produce and spread toxins in termite colonies. The scientists have constructed killer strains of yeast to produce toxins that specifically destroy the gut flora that termite workers need to digest wood. Termites will starve without gut flora. Termite feeding and grooming habits spread these microorganisms naturally throughout the termite colony. This system is a promising approach to eliminate termite colonies in a cost-effective way.
Claudia Husseneder, Assistant Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.
(This article was published in the fall 2007 issue of Louisiana Agriculture.)