Termiticide Treatments: Thick Vs. Thin

The termite treatment performed during a structure’s pre-construction phase is a crucial element of the construction process. Without a termite treatment during construction, the integrity of the structure may be compromised even within one year after construction is completed. Furthermore, an incomplete or faulty treatment is nearly as devastating as not performing one at all. Sometimes, even after a complete and thorough termiticide application, termites still manage to find their way through the treatment. While there are numerous construction practices that may result in termite treatment failures, blame is sometimes placed on the longevity or durability of the currently available termiticides. Therefore, it is important to understand what the termiticides PCOs are using are capable of in the field. Termiticide penetration into soil during application and the resulting termiticide concentration in the soil are probably the two most important factors influencing the success of the termiticides.

More than 30 years ago researchers demonstrated that 1 day after applying liquid termiticides to Florida soils, 95 percent of the termiticide was present in the upper 1.9 cm portion of the treated soil. A more recent study found that sub-slab termiticide concentrations decreased as horizontal and vertical distance from termiticide injection point increased. Additionally, termiticide may run off certain "hills" under the slab and puddle in "valleys." This results in uneven treatment thickness or concentration, with the "hills" having thinner treatment thickness or lower concentration than the "valleys," even when the termiticide is applied correctly.

Termiticides also break down in time after application. Breakdown is faster in wet soil, high temperatures and in sunlight. As termiticides break down, the concentration of termiticide in the soil declines from the label rate to lower concentrations. Also, as sunlight, rain and soil disturbance disrupts the treatment along the perimeter of the building, treatment thickness also becomes thinner. Even if a treatment is put down completely and uniformly, through time treatments become non-uniform in thickness and concentration due to weathering and disturbance. Therefore, thickness of termiticide treatment after pre-construction termite treatments may be a few centimeters or less so that relatively thin layers of treated soil (at varied concentrations) would be relied upon to prevent subterranean termite attack.

THE RESULTS. In the past, treatment thickness was not thought to be an issue for repellent pyrethroid termiticides because research demonstrated that the pyrethroids were repellent at very narrow thickness. However, the trend toward development of non-repellent termiticides posed some questions. Could termites penetrate through narrow treatment thickness and survive? Or, do "kamikaze" termites blaze a trail through the termiticide treatment for other termites to exploit without harm? A study was recently undertaken at the University of Florida to answer these questions and provide insight into termiticide performance in situations simulating pre-construction termite treatments.

By sandwiching a desired thickness and concentration of termiticide-treated soil between two segments of untreated soil in a large test tube, the penetration of the eastern subterranean termite, Reticulitermes flavipes, into the termiticide-treated soil and resulting mortality could be assessed after seven days. Treatment thicknesses evaluated were 1, 2.5, 5, 10, 25 and 50 millimeters. Each termiticide was evaluated at label rate and as low as 100 times less than label rate.

At label rate concentrations of termiticides containing permethrin or bifenthrin, subterranean termites did not penetrate into any thickness of termiticide-treated soil. This can be attributed to the repellent nature of these termiticides that drives termites away from the termiticide. On the other hand, termites penetrated into all thickness of chlorpyrifos and completely penetrated narrow thickness of 1.0 and 2.5 millimeter. These results demonstrate the non-repellent activity of chlorpyrifos toward termites. Two other non-repellent termiticides, fipronil and imidacloprid, also were penetrated by subterranean termites. In fact, these termites were able to penetrate these termiticides significantly more than chlorpyrifos. Termites generally completely penetrated label rate concentrations of fipronil and imidacloprid termiticides at thickness of 10 millimeters or less. It would appear from these results that, at narrow thickness like those that may be present after a pre-construction treatment, non-repellent termiticides containing fipronil or imidacloprid would not be effective. However, after observing termite mortality at the end of the test period, we realized that was not the case.

At label rate, pyrethroid termiticides containing permethrin or bifenthrin were not penetrated and termite mortality was less than 35 percent at the end of 7 days. However, each of the non-repellent termiticides (fipronil, imidacloprid, and chlorpyrifos) caused 100 percent mortality — even at 1.0 millimeter treatment thickness. So, while some of the non-repellent termiticides were completely penetrated by termites, the termites were killed by termiticides as they traveled over, tunneled into, contacted and ingested the termiticide-treated soil particles.

At the full label rate every termiticide tested in this study was effective against termites at thickness as narrow as 1 ion differed. Pyrethroids protected because termites could not penetrate through a thorough treatment applied at the label rate. Chlorpyrifos, imidacloprid and fipronil protected by killing termites as they penetrated barriers applied at label rate.

As the concentration of pyrethroid termiticides decreased to 100 times less than label rate their repellency was lost. Termites penetrated easily into all thickness of permethrin- or bifenthrin-treated soil. Termites also easily penetrated all thickness of soil treated with the non-repellent termiticides containing fipronil, imidacloprid or chlorpyrifos (Figure 6). While these termiticides were all penetrated by termites, a few of these termiticides were still effective due to their toxicity at low concentrations.

Termiticides containing permethrin, bifenthrin or imidacloprid at 100 times less than the label rate caused low subterranean termite mortality and therefore were not effective termiticides. Alternatively, termiticides containing chlorpyrifos and fipronil still killed subterranean termites. At 100 times less than label rate, chlorpyrifos killed from 62 to 97 percent of the termites and fipronil killed 67 to 94 percent. Even 1 millimeter thickness of chlorpyrifos- and fipronil-treated soil caused high termite mortality.

CONCLUSIONS. This study indicates that pyrethroid and non-repellent termiticides at label rate are effective at either repelling or killing termites at thickness as narrow as 1.0 millimeter. However, the termiticides tested differ in their ability to be effective at lower concentrations.

Products containing fipronil or chlorpyrifos killed termites at concentrations 100 times less than label rate while the repellent (pyrethroid) termiticides lost their repellency and any toxic effects at 10 to 100 times less than label rate. Pyrethroid termiticides don’t kill termites and seem to be effective only as long as they possess their repellent nature. Imidacloprid effectively killed termites at the label rate but failed to kill termites at 100 times less than the label rate. Termiticides containing fipronil or chlorpyrifos have a wider concentration range at which they are effective at killing termites even at very narrow treatment thickness.

No matter which type of termiticide (repellent or non-repellent) PCOs choose to use, a uniform and complete termiticide application at the full label rate is critical for stopping termites. Steps should also be taken to ensure that the integrity of the termiticide application is not compromised after the treatment is complete. It is comforting, however, to understand what the currently available termiticides are capable of and what to expect in the field.