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New termite baiting technologies for the preservation of cultural resources:RESULTS OF FIELD TRIALS IN THE NATIONAL PARK SYSTEM

By Mark Gilberg and Nan-Yao Su

Figure 1. The Statue of Liberty is one of five sites in the NationalPark System where scientists tested hexaflumuron, an insectgrowth regulator, as a means of controlling termites.

Since 1995, the

National Center for

Preservation

Technology and

Training

(NCPTT)—part of

the NPS Directorate of

Cultural Resource

Stewardship and

Partnerships—has spon-

sored a series of field trials to

assess the efficacy and practical appli-

cation of a new technology for the

control of termites in historic structures

and buildings. Many of these field trials

have been conducted in our national

parks where cultural resources are severely

threatened by termite activity (fig. 1).These

sites have proved ideal settings for this evalua-

tion and for testing other new technologies.

Moreover, the field trials have helped ensure

the preservation of important cultural

resources.

BackgroundTermites are a significant structural pest in the United

States, costing the public nearly $1.5 billion in damageeach year. The bulk of this damage can be attributed tosubterranean termites. Historic buildings and structuresare particularly vulnerable to subterranean termite dam-age, given the traditional use of wood as a building mate-rial. Termite damage to historic buildings is both costlyand irreversible and can diminish the historic signifi-cance of the structure through the loss of original build-ing fabric. Cultural landscapes are also vulnerable to ter-mite damage. In New Orleans, many of the historic oaktrees that add shelter and beauty to the city are threat-

ened by an introduced species,the Formosan subterranean ter-mite, Coptotermes formosanus.

“Termite damage to historic buildings is both costly and irreversible

and can diminish the historic significance of the structure....”

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This species can construct nests within the dead heart-wood of the tree eventually weakening it to the pointwhere it is unstable and falls in bad weather.

Conventional methods for the control of subterraneantermite infestations rely heavily on the use of organic(i.e., carbon-based) insecticides to provide a barrier forthe exclusion of soil-borne termites from a structure.Typically, large volumes of liquid insecticide are appliedto the soil beneath and surrounding an infested building.Poisoning the soil is not a sustainable practice and maycontaminate groundwater as well as pose health andsafety hazards. Moreover, such an approach is not alto-

gether effective. Creating an uninterrupted barrier oftreated soil beneath an existing structure is extremelydifficult, and gaps in the barrier invariably allow access tothe structure. Also, because the soil treatment onlydeters termite attack, the vast majority of subterraneantermites are unaffected. Conventional soil treatmentsoften result in physical damage to the structure; theyrequire the drilling of often disfiguring and unsightlyholes in the foundation floor before liquid insecticidesare injected into the soil.

New termite baiting technologiesIn response to these concerns, a number of new baiting

technologies have been developed in recent years as analternative to conventional liquid insecticides. Of these,baits containing the insect growth regulator, hexaflu-muron, have proved most promising in successfully elim-inating subterranean termite populations at or nearstructures. Hexaflumuron inhibits the synthesis ofchitin, which is essential for the formation of insectexoskeleton, but is virtually harmless to vertebrates(LD50>5,000mg/kg1). The treatment uses a monitoringand baiting procedure, whereby hexaflumuron is deliv-ered by foraging termites to eliminate the entire colonypopulation. The procedure is marketed currently as the

Sentricon® Termite Colony Elimination System (DowAgroSciences, Indianapolis, Indiana) to authorized pestcontrol operators. Studies using the Sentricon system or

its commercial prototypes have confirmed that termitecolonies of several million individuals can be suppressedto the point of inactivity (or observed elimination) usingless than 1g of hexaflumuron. Moreover, elimination ofcolony populations creates a zone of termite-free soil sur-rounding a building for several years.

The Sentricon system employs a cyclical process ofmonitoring and baiting for termite activity. Initially, atechnician installs Sentricon stations containing monitor-ing devices in the soil surrounding a structure. When ter-mite activity is discovered in a station, the monitoringdevice is replaced with bait containing 0.5% hexaflu-

muron (fig. 2A). Foraging termites feed upon thebaits and thoroughly distribute the hexaflu-muron throughout the colony population.Unlike conventional termiticides, hexaflumuron

is a slow-acting toxicant that kills termites only when theymolt, every 1-2 months. Thus, dead termites do not accu-mulate around the bait that would otherwise repel otherforaging termites and prevent further uptake of the bait.Several months may be required to achieve control butthe end result is complete elimination. Once the colony iseliminated, a return to monitoring continues to detectfurther termite activity.

