Richard Murray, Research and Development Manager for PCT International, outlines the findings of the latest research into German cockroach resistance from the US.
The German cockroach is probably the most widespread and successful cockroach that coexists with humans. It is the most prolific breeder of all the pest cockroaches, taking as little as 40 days to develop from hatching to adulthood. Due to their quick action, low cost, and availability, gel bait insecticides have been extensively used in cockroach management. However, after decades of continued use, issues with reduced effectiveness of such formulations are becoming increasingly common throughout the world. Multiple cases of resistance towards baits have been reported in the US in the past decade, and with evidence of cross resistance, the use of product rotation and mixture strategies, which can minimise the development of resistance in non-resistant populations, appear to be less effective in controlling populations already exhibiting high levels of resistance.
Many US pest managers and housing authority personnel, particularly in several Californian cities, have been reporting the reduced performance of cockroach treatments at residential sites.
The last survey on insecticide resistance occurred 30 years ago with limited testing of a single insecticide (chlorpyrifos) and before the widespread adoption of bait insecticides. To address this, five eminent researchers including Dr Chow-Yang Lee (known to many Australian pest managers) conducted a study to test cockroach resistance to gel baits, with the paper published in the Journal of Economic Entomology in December 2021.1
In the study the researchers collected five samples of German cockroaches from the field and evaluated them against five commercial bait products. A lab-reared strain of German cockroach with no previous insecticide exposure was used in this study to determine the baseline toxicity of insecticides and served as a standard for comparison. The five commercial bait products were respectively 0.05% fipronil, 1% clothianidin, 0.6% indoxacarb, 0.1% emamectin benzoate and 2% hydramethylnon.
In the trials, cockroaches were allowed to feed freely on bait or dog food over a period of 14 days. They were removed from the arena when dead or unable to walk, and the number of dead counted at the end.
The level of effectiveness of the gel baits varied greatly between the five cockroach samples. However, it was clear that the field-collected German cockroaches showed resistance to four of the five baits when compared to the lab-reared strain. The best performing bait was 0.1% emamectin benzoate, which achieved a mortality rate of 93-100% compared with 50-80% of the 0.05% fipronil, the weakest performer.
As a follow up, the researchers decided to assess the mortality of the cockroaches when the insecticides were applied directly as a droplet, at high and low concentrations. In addition to the insecticides previously tested, the researchers also tested abamectin (used in place of emamectin benzoate) and deltamethrin, which is absent in baits, but commonly found in many residual insecticide spray products.
The field-collected cockroaches demonstrated almost total resistance to deltamethrin. With fipronil, all field-collected cockroaches showed resistance, with mortality ranging from 20-70% at the higher dose. Resistance towards clothianidin and indoxacarb varied between cockroach samples, with mortality varying between 53-100% for both insecticides. All cockroaches were found to be resistant to hydramethylnon at the lowest level, with mortality between 70–83%. Abamectin performed the most effectively, achieving total mortality across all cockroaches when applied at the highest concentration, and more than 80% when applied at the lowest concentration.
In summary, the gel bait containing 0.1% emamectin benzoate and the direct application of abamectin achieved the highest overall mortality rates.
The findings suggest that abamectin is a useful tool in cockroach management programs. A member of the family of macrocyclic lactones (chemicals derived from soil microorganisms), the researchers noted that abamectin may have particular use “treating populations with high resistance towards other insecticides or those which tend towards higher proportions of survivors after treatment”.
Australian pest managers are fortunate to have access to an abamectin-based cockroach gel bait from PCT in Avert Cockroach Gel Bait containing 0.5 g/kg abamectin for use in resistance management programs.
Pest managers must be aware that consistent exposure to excessive lethal doses can increase the potential of high resistance levels, and research by scientists has demonstrated that judicious use of cockroach insecticides can result in further overall resistance development.
The scientists conducting this study have concluded that more research is required in several key areas. For example, there is the possibility of incorporating synergists to improve the efficiency of individual insecticides, and screening for insecticide resistance in the field would inevitably improve product selection. Such novel strategies to combat resistance will be vital for the pest control industry’s continued reliance on cockroach baits not only in the US, but here in Australia too.
1 Lee, Shao-Hung & Choe, Dong-Hwan & Rust, Michael & Lee, Chow-Yang. (2021). Reduced Susceptibility Towards Commercial Bait Insecticides in Field German Cockroach (Blattodea: Ectobiidae) Populations From California. Journal of Economic Entomology. 115. 10.1093/jee/toab244.
Richard Murray, Research and Development Manager, PCT International