The German cockroach (Blattella germanica) is a common pest in buildings throughout the world. It spreads pathogens that can cause diarrheal disease and allergens that can cause lung hypersensitivity in vulnerable people. Insecticide sprays and insecticide-loaded baits have been used to control B. germanica for decades, but there is evidence that some populations have developed resistance to insecticides.
Little is known about the extent of resistance, and the last study of insecticide resistance in German cockroaches in California was conducted more than 30 years ago. To remedy that, a group of researchers at the University of California, Riverside, tested the efficacy of a range of different baits and insecticides to determine their degree of resistance. Their findings were published in December in the Journal of Economic Entomology.
The reproductive cycle of B. germanica consists of an adult female laying eggs, eggs developing into nymphs, and nymphs proceeding through a series of instars and then becoming adults. The reproductive capacity of the species is huge: Given unlimited resources and under optimal environmental conditions, one pair of adults and their offspring can generate 10,000 cockroaches in one year.
The magnitude of the German cockroach problem is daunting. “Pest Control Technology magazine’s 2019 survey among pest management professionals ranked cockroach control (23 percent) as the most important service revenue generator, more even than pest ants (22 percent). And, for every four cockroach service calls pest management professionals do, three jobs are related to German cockroaches,” says Chow-Yang Lee, Ph.D., BCE, professor and endowed presidential chair in urban entomology at UC Riverside and senior author on the study.
The impact of German cockroaches is more than just economic—they also can negatively affect public health. “There are two significant health risks associated with a German cockroach infestation in homes,” Lee says. “They can transmit pathogenic microbes (which can also spread antibiotic resistance genes), and they can produce compounds that can trigger allergies and asthma.” Blatella germanica can leave pathogenic microbes on the food they walk over, spreading food-borne illnesses such as E. coli, Salmonella, and Rotavirus.
Personnel attempting to control B. germanica usually spray residual insecticides or leave gel baits in areas the cockroaches frequent. “Application of gel baits offers the advantages of being precisely placed, being more target-specific than a residual spray, and using less insecticide,” Lee says. “However, insecticide resistance in the German cockroach has become a significant challenge to the pest management industry.”
To test the degree of resistance to commonly used insecticides and bait formulations, Lee and colleagues tested six insecticides and five commercial baits against five strains of field-caught B. germanica from different locations in California. As a control, they also tested the insecticides and baits against one control strain of B. germanica from the lab that was never exposed to insecticides and is known to not be resistant to any insecticides. The insecticides tested were fipronil, clothianidin, indoxacarb, abamectin, hydramethylnon, and deltamethrin. The baits tested were Maxforce FC Magnum, Maxforce Impact, Advion Evolution, Optigard, and Siege.
In their experiments evaluating baits, Lee and colleagues used adult males because of their consistent size. Males were added to experimental arenas containing dog food and a paper cup for shelter. After two days, insecticide bait was added to each arena on a small plastic tray.
Of the baits tested, Maxforce Magnum, Advion Evolution, and Siege all showed the most pronounced decreased effectiveness against field-caught insects relative to lab-reared (control) insects. Mortality for Maxforce Magnum in field-caught German cockroach strains tested was in the range of 50–80 percent, mortality for Advion Evolution was 80–100 percent, and mortality for Siege was 60–90 percent.
The researchers determined diagnostic doses by diluting insecticides with acetone and applying the insecticides to the abdomens of male cockroaches. Controls had only acetone applied to their abdomens. They then recorded mortality after 72 hours (and after 120 hours for hydramethylnon). They used these data to calculate the lethal dose that would kill 95 percent of a population for each insecticide, something called the LD95. Then they screened each strain using diagnostic doses of three times the LD95 with each insecticide (3xLD95) and with 10 times the LD95 with each insecticide (except for hydramethylnon). “The mortality was measured at 120 hours for hydramethylnon,” Lee says, “because it is a slow-acting insecticide. If we measure the mortality too early, it will give us an impression that the active ingredient is not effective.”
Mortality for cockroaches receiving a 3xLD95 insecticide concentration was 0 percent for deltamethrin, 0–3 percent for fipronil, and 13–27 percent for clothianidin. In other words, mortality was low and insecticide resistance high for all three of these insecticides.
Mortality for cockroaches receiving a 3xLD95 insecticide concentration was 13–63 percent for indoxacarb, 70–83 percent for hydramethylnon, and 80–100 percent for abamectin. Thus, mortality was consistently high, and resistance consistently low for abamectin.
Mortality of five field-caught cockroach strains (WM, RG386, Ryan, CDR, and SY) and a control strain (UCR) following treatment with six insecticides. Darker boxes indicate lower mortality, and thus greater insecticide resistance. Note that insecticide resistance was found for all insecticides tested except for abamectin. (Figure originally published in Lee et al 2021, Journal of Economic Entomology)
Resistance was found for all insecticides tested except for abamectin. This insecticide, which is part of the avermectin family of insecticides, is created by Streptomyