[Focus On Technology] Chain Of Events

Many pest management professionals agree that evolution is essential for the growth and prosperity of the pest control industry. Insecticide resistance, stringent regulations, and stronger demand for quicker, more effective pest control methods increasingly dictate the need for improved insecticide technologies.

Though most insecticides work primarily through direct contact and often require widespread application, some current insecticides offer a unique control effect known as "secondary kill," the "Domino Effect" or "horizontal transfer." This feature, where one insect exposes another to an insecticide, provides greater control for a variety of insects while using less product, saving time and reducing costs.

Secondary kill uses insects’ own behavioral and biological patterns against them, allowing them to bring active ingredients (i.e., imidacloprid, fipronil, hydramethylnon, boric acid, chlorfenapyr and abamectin) back to the source of the infestation — the colony or nest. Especially effective against social insects like ants and termites, secondary kill also can play an important role in controlling other pests — such as cockroaches — that contact each other while crowded together in favorable harborages.

Insects expose each other to pesticides in a variety of different ways including:

Physical Contact: On the outside of their bodies, insects can transfer insecticides by grooming or simply touching each other. For termites, their natural grooming behavior and contact in the tight confines of the galleries is an important mechanism for the transfer of non-repellent termiticides.

Trophallaxis: Food exchange for mutual benefit via the mouth and anus is observed in a variety of insect pests and can result in significant secondary kill. Ants rely extensively on trophallaxis to exchange nutrients between colony members. More specifically, termites often consume anal excretions to acquire cellulose-digesting symbionts.

Coprophagy: Cockroaches are widely known to feed on their own species’ feces. After eating hydramethylnon, for example, feces from a single cockroach can kill up to 44 other roaches (Silverman 1991).

Emetophagy: More recently, Dr. Coby Schal of North Carolina State University has found that cockroaches demonstrate emetophagous behavior — feeding on their own species’ vomit to gain vital nutrients. After feeding on bait containing the active ingredient fipronil, cockroaches vomited just before dying, other roaches then quickly consumed the vomit.

Cannibalism: Cockroaches often feed on dead and dying cockroaches to obtain nutrients. For example, Aparicio (1996) discovered that all stages and sexes of German cockroaches cannibalize even when there is sufficient food, water and harborage.

FROM PROCESS TO CONTROL. Armed with an understanding of the processes in which insects can distribute nutrients and toxicants to other members of the colony, the following text reviews how secondary kill positively impacts the control of three important pests: cockroaches, ants and termites.

Cockroaches. Researchers have long been aware of the link between cockroach behavior and secondary kill. Similarly, they have acknowledged through studies that certain active ingredients in baits are good for achieving secondary kill for a variety of cockroach species, including German, American, Oriental, Asian and brown.

Silverman et al. (1991) found that Maxforce baits containing hydramethylnon provided secondary kill in all stages of German cockroaches that fed on cockroach feces from treated insects (coprophagy). In studies conducted in cockroach-infested apartments, Kopanic and Schal (1997) demonstrated that secondary kill with hydramethylnon baits, most likely through coprophagy, played a major role in controlling German cockroach nymphs.

Gahlhoff et al. (1999) examined secondary kill of German cockroaches via cannibalism of nymphs fed toxic baits. The study found that all of the baits tested — fipronil, hydramethylnon, boric acid, chlorpyrifos and abamectin — led to secondary kill. Baits containing the active ingredients fipronil and chlorpyrifos were the fastest acting with LT50 values (lethal time to kill 50 percent of test insects) of 1.22 days for first and second instar cockroaches.

Fipronil, the active ingredient in Maxforce FC Roach Gel and Stations, controls cockroach populations through both contact and ingestion via the Maxforce Domino Effect.™ This chain reaction extends the killing power of bait placements deep into hard-to-reach areas normally inaccessible to conventional control methods.

Buczkowski and Schal (2001) examined the horizontal transfer of fipronil among various developmental stages of German cockroaches using topical (direct application to cockroaches), residual (fipronil on glass surfaces) and oral delivery methods using baits. Fipronil was most effectively transferred to untreated cockroaches when ingested in baits. The authors concluded that secondary kill with fipronil may be caused by ingesting excreted fipronil residues, cannibalism of poisoned cockroaches, and to a lesser extent contact with fipronil contaminated surfaces.

Ants. Researchers worldwide have demonstrated that only a small percentage of an ant colony actively forages away from the nest for food and water. Therefore, baits with a delayed action kill can offer control for a wide variety of different ant species. Here’s how it works:

• Workers locate food and water and return it to the colony and feed solids to larvae.

• Larvae digest the solids, producing a liquid.

• Other members of the colony take this liquid and feed each other and the queen.

• Therefore, when larvae consume insecticide bait as food, their liquid output contains the active ingredient that is distributed throughout the colony.

Termites. Recent studies have demonstrated the significant secondary kill effect that results from several non-repellent termiticides. (Editor’s note: the study discussed here did not include chlorfenapyr, another non-repellent termiticide.) Results from a recent study by Thomas G. Shelton and J. Kenneth Grace from the University of Hawaii, slated to appear in an upcoming issue of the Journal of Economic Entomology, shows that imidacloprid is transferred among termites when used at soil concentrations typically found in a trench soil application. Just five termites, previously exposed to imidacloprid-treated soil ("donor" termites), when released in an untreated environment with 95 unexposed termites ("recipient" termites) can cause great mortality to termites that were not directly exposed to treated soil. After 15 days, 80 percent of the unexposed termites were dead.

Overall, these results documented lethal transfer from exposed to unexposed C. formosanus workers when donors were exposed to as little as 100 ppm of imidacloprid or fipronil in treated soil for as little as one hour. Continuing studies will examine the role of other factors (how long donor termites are exposed to treated-soil, the ratio of donor to recipient termites, individual colony variation in transfer susceptibility, etc.) in the extent of this transfer phenomenon for non-repellent termiticides.

A high priority for future research on this transfer phenomenon in subterranean termites will be to understand the exact mechanisms at play in achieving this "remote kill" of termites far removed from the treated zone. Some important possibilities are termite-to-termite contacts that occur as a normal consequence in foraging and tunneling; trophallaxis or food/fluid exchanges between termites (which is highly evolved in ants); the direct transfer of treated soil to remote areas of termite "workings" or nest systems; and the role of cannibalism, when dead or dying termites are recycled by the colony.

With a deeper understanding of exactly how these mechanisms operate, and how they may differ subtly between different types of non-repellent termiticides, we can hope to see technologies developed to maximize this effect, making non-repellent termiticides even more effective than they are today.

VALUABLE BENEFITS. Secondary kill undoubtedly provides enhanced control of the aforementioned important pests. Taking advantage of insect behavioral processes enables pest management professionals to use secondary kill to help implement more targeted control as part of an Integrated Pest Management (IPM) program.

In response, proper training and education is vital if pest management professionals are to harness this unique control effect and continue to succeed in the evolving pest management marketplace. However, it is important for pest management professionals to remember to apply insecticides according to the EPA label directions to avoid negative impact on non-target pests, humans and the environment.

Editor’s note: The active ingredients in the termiticide portion of this story are found in the following products:

• Imidacloprid (Premise by Bayer ES)

• Fipronil (Termidor by Bayer ES)

• Chlorfenapyr (Phantom by BASF)

Gary Braness, Ph.D., and Byron Reid, Ph.D., are both product development managers for Bayer Environmental Science. Joe Barile, a board certified entomologist, works in the technical services department of Bayer Environmental Science. Braness can be reached at gbraness@pctonline.com.

References:

Aparicio, M. L. 1996. Cannibalistic response of the German cockroach, Blattella germanica (L.) to differentiate density and developmental stage of conspecifics, M.S. thesis, University of Florida, Gainesville.

Buczkowski, G. and C. Schal. 2001. Method of insecticide delivery affects horizontal transfer of fipronil in the German cockroach (Dictyoptera: Blattellidae). J. Econ. Entomol. 94(3): 680-685.

Gahlhoff, J.E., D. Miller, and P. Koehler. 1999. Secondary kill of adult male German cockroaches (Dictyoptera: Blattellidae) via cannibalism of nymphs fed toxic baits. J. Econ. Entomol. 92 (5): 1133-1137.

Kopanic, R.J., and C. Schal. 1997. Relative significance of direct ingestion and adult-mediated translocation of bait to German cockroach (Dictyoptera: Blattellidae) nymphs. J. Econ. Entomol. 90(5): 1073-1079.

Silverman, J., G.I. Vitale, and T.J. Shapas. 1991. Hydramethylnon uptake by Blattella germanica (Orthoptera: Blattellidae) by coprophagy. J. Econ. Entomol. 84(1): 176-180.

Thomas G. Shelton and J. Kenneth Grace. 2003. Toxicant transfer among C. formosanus.