Hexaflumuron targets only subterranean termites;drywood termites and other insect species remain unaf-fected. Also, it only impacts those colonies at or near thesite to be protected. Hexaflumuron can not be spreadover a large geographical area and thus threaten theextinction of C. formosanus as a species. In fact, experi-mental results to date suggest that re-infestation alwaysoccurs but at a slower rate than in the absence of hexa-flumuron due to an overall decrease in termite popula-tion levels around the site.

Field trialsNCPTT-sponsored field trials involving the use of

Sentricon have been conducted at a number of NationalPark System sites particularly in the Southeast and theCaribbean where subterranean termite activity is mostpronounced and threatens many historically significant

structures. In the greater New Orleans area theannual cost of termite damage and treatment isestimated at $300 million. The historic FrenchQuarter is particularly threatened because of

the widespread use of wood as a building material andshared-wall construction practices that make pest con-trol difficult (Freytag et al. 2000). Much of this damagecan be attributed to the Formosan subterranean termite,which was introduced from Asia after World War II. Thisspecies is characterized by extremely large colonies and,unlike other subterranean termites, is capable of formingaboveground nests.

“Poisoning the soil is not a sustainable practice and may contaminate

groundwater [and] pose health and safety hazards.”

“Termite colonies of several million individuals can be suppressed to the

point of inactivity ... using less than 1g of hexaflumuron.”

1 Lethal Dose, 50%, refers to the amount of insecticide that, if admin-istered to a population, will cause 50% of the population to die. It isusually expressed in terms of milligrams of insecticide per kilogram ofsubject body weight.

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To date, the National Center for PreservationTechnology and Training has sponsored field workinvolving the use of Sentricon at San Juan NationalHistoric Site, Statue of Liberty National Monument,Cane River Creole National Historical Park, VirginIslands National Park, and New Orleans Jazz NationalHistorical Park. These trials represent a joint effort byNCPTT and its partners—the University of Florida, theNew Orleans Mosquito and Termite Control Board, andDow AgroSciences—to advance our knowledge of sub-terranean termite control in historic structures. In addi-tion to establishing the efficacy of Sentricon, these trialsyielded considerable information regarding the ecologyand behavior of subterranean termites. They also provid-ed opportunities to evaluate several new technologies fordetecting termite activity, including thermal imaging andacoustic emissions. We highlight below themethods used to (1) identify termite infes-tation, (2) measure termite activity for baitefficacy assessment, and (3) apply baits,and the results from several of these trials.

Identifying termite infestation

The first step is to identify the whereabouts of termiteactivity or damage. At the Statue of Liberty NationalMonument, signs of termite activity such as swarming,wood damage, and mud-tubes as noticed by park per-sonnel led us to identify three sites of live termite activ-ity in the structure (fig. 3): boiler room (BOL), displaycase (DIS), and sally port (SAL). Another importanttool in identifying termite activity in soil is the surveyusing wooden stakes (Su and Scheffrahn 1986).Researchers drove spruce stakes in soil surrounding theexterior walls of the monument to detecttermite activity. The survey revealed twoactivity sites, one in the soil outside theboiler room and the other at the sally portexit (EXT) (fig. 3). As shown in figure 3,researchers identified four clusters orpopulations of the eastern subterraneantermite, Reticulitermes flavipes, at theStatue of Liberty (Su et al. 1998).

Figure 3. Site inspectionand stake surveyrevealed four clusters ofR. flavipes activity in theStatue of LibertyNational Monument.Termite activity wasmeasured using under-ground (T) and above-ground (AGM) monitor-ing stations. Three typesof bait stations deliv-ered hexaflumuron: in-ground Sentricon sta-tion (S), abovegroundbait station Recruit AG(AGH), and soft-styleaboveground bait sta-tion (AGS).

Figure 2. The field trialsused three types of baitstations to deliver thehexaflumuron baits: (A)in-ground Sentricon sta-tion, (B) hard-styleaboveground bait sta-tion, Recruit AG, and (C)soft-style abovegroundbait station.

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Measuring termite activityTermite activity must be quantified before, during, and

after bait application in order to properly assess theeffects of the baits on the populations. Researchers usedseveral techniques to measure termite activity.

