To Kill Or Not To Kill

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July 3, 2001

This article appeared in the March 1999 issue of PCT Magazine.

The goal of any subterranean termite treatment is to apply a complete treatment to establish an unbroken chemical barrier underneath and around the structure. But what does that treatment do once it is placed in the ground? Most researchers and pest control operators would agree that termites should not penetrate the barrier because they might damage some wood. But if they penetrate and die, is that bad? Also, if they don’t penetrate and live to find gaps in treatment, is that bad? Pest control operators need to take a stand and decide whether they want "to kill or not to kill" termites.

Any treatment requires a completely thorough application. Attaining a complete treatment is difficult, but doing so has been facilitated in the last decade by the development of treatment equipment such as foam machines, sub-slab injectors and lateral discharge nozzles. Regardless of the application equipment or which type of termiticide is chosen for use, all termiticides are applied exactly the same.

However, not all termiticides are the same. Although all registered termiticides have passed the Forest Service United States Department of Agriculture field tests, there are several critical differences between termiticides. For this reason PCOs should select a termiticide with several questions in mind: How do termiticides kill termites? What is repellency? What are the advantages and disadvantages of repellent and non-repellent termiticides?

AVAILABLE OPTIONS. How do termiticides kill termites? That depends on the type of termiticide. Currently there are about a dozen registered termiticides on the market and at least two more will be available in the next two years. Chlordane, heptachlor and aldrin were chlorinated hydrocarbons that killed "really good" but were banned in 1988. That left Dursban TC, an organophosphate, as the only available termiticide. However, in the early 1990s a host of pyrethroid termiticides (Demon TC, Dragnet, Prevail, Prelude, Torpedo, Tribute, Biflex and others) were developed and were the most commonly used termiticides during the past five years. Recently, Premise, a new termiticide, was introduced. Premise contains the active ingredient imidacloprid, a chloronicotinyl insecticide.

Two new products will be available in the next one to two years. Termidor, containing fipronil, a phenyl pyrazol insecticide, and Phantom, containing chlorfenapyr, a pyrrole insecticide, are in the final stages of development. So the dozen or so termiticides boils down to five classes of chemistries — organophosphates, pyrethroids, chloro-nicotinyls, phenylpyrazols and pyrroles. A brief review of each of these classes of chemicals follows.

Organophosphates. Organophosphates, commonly known as OPs, are widely used insecticides that affect the nervous system of termites, causing continual firing of the nerves and death of the insect within hours of contacting treated soil. Additionally, because chlorpyrifos is somewhat volatile, vapors associated with chlorpyrifos may also kill termites. Chlorpyrifos is about 10 times more toxic than chlordane to topically treated termite workers.

Pyrethroids. At least five pyrethroids are used in termiticide formulations. Pyrethroid insecticides also affect the nervous system of termites, are fast acting and kill treated termites within minutes. They are also highly toxic to termites at very low concentrations. Both pyrethroids and chlorpyrifos induce tremors, convulsions and paralysis in insects. Most pyrethroids are up to 150 times more toxic than chlordane to topically treated termite workers.

Chloronicotinyls. Premise, a chloro-nicotinyl containing imidacloprid, also affects the nervous system of insects but results in sluggish and disoriented insects that are not able to carry on normal behaviors such as feeding and movement. This mode of action causes death in termites in about three to seven days after contact with treated soil.

Phenylpyrazol. Termidor, a phenyl-pyrazol containing fipronil, is another new insecticide with a new mode of action. It affects the nervous system much like pyrethroid insecticides but much more slowly. Termites usually die about one day after contacting treated soil.

Pyrrole. Phantom, a pyrrole containing chlorfenapyr, is the only termiticide that does not affect the termite’s nervous system. Instead, chlorfenapyr targets the biological process that insects and other animals use to make energy. Termite workers that contact chlorfenapyr-treated soil become very sluggish for several days and die within one week.

WHAT IS REPELLENCY? The term repellency means to "drive back, ward off or keep away." How does this apply to termi-ticides? If a termiticide is repellent a termite will approach the treatment and avoid the treated area. Termites will not tunnel through or walk on a repellent termiticide. Pyrethroid termiticides are highly repellent to termites even at very low concentrations. The repellency is not from vapors that termites detect from a distance but is actually caused by the termites’ avoidance of treated soil. Therefore, termites can differentiate one grain of pyrethroid-treated sand from untreated sand. Because termites do not contact the pyrethroid-treated soil, they do not die. On the other hand, for non-repellent termiti-cides, termites cannot differentiate between non-repellent treated soil and untreated soil so these termites contact the treatment and die.

There are several laboratory tests that determine whether a termiticide is repellent or not. One laboratory test that is often used to determine repellency is the tube test. A tube is filled with a 5 cm plug of soil sandwiched between two plugs of agar (gelatin) that separate the termites from direct contact with the treated soil. Termites are placed in one end of the tube and allowed to tunnel for about seven days. Then the distance tunneled into the soil and mortality are measured. Termites tunnel into chlordane, heptachlor and aldrin and die. That behavior is typical of non-repellent termi-ticides.

Based on these studies, we conclude that chlorinated hydrocarbons were non-repellent. That same response (penetration of the soil treatment and high mortality) is seen with all the other non-repellent termiticides such as Dursban, Premise, Phantom and Termidor.

However, termites avoid soil treated with pyrethroids, never tunnel into the soil and very rarely die because they avoid contact with the treatment. Pyrethroids do kill termites in a laboratory environment when they are placed directly onto treated soil. However, such a situation is unlikely to occur in the field because termites usually have a choice of remaining in untreated soil or tunneling into pyrethroid-treated soil. In the field, termites would approach a repellent insecticide and redirect their foraging to avoid the treatment. Therefore, although the pyrethroids are the most toxic termiticides available, it is unlikely they will kill foraging termites in the soil and termites avoid the treatment.

Although solvents in the Dursban TC formulation (and possibly other products) may be repellent for a week or longer after the initial application, the active ingredients in Dursban TC, Premise, Termidor and Phantom are considered non-repellent to termites. When the repellent solvents in Dursban TC evaporate, the active ingredient left behind leaves a non-repellent residue. Therefore, termites unknowingly and unwittingly forage into the termi-ticide treatment and die.

PROS & CONS. What are the advantages and disadvantages of repellent termiticides? Use a repellent (pyrethroid) termiticide if a very complete and thorough treatment can be obtained. Attaining a complete barrier will prevent termites from entering the structure. However, this barrier must be applied thoroughly and efforts should be made to keep the barrier free of gaps for years after the treatment. This is because research at the University of Florida and Texas A&M University indicates that ter-mites are able to find gaps in pyrethroid treatments (see photo at bottom left) These types of gaps can occur where treatments were not thoroughly applied or where construction practices or homeowner maintenance disrupted the treatments.

Pyrethroid termiticides may be a good choice for creating horizontal barriers applied during pretreatment because such treatments may be less than ½-inch thick. A very thin layer of pyrethroid-treated soil will be effective at repelling termites from the structure. The effect of a non-repellent termiticide at such narrow thicknesses is not yet known. Can non-repellent termiticides continually kill termites after the treatment is penetrated? Can these treatments withstand foraging pressure at nearly paper-thin treatment thicknesses, especially as they degrade in the soil through time? Also, are there kamikaze termites that may forage through a thin treatment, thus sacrificing themselves so others can get through? These questions have not yet been answered.

What are the advantages and disadvantages of non-repellent termiticides? Studies at the University of Florida demonstrated that termites cannot find gaps in non-repellent treatments, meaning the termites could not tell the difference between non-repellent treated soil and untreated soil (see foraging arenas on page 24). Therefore, the termites died as a result of contact with the non-repellent termiticide treated soil. This indicates that structures can be protected even in the presence of gaps. However, non-repellent termiticides will not make up for sloppy treatments; the more thorough the treatment, the less chance of failure.

Sometimes, as in the case of Dursban TC, termites may die quickly within the treated area. As numerous termites die they decompose and produce acidic chemicals that are more repellent than some pyrethroids. Other termites may be repelled by the dead bodies so they do not contact the treated soil. Therefore, a non-repellent termiticide may cause a repellent barrier underneath and around the building as termites die and decompose.

Certain construction flaws or other special circumstances could be overcome by the use of a non-repellent termiticide. For example, suppose termites went unnoticed for several years in the wall of a structure. Chances are good that thousands of termites are living in the wall. As stated earlier, the application of a pyrethroid in this situation would not only repel termites in the soil from entering the structure but also repel termites back up into the wall to live for an extended period of time. In fact, according to Dr. Brian Forschler of the University of Georgia, the number of termites could actually increase because supplementary reproductive termites capable of laying eggs could be present or produced in the wall. In this situation, termites could remain in the wall for months if not years after the initial treatment. However, the use of a non-repellent termiticide in this situation would allow termites to forage into the treatment and die.

Most pest control operators are no longer used to killing termites instead of repelling them. In December, at the Florida Pest Control Association’s Annual Termite Symposium, Paul Hardy, senior technical manager for Orkin Pest Control and one of the most respected names in the termite control market, told a story about an experimental treatment they performed with Phantom, a non-repellent termiticide. After a soil application of Phantom the homeowners complained of a foul smell from within the walls in the home. "It smelled like a dead rat," Hardy said. The odor, in fact, was that of thousands of decaying dead termites in the walls. Because Phantom is slow acting, termites were able to transfer treated soil from the treated area into the walls, exposing the termiticide to all the termites in the wall. Although the aroma of a dead rat is not a desirable side effect, it was evident that termites were being killed. Similar results can be obtained with pyrethroids, but direct application by drilling and treating infested wood may be necessary to kill termites quickly on contact.

A combination approach to selecting termiticides may be justified. Perhaps using a pyrethroid termiticide for the horizontal treatment during preconstruction and applying a non-repellent termiticide as a vertical barrier is a reasonable approach. Much is yet to be discovered about the nature of non-repellent termiticides but the trend toward non-repellent termiticides is being followed by many chemical manufacturers as evidenced by the development of Termidor and Phantom. Although it is too early to tell what effect they will have on reducing retreatments, non-repellent termiticides are, at the very least, another important weapon in the arsenal against termites.

The authors are with the Department of Entomology & Nematology at the University of Florida, Gainesville, Fla.