A New Technique for the Rapid Prediction of Insecticide Resistance in German Cockroaches

The German cockroach, Blattella germanica L, has been infamous in the insect world for its rapid development of resistance to pesticides (Brown 1971, Cornwell 1976 and Cochran 1982). Some strains of German cockroaches are resistant to organophosphate, carbamate or pyrethroid insecticides (Cochran 1989); other strains are resistant to two or three of these insecticide groups.

Insecticide resistance in cockroaches is the result of long-term exposure to low levels of pesticide (Cornwell 1976). The low levels of pesticides usually are a result of product breakdown during storage, faulty preparation of the pesticide before application or poor application procedures. When populations of cockroaches are exposed to these insufficient residues, susceptible cockroaches are killed but cockroaches with genes for resistance remain alive, resulting in control failures.

Most pest control companies blame control failures on technicians. Technicians occasionally are rushed and do not mix the chemical properly or apply it thoroughly in accounts. Correction of technician error is straightforward. Basically technician training, worker incentives and quality control solve the problem. Usually supervisors tell technicians to formulate new material, increase the application rate, do a better job of application or, if all else fails, change the chemical in hopes of achieving satisfactory control. All these options are time consuming, expensive and do not guarantee success (Bennett and Owens 1987) or satisfied customers. Customers want the insects eliminated with the least amount of toxic material applied to their home or yard.

Most university researchers think control failures are primarily due to insecticide resistance. If a control failure is due to insecticide resistance, technician training and incentives will not solve the problem. Much time and money could be saved for the pest control industry if there were an easy way to determine whether a control failure was due to technician error or insecticide resistance.

PREVIOUS TESTING METHODS. Insecticide resistance has usually been documented in a research laboratory using either direct application of the pesticide to the insect (e.g., topical, immersion or spray treatments) or indirect application (insects contaminate themselves by coming in contact with a treated surface or consuming a treated food). These standard testing techniques used for insecticide resistance evaluations usually require large numbers of insects. German cockroaches must be captured alive in homes, restaurants and apartments. The captured cockroaches are sent to a laboratory where they are raised in sufficient numbers so they can be directly or indirectly treated with the chemical. The results of these treatments then allow the scientist to determine if resistance is present. LD50s and LD90s (or the LT50s and LT90s) are used to determine the presence and level of resistance in a cockroach population.

The entire testing procedure is cumbersome, time-consuming and difficult to implement. Pest control operators do not have the equipment, facilities or trained personnel to handle live cockroaches. Many times insufficient cockroaches are collected, collected cockroaches die of insecticide contamination, heat or cold in the service vehicle; or the field-collected colony is lost before any evaluations are done. When field-collected cockroaches are successfully given to a scientist, they are held in the laboratory for several generations so that enough cockroaches are available for testing. Laboratory colonization often results in insecticide resistance changes before testing. If everything goes perfectly, the pest control operator will find out whether his control failure was due to resistance in six to 12 months. Unfortunately, the property owner who is troubled by cockroaches wants control immediately, not six months or a year later. Consequently, there is a lot of trial and error use of insecticides by pest control operators, as well as new chemicals being tried to overcome the control failure.

A SOLUTION? There are several potential solutions to the current problems with assaying insecticide resistance. The World Health Organization test kit - or a modification of it - used field collected or laboratory-reared individuals which are exposed to insecticide residues. Mortality is recorded at different times. Unfortunately, the pest control operator would have to collect and do experiments on live cockroaches. Biochemical assays for specific insecticide resistance mechanisms could provide instantaneous results, but usually require years and a lot of expensive equipment to develop. Although a simple bioassay can indicate insecticide resistance, it can be very misleading unless proper precautions are taken. To get the most accurate readings the insects must be the same age and sex.

The USDA-ARS Medical and Veterinary Entomology Research Laboratory has been working on the development of a simple method of evaluating insecticide resistance in German cockroaches. In this paper we report the development of a new method for the evaluation of German cockroach insecticide resistance which can be easily used by the pest control operator.

Sticky traps and/or glue boards have been used for years to monitor and control German cockroach populations (Ballard and Gold 1982, Owens and Bennett 1988). If placed in the proper locations, large numbers of cockroaches can be captured in a very short period of time. It has been estimated that a standard cockroach sticky trap which is about 2 x 4 x 6 inches will capture only 2 percent of the population in a 24-hour period (Koehler et al. 1985). Therefore, sticky traps are not very practical for controlling cockroaches but are very useful for monitoring populations.

Because German cockroaches caught in the glue of these traps can live for several days, we decided to incorporate insecticide into test sticky traps and put German cockroaches onto the glue (e.g. sticky traps with various insecticides incorporated into them have been used to monitor insecticide resistance in agricultural arthropod pests [Haynes 1987]). Our theory was that resistant cockroaches caught in the insecticide-treated traps would survive and susceptible ones would die. Therefore, pest control operators could quickly determine broad resistance profiles of wild cockroaches.

Because we maintain a number of different German cockroach strains in culture, with various degrees of insecticide resistance, it was fairly easy to evaluate the new type glue/toxicant traps. We have determined the LC50s of insecticides in glue for chlorpyrifos bendiocarb and cypermethrin. By using insects which have different levels of insecticide resistance we calculated resistance ratios (LC50s of resistant/LC50 or susceptible). The three major insecticide groups were evaluated, the organophosphates (chlorpyrifos), carbamates (bendiocarb) and the pyrethroids (cypermethrin). These insecticides were then mixed into glue which had been thinned with a solvent. The insecticide-glue mixture was painted on cardboard disks (100 mm diameter). The solvent was allowed to evaporate for 24 hours in a fume hood, leaving the insecticide-impregnated glue. Disks with the various dosages of the pesticides were prepared so that a dose mortality curve could be developed for each insecticide. Adult male cockroaches gave the most consistent results so they were used for our standard bioassay animal. Adult male cockroaches from one to seven days of age were placed on the glue/insecticide cardboard disks which were then placed in a petri dish. These dishes of cockroaches stuck in the glue mixture were held at 26 degrees Centigrade. Mortality of the cockroaches was determined, at various time intervals, by prodding them with a probe. The cockroaches were considered dead if they did not move at all.

Topical insecticide applications were performed on the strains of cockroaches so that we would have a conventional assessment of insecticide susceptibility for comparison. The insecticides were applied in acetone solution between the hind legs of adult male cockroaches which had been immobilized with carbon dioxide. Cockroaches were counted as dead if they could not right themselves 24 hours after the insecticide application. Resistance ratios were calculated from the LD50 value in the same manner used for the glue/insecticide bioassay. We compared the resistance ratios for topical application of insecticides with those for insects in insecticide/glue mixtures for the three insecticides. The results were comparable (Table 1).

There were some differences in the ratios, but the same trend was apparent in all cases. The ratios were the greatest in strain B with the cypermethrin. The glue/insecticide trap did give a clear indication of the high degree of resistance in strain B to the pyrethroid insecticides. Based on findings of Atkinson et al. (1991), the B strain of German cockroach is resistant to all pyrethroids. One interesting fact is that Strain A originally had a seven-fold resistance ratio to bendiocarb when it was tested in 1988. When the glue/insecticide traps were tested, no resistance to bendiocarb was seen. Topical studies were rerun on all the strains and it confirmed what was shown by the glue/toxicant trap, the Strain A had indeed lost its resistance to bendiocarb.

FIELD STUDIES. Our next step was to try our glue/toxicant trap in the field to see if it could be used to predict the success or failure of a compound if it was applied correctly and according to label rates. One percent chlorpyrifos or cypermethrin glue/insecticide traps were placed in an apartment in a low-income public housing project. The traps were collected the next morning and held for another 24 hours at 26 degrees Centigrade before being evaluated. The apartment was then treated with either chlorpyrifos or cypermethrin at the labeled rate. The glue/chlorpyrifos trap predicted a kill of 17 percent; chlorpyrifos treatment killed 32 percent of the cockroaches in the apartment. The glue/cypermethrin trap predicted a kill of 85 percent; cypermethrin treatment resulted in a 95 percent kill of the cockroaches in the apartment.

Thus a simple technique has been developed to easily and quickly determine if you are dealing with a application problem or true insecticide resistance and what group of chemicals you should switch to. It is easy, safe, inexpensive and reliable for the pest control operator to handle.

We anticipate that this monitoring system will be a useful research tool, however the main purpose of its design was to provide a means for pest control operators and large institutions to forecast which insecticides would give control or predict failure. Baited cockroach traps with insecticide-treated glue will be distributed to pest control companies. The dose of insecticide in the traps will be designed to kill non-resistant cockroaches and leave resistant cockroaches alive. Pest control operators should place these traps in infested or problem accounts.

Several traps, each containing a different insecticide which could be used in the situation, should be placed in infested areas of the account. The traps should be left overnight, retrieved and returned to the office. On the day after pickup, the technician should count the total number of live and dead cockroaches in the trap. Percent survival in each trap would predict the percentage of cockroaches which would survive the treatment with that insecticide. The pest control operator could then make informed decisions to choose the effective insecticide for an account. This procedure would result in a reduced number of call-backs and treatment failures.

James I. Moss and Richard S. Patterson are with the USDA-ARS, Medical and Veterinary Entomology Research Laboratory, Gainesville, Fla. Philip G. Koehler is with the Department of Entomology and Nematology, University of Florida, Gainesville, Fla.

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