Underground monitoring stationAt some sites, researchers replaced survey stakes with

underground monitoring stations composed of a plasticcollar containing a feeding block (Su and Scheffrahn1986) (fig. 4A). Termite activity is quantified by measuringthe wood weight loss of the feeding block. The monitor-ing stations also provide opportunities to conduct amark-recapture procedure to identify the foraging rangeof the termite colony. In the Creole House of the Cabildocomplex in New Orleans, for example, researchers col-lected workers of the Formosan subterranean termitefrom a station in the courtyard and stained them with ablue dye before releasing them back into the same station(fig. 5). During the follow-up inspection they found bluetermites in the wooden floor of the second-floor office(Su et al. 2000).

Aboveground monitoring station Soil was not always accessible. Therefore, researchers

used an aboveground monitoring station similar to thatdescribed by Su et al. (1996) (fig. 4B) tomeasure the termite activity in some sitessuch as San Cristobal of San Juan NationalHistoric Site.

Acoustic emission deviceIn addition to the monitor-

ing stations, researchers usedother methods such asacoustic emission detectors(AED) to measure termitefeeding in wood (fig. 6). Thedetector recorded soundwaves of ultrasonic frequencythat were generated when ter-mites broke wooden fibers.Researchers used the deviceto quantify termite activity inthe wooden floor of the dis-play case in the Statue ofLiberty National Monument(Su et al. 1998), and in wood-en beams of the FortChristiansvaern of VirginIslands National Park.

Figure 4. The field trialsused underground (A)and aboveground (B)monitoring stations tomeasure termite activityat several historic sites.

Figure 5. Termites marked with blue dyewere released into a monitoring stationto delineate colony foraging range.

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Other methods of quantifying termite activityAt some historic sites, none of these tools could be

used because of preservation concerns. For these sites,researchers counted the number of active monitoringstations and any other signs of termite activity. In FortChristiansvaern of Virgin Islands National Park, forinstance, researchers counted newly emerged foragingtubes of the subterranean termite Heterotermes sp. andremoved them at each visit so that any new termite activ-ity would be recorded.

Bait applicationIn some places where soil access was

limited, researchers used abovegroundstations (Recruit®AG), which consistedof a plastic box containing hexaflumuronbait (fig. 2B, page 19). Researchersattached the open-side of the Recruit AGbait box over an active infestation so thatthe bait was accessible to foraging ter-mites. Researchers also experimentallyconstructed another type of aboveground bait station,the soft-style station, for use in several historic sites. Thisstation consisted of a flexible plastic pouch containinghexaflumuron bait (fig. 2C, page 19) and, on its back side,a removable flap surrounded by flexible adhesive. Softstations were attached over active infestations and theremovable flap was pulled to expose the bait. Because ofits flexibility, the soft station was adaptable to flat,curved, or contoured surfaces.

Effects of hexaflumuron baits on termite populations

Researchers measured termite activity at the Statue ofLiberty National Monument (see fig. 3) using under-ground (T) and aboveground (AGM) monitoring sta-tions. Additionally, they used three types of bait stationsto deliver baits to termite populations, including the in-ground Sentricon station (S), the aboveground bait sta-tion, Recruit AG, (AGH), and the soft-style abovegroundbait station (AGS). Termites fed on hexaflumuron baits assoon as researchers placed a bait station inside the dis-play case in August 1996 (fig. 7, DIS). The acoustic emis-sion device detected 20–30 feedingepisodes per minute from the woodenfloor of the display case through fall 1996to spring 1997, during which R. flavipescontinued to feed on the baits. By March1997, no termites were found in the baitstation, and the feeding activity in thenearby wooden floor also ceased.

Figure 6. The acousticemission detector (AED)recorded sound wavesthat were generatedwhen wooden fibers werebroken by termitemandibles, and was usedto measure termite feed-ing in wood.

Figure 7. The researchers quantified termite activity as the acoustic emission count (perminute) in the display case of the Statue of Liberty National Monument (DIS), wood con-sumption rate (g wood per station per day) in the utility room of San Cristobal of San JuanNational Historic Site (SCI), and number of active sites such as newly emerged foragingtubes of Heterotermes sp. in a storage room of Fort Christiansvaern, Christianstead NationalHistoric Site (CHRI3). Arrows depict applications of hexaflumuron baits. Number of activestations was also included to measure the overall termite activity during and after baitapplications.

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In March 1997, the subterranean termite Coptotermeshavilandi fed extensively on wooden blocks in two mon-itoring stations in the utility room of San Cristobal (fig. 7,SCI). In April, researchers applied hexaflumuron baits tothese stations, and after one month, C. havilandi activityhad declined significantly. By July no termites werefound in the monitoring station. Slight feeding on hexa-flumuron baits continued in July and August. During thistime researchers found C. havilandi individuals in thebait station that exhibited apparent symptoms of hexa-flumuron effects such as marbled coloration on theworker’s abdomen. Since September 1997, no termite hasbeen found in this location.

Unlike Coptotermes sp. that is more susceptible to hexa-flumuron, the response of Heterotermes sp. in FortChristiansvaern of the Virgin Islands National Park wasmore erratic. After the initial baiting in a storage room inAugust 1996, termite activity started to decline inDecember, but new activity emerged in spring 1997 (fig. 7,CHRI3). Despite repeated applications of baits through-out 1997, termites continued to feed on baits and new for-aging tubes kept appearing. Our persistent efforts seemedto pay off when this second wave of termite activityceased in spring 1998. The cessation lasted for threemonths, but in October 1998 termites reappeared in onebait station. This third wave of light activity, however, didnot last as long as before. Researchers have found no ter-mites or additional foraging tubes in this room sinceDecember 1998, two months after termites began feedingon hexaflumuron baits. The repeated cycles of activityduring bait application appeared to be common forHeterotermes sp., which tend to have many small coloniesin one area.

Monitoring and inspectionAfter successfully eliminating termite populations at a

historic site, resource managers must establish a monitor-ing program to continue protecting the site from furthertermite infestation. At the Statue of Liberty NationalMonument, for example, Sentricon stations installed insoil surrounding the exterior wall of the monument havebeen monitored quarterly since 1998, and no termites havebeen found on Liberty Island. Termites are abundant in thetropics and subtropics. Even after successfully eliminatingall detectable populations of Heterotermes sp. at FortChristiansvaern, Virgin Islands National Park, re-infesta-tion by neighboring populations is likely. To date, the rou-tine inspections by Park Service personnel have notdetected any new termite activity. If any sign of a new infes-tation is detected, the baiting program will resume to elim-inate the new population before severe damage occurs.

ConclusionsNational Park System sites and monuments are ideal

environments for evaluating many new technologies forthe preservation of cultural resources. Recent studiesinvolving the use of baits containing the insect growthregulator, hexaflumuron, have demonstrated that theyare safe and effective in protecting historic buildingsand structures against subterranean termites with noadverse effect upon the cultural or surrounding naturalresources. Moreover, the introduction of baits did notinterfere with visitor services or the quality of the visitorexperience at the sites or monuments.

ReferencesFreytag, E. D., M. K. Carroll, and E. S. Bordes. 2000. Control of Formosan

subterranean termites in Perserverance Hall in New Orleans,Louisiana. APT Bulletin XXXI:71–75.

Scheffrahn, R. H., W. P. Robbins, P. Busey, N.-Y. Su, and R. K. Mueller.1993. Evaluation of a novel, hand-held, acoustic emissions detectorto monitor termites (Isoptera: Kalotermitidae, Rhinotermitidae) inwood. Journal of Economic Entomology 86:1720–29.

Su, N.-Y., and R. H. Scheffrahn. 1986. A method to access, trap, andmonitor field populations of the Formosan subterranean termite(Isoptera: Rhinotermitidae) in the urban environment. Sociobiology12:299–304.

Su, N.-Y, P. M. Ban, and R. H. Scheffrahn. 1996. An aboveground stationfor monitoring structure-infesting populations of the Formosan sub-terranean termite (Isoptera: Rhinotermitidae). Sociobiology 27:39–45.

Su, N.-Y., and R. Scheffrahn. 1998. A review of subterranean termitecontrol practices and prospects for integrated pest management pro-grams. Integrated Pest Management Review 3:1–13.

Su, N.-Y., J. D. Thomas, and R. Scheffrahn. 1998. Elimination of subter-ranean termite populations from the Statue of Liberty NationalMonument using a bait matrix containing an insect growth regulator,hexaflumuron. Journal of the American Institute for Conservation37:282–92.

Su, N.-Y., E. Freytag, E. Bordes, and R. Dicus. 2000. Control of theFormosan subterranean termite infestations in historic Presbytereand the Creole House of the Cabildo, French Quarter, New Orleans,using baits containing an insect growth regulator, hexaflumuron.Studies in Conservation 45:30–38

About the authorsMark Gilberg is Research Director, National Center for Preservation

Technology and Training, 645 College Ave., Natchitoches, LA 71457; 318-356-7444; mark_gilberg@nps.gov. Nan-Yao Su is Professor ofEntomology, Fort Lauderdale Research and Education Center, Institute ofFood and Agricultural Sciences, University of Florida, 3205 College Avenue,Fort Lauderdale, FL 33314; 954-577-6339; nysu@ufl.edu.

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