There is no doubt that over the past several years, concern over the efficacy of bait formulations in controlling German cockroaches has been on the rise. In late 2002, six leading urban pest management researchers, who have spent their lives studying German cockroaches, were contacted, and only one indicated that pest management professionals in his area were not expressing similar concerns. And since that time, the frequency of this problem has gotten worse.
The question is: What is the underlying cause for bait failures? Feeding behavior appears to be the problem when control failures occur, indicating behavioral as opposed to physiological resistance. Perhaps, through the overuse of baits, we have caused the development of German cockroach populations that are sensitive to the texture, a component (e.g., glucose), placement, or contaminant of the gel baits.
In their chapter on cockroaches in the 8th edition of the Handbook of Pest Control — Mallis Benson and Zungoli state, "Unfortunately, insecticide resistance is a primary topic for researchers, but is seldom recognized or even recognized in the field by pest management professionals." However, since 1997 when the 8th edition was published, German cockroach bait aversion (behavioral resistance) has become well recognized in the field and has been documented through independent research.
WHAT IS RESISTANCE? Resistance is the development and heritable ability of insect populations to tolerate doses of toxicants that would prove lethal to the majority of individuals in that population. Typically the frequency of resistant individuals is low, but increases greatly over several generations by insecticide selection, i.e., the strong or averted ones survive (Braness, 2004).
Resistance develops much more rapidly in insect populations, e.g., German cockroaches, house flies and mosquitoes that exhibit the following characteristics:
• high reproductive potential,
• short developmental times, and
• minimal migration patterns.
Resistance can take two forms, physiological and behavioral.
• Physiological resistance occurs when an insect population develops resistance mechanisms such as thickened cuticle, detoxification enzymes, absorption, etc.
• Behavioral mechanisms include repellency, avoidance and habitat isolation.
For the most part, physiological resistance mechanisms have been well documented with traditional insecticide formulations and older active ingredients. German cockroach physiological resistance to chlordane was first reported by a pest control operator from Corpus Christi, Texas, in 1951. Physiological resistance to organophosphate and carbamate insecticides was reported from the mid-1970s through the mid-1980s, followed by reports of widespread resistance to most of the pyrethroids since that time. Most recently resistance has been reported to the macrocyclic lactone glycosides and fluorinated sulfonamides (active ingredients found in some cockroach baits), respectively and more commonly known as avermectin and sulfluramid.
Repellency and behavioral mechanisms can negatively impact cockroach management strategies. For instance, Brett and Ross (1985 and 1986) demonstrated insecticide induced dispersal and grooming behavior in a field collected strain of German cockroaches. Silverman and Biemann (1993) found that glucose aversion was responsible for baiting failures in two field collected strains. Further work by Silverman and Ross (1993) suggested that behavioral resistance to bait may become a significant factor in the long term effectiveness of baits.
The overwhelming shift in German cockroach management through baiting and the test of time has confirmed their hypothesis. For example, a consequence of bait failure has caused the bait matrix in the most widely used gel bait in the industry, Maxforce, to be reformulated two times. Additionally, reformulation has involved the use of two active ingredients, hydramethylnon and fipronil. Likewise, one can only assume that similar problems have occurred with the over-the-counter product, Combat.
To satisfy our own curiosity regarding persistent cockroach problems in two apartment buildings we have continuously baited since 1995, we set-up a study to determine the effectiveness of our baiting protocol using Maxforce FC gel bait and Maxforce FC bait stations. The study was conducted over a 28-day period using four sticky monitors per efficiency apartment. (See Table 1 on page 56.)
• Group 1 — approximately 70 grams (two tubes) of gel bait were applied per apartment.
• Group 2 — 12 containerized bait stations were placed in each apartment.
Week one is the pre-treat count of German cockroaches. The data is reported as the average number of German cockroaches collected per recovered trap (occasionally monitors were destroyed or removed by the tenant). Data marked "na" indicates all of the monitors were missing at the time of data collection.
The results in Table 1 reflect the raw field data and have not been analyzed for statistical significance. In reviewing the data, it becomes readily apparent that the baits did decrease the German cockroach populations in the treated apartments; however, in an environment where the expectation is no cockroaches these results are unacceptable. It is important that we gain a better understanding of why this is occurring.
In 1995, Rust, Owens and Reierson indicated that no good examples of behavioral resistance had been documented in cockroaches. However, they further indicated that the best opportunity to investigate behavioral resistance in cockroaches lies with the use of baits. They hypothesized this could be demonstrated by offering a suspect strain the choice of treated and untreated baits. If this strain only ate the untreated bait — while the susceptible strain ate both baits indiscriminately — this would be a strong indication of a genetically based behavioral resistance.
CURRENT RESEARCH. Until recently, neither physiological nor behavioral resistance has been suspected as playing a role in control failures with baits. Our assumptions have been if you place it close enough to the cockroaches and they eat it, they will die. This may no longer be the case.
The following is the little that can be drawn from the current body of research:
• Glucose aversion was demonstrated in earlier formulations of hydramethylnon bait (Maxforce).
• Cross resistance with the earlier and heavily used cyclodiene insecticides (site of action) has been found with fipronil (Dr. Schal) and avermectin (Dr. Bennett).
• The potential for physiological and behavioral resistance has been demonstrated in the lab (Don Reierson).
• In field and lab tests, fipronil provides the quickest knockdown and the highest mortality of all active ingredients (Dr. Bennett).
• If German cockroaches eat any of the currently formulated baits they will die.
The following research was conducted by Dini Miller, VPI and SU. German cockroaches were evaluated in the laboratory for behavioral avoidance of gel bait formulations. The Bubba strain German cockroach was collected from a large state government cafeteria facility in the District of Columbia. The pest management company for this facility was having difficulty controlling the infestation with gel baits (Maxforce, Maxforce FC and Avert). This strain was suspected to be averse to gel bait formulations.
