FOCUS ON TERMITE CONTROL: Chlorpyrifos Update

Chlorpyrifos was (and may still be) one of the most widely used termiticide pretreatment products. And although most uses of chlorpyrifos have been restricted by manufacturers and EPA, a couple of uses still exist for the pest management industry (see chart).

The door is closing on the chlorpyrifos era but research continues on this active ingredient, some of which is detailed in the following text. And some of the most interesting data revolves around the combination of chlorpyrifos and cellulose in formulation.

The Environmental Protection Agency (EPA) registered Termiticide T/C™ in 1997. It is a 42.40% formulation of chlorpyrifos. The formulation is a mixture of the active ingredient (chlorpyrifos) with methyl cellulose. The product has been marketed by Vopak as Masterline Termiticide T/C. It is registered in 28 states for termite control including pre- and post-treatment applications by professional applicators. The registered final use dilutions can be as low as 0.5% or 0.25%.

Chlorpyrifos has been one of the most widely used pesticide active ingredients in the world. It has many uses, which until recently, have been widely used by professional pest control companies, as well as homeowners, for structural, lawn and garden applications. Its use as a termiticide in the pest control industry was initiated around 1985, and until recent years it has been the major product used for termite control.

Chlorpyrifos emulsifiable concentrate (EC) formulations have been registered for use at rates of 1.0% and 0.75% for pre-construction termite control. In 2000, EPA also registered reduced rates at 0.5%. But it wasn’t until the fall of 2000 that the industry learned that a 0.25% proprietary additive formulation was also registered. This 0.25% formulation is the topic of the evaluation reported here.

There have been several tests conducted over the years about the amount of chlorpyrifos found in soils after application of registered termite control products. In the case of the Termiticide T/C product, soil tests have also been conducted and evaluated since 1996. In addition, regulatory evaluations of treated structures have been performed in more than 3,000 structures treated in Arizona alone. These treatments included pre- and post-construction applications.

CHLORPYRIFOS AS A TERMITICIDE. The industry quickly moved to chlorpyrifos in the 1980s and it is estimated that the soil around (and under) more than a million structures have been treated. The field data on the product is largely anecdotal with little field data to ascertain the relative effectiveness of chlorpyrifos to other termiticides brought into the market in the late 1980s. Where there have been reported “failures” there is no documented investigation as to why the treatment failed.

The only data considered valid in registering products, used in “comparisons” and discussing efficacy, has been testing performed through the USDA Forest Service, commonly referred to as “Gulfport” data. This historical report has shown that chlorpyrifos holds up well at 1.0 and 0.75% concentrations. Data at 0.5% is not as conclusive. All these formulations tested and reported on have been (and are) emulsifiable concentrates. No other test data has been produced, nor made public, on other formulations. Recent studies in other states by well-known researchers have shown discrepancies in the longevity and distribution of various termiticide formulations in different soil types and treatment scenarios. At the same time, the application rates of chlor-pyrifos used were higher than other termiticides and generally left more residual soil levels (ppm) than other chemicals.

The most recent data from Gulfport studies (through 2001) show that EC formulations of chlorpyrifos established in plots in 1971 have given three to eight years of “control” at 0.5% and six to 13 years for 1.0% rates. This data is the percentages reported for “concrete slab” (CS) tests. The data for ground board testing (GB) is less encouraging, having a two- to nine-year effectiveness at 1.0%, in different locations in the U.S. The GB test is by far the most extreme test under field conditions. It exposes the termiticide-treated soil to the changing climatic conditions, similar to the effects that soil found around the exterior of a structure would experience.

It should be cautioned here that this is not “real-world” testing. Applications in these studies are done under strict protocols on soil barren of structures. Also, if these studies take into account the variation in climatic and geological factors that can affect the chemical residual in the soil, they are often not discussed or included in published reports.

CELLULOSE AS AN ADDITIVE. One of the oldest stories told to PCOs is that termiticide actives will bind to organic matter in the soil. If this happens then the termiticide could be tied up by that material and not be available to the termite. This is an interesting theory, but it is not supported by scientific data or results. Organic material can be a wide variety of sources, but what if the material was a food source for the termite?

Cellulose is, of course, the main source of food for the termite. It is also a source of organic carbon. The molecular structure of cellulose makes it an interesting reactionary molecule to which a material can chemically bond. Studies in soils indicate that where the product was used there are marked increases in organic carbon. In addition, there is a marked increase (residual) in the total chlorpyrifos found in those samples. If chlorpyrifos were bonding with the cellulose as it appears, and the cellulose is concentrating in the soil, then termites encountering such a concentration would first be enticed to eat the cellulose, and second, be exposed to a high concentration of a termiticide that kills rather than repels.

From analyses performed on soils treated with the Termiticide T/C formulation at 0.25%, the levels of chlorpyrifos in the soil are higher and more concentrated than in soils treated with higher rate materials. In addition, residual studies indicate a half-life of more than three years for this product in the soil, making the effective life of the product projected beyond the five years “required” by EPA. This can be correlated back to studies already completed in Gulfport and relied upon for regulatory decisions on labeled amounts of termiticides in the soil, and the levels required in regulatory actions.

Many say that Gulfport data is science conjecturing on the eventual life of a product. What about real-world incidences of termites invading structures treated with this (or any) formulation? Termiticide T/C was granted an experimental use permit from Arizona’s Department of Agriculture (ADA) in 1996. Several structures were treated in 1996 and monitored by ADA . The most recent soil sampling (in December 2001 [five years after treatment]) showed significantly higher amounts of chlorpyrifos in the soil than structures treated with other chlorpyrifos EC formulations. Also, there were no breakthroughs reported to the Arizona Structural Pest Control Commission (SPCC) in the T/C treated structures.

In a plot study following Gulfport protocols for GB testing, and under the guidance of Dr. Paul Baker at the University of Arizona, soils were treated with chlorpyrifos EC at 0.5 and 0.75% and the Termiticide TC product at 0.5 and 0.25%. Evaluations for termite infestation were performed and termites infested the wood in the control and EC treated plots within three to six months after treatment. At the end of one year, the EC plots had 60 to 100% infestation while the TC plots still had no infestations.

The majority of the field applications of this product have been in Arizona. In the more than 3,000 treatments performed since 1998, eight breakthroughs have been documented on post-construction treatments (about 1,300 treatments [0.006%]) and all of these have been attributed to homeowner alterations to the perimeter soil. There have been no breakthroughs in more than 1,700 pre-construction treatments as documented by the SPCC. Evaluations on pre-treatments performed by the ADA have not found any structures to currently have termites. There have been no registered complaints to date from consumers with Termiticide/TC treatments that can substantiate that the chemical treatment failed.

CONCLUSION. This is significant. I have been involved in two class-action lawsuits in Arizona. In both, termites had infested structures in subdivisions within three years of their completion. A variety of termiticides were used in treating these structures and the infestation rate within three years approached 25%. The Termi-ticide/TC product has a highly significant track record when compared to other situations in Arizona.

In June of 2001, I inspected structures within subdivisions that had been treated with the Termiticide T/C product. There are no documented breakthroughs in any of these structures. At the same time, within these same subdivisions, sometimes the house next door to a T/C treated house, there have been reports of termite activity even though these structures had been treated with other materials.

The author is president of George Rambo Consulting Services, Herndon, Va. and can be contacted at grambo@pctonline.com