To determine if the Bubba strain was truly averse to gel baits, bait consumption and subsequent mortality was compared in no choice tests with other German cockroach strains: a known susceptible laboratory strain [Virginia Polytechnic and State University ("VPI")] and a field strain ("Jeff Wilson") known to be cyfluthrin resistant.
Evaluations to determine baseline consumption (mg/ 2 h) of dry dog food for each strain indicated that there were no strain related differences in consumption. (See Table 2 on page 58.)
From the preliminary data we were able to conclude that the Bubba strain did consume significantly less of the gel bait formulations than the other two strains. (See Table 3 at left.) However, consumption of the PreEmpt gel bait was not significantly different for any of the strains tested. Because of the reduced consumption, mortality of the Bubba strain was also significantly lower than that of the VPI strain for all gel bait formulations. (See Table 4 on page 119.) However, cockroach mortality for the Bubba and Jeff Wilson strain when exposed to PreEmpt bait was not significantly different.
OBSERVATIONS. In performing these experiments we made several observations. The first was that the Bubba strain cockroaches were attracted to the baits even though they did not consume them. Immediately after the bait was put into the test arenas the cockroaches would leave the harborage, wave their antennae and search for the bait. Once the cockroaches had located the bait they would "taste" it several times with the palps. The antennae would continue to wave as the cockroaches made multiple contacts with the mouthparts on the bait. However, after several "tastes" the cockroaches would walk away.
These behaviors suggest that the antennal receptors of the Bubba strain recognize the gel bait as a potential food resource. However, the receptors on the palps do not. Our observations indicate that it is the reduced palatability of gel formulations, as opposed to repellency or cockroach learned behaviors, that is causing the bait avoidance and increased survivorship of the Bubba strain German cockroaches.
Benson and Zungoli (1997) indicate the traditional management of German cockroach resistance involves several strategies:
• Increasing the volume or concentration of insecticide. Increasing the volume of material applied is feasible but impractical if the population can tolerate much higher doses. The concentration at which a pesticide can be applied is limited by the label and in most resistant cases it is already being maximized.
• Rotation between different classes of insecticides. Theoretically the systematic rotation of insecticides should prevent selection for resistance to insecticides in each class included in the rotation.
• Mixtures. This is a process by which two or more different classes of insecticides with different modes of action are applied concurrently (a shotgun approach) in the hope that each will overcome the shortcomings of the others.
In the case of bait aversion these strategies have little application. The reason is that the behavioral mechanism(s) involved in bait aversion to multiple products are apparently not linked to the active ingredient. Rust, Owens and Reierson (1996) indicate at the present time there are no direct means available to overcome a strictly behavioral resistance mechanism.
Gary Bennett (personal communication) of Purdue University summed up what we are hearing from pest management professionals, i.e., based on lab and field studies baits are not performing as well as they were 10 years ago. He further suggested that pest management professionals should begin practicing cockroach bait management — don’t overuse them and carefully select products for use.
Keep in mind the aversion problem associated with cockroach baits involves isolated populations and does not involve every population of German cockroaches. It is important to rule out other reasons for perceived bait failures, e.g., not enough bait, unbaited areas, contamination, etc., before assuming the problem is bait aversion.
In our experience we have found that these German cockroach strains respond differently to dry bait formulations, e.g., Avert flowable, Niban FG, and we have had some success in further reducing cockroach populations. Alternatively, in areas of repeated bait failures, we are considering reverting to the traditional clean-out crack and crevice spray. Theoretically, if we can destroy these behaviorally resistant populations, we can revert to baiting.
Some manufacturers are keenly aware of bait aversion problems, as evidenced by the second reformulation of Maxforce cockroach gel bait (Maxforce FC Select) and are diligently trying to stay ahead of the cockroaches (Morrison, Barile and Macom, 2004).
Austin Frishman has always said it best, "Bet on the roaches."
The authors are director of technical services, American Pest Management, Takoma Park, Md., and professor, department of entomology, Virginia Tech, Blacksburg, Va.
Benson, E.P. and P.A. Zungoli. 1997. Cockroaches. In: Mallis, A.: Handbook of Pest Control. Mallis Handbook and Technical Training Co., Cleveland, Ohio, pp. 122-203.
Braness, G. A. 2004. Insecticides & pesticide safety. In: Mallis, A.: Handbook of Pest Control. GIE Media Inc., Cleveland, Ohio, pp. 1098-1163.
Brett, B. and M. Ross. 1985. Insecticide-induced dispersal in the German cockroach, Blattella germanica (L.) (Orthoptera: Blattellidae). J. Econ. Entomol. 78: 1293-8.
Brett, B. and M. Ross. 1986. Behavioral responses of German cockroach, Blattella germanica (L.) (Orthoptera: Blattellidae), to propoxur formulation. J. Econ. Entomol. 79: 426-30.
Morrison, G., J. Barile and T.E. Macom. 2004. Roaches take the bait again. Pest Control Technology. 32 (2): 62, 64, 66, 68, 70-71.
Rust, M.K., J.M. Owens and D.A. Reierson. 1995. Understanding and controlling the German cockroach. Oxford University Press, New York, N.Y. 430 pp.
Silverman, J. and D.N. Bieman. 1993. Glucose aversion in the German cockroach, Blattella germanica. J. Insect Physiol. 39: 925-933.
Silverman, J. and M. A. Ross. 1994. Behavioral resistance of field-collected German cockroaches (Blattodea: Blattellidae) to baits containing glucose. Environ. Entomol. 23(2): 425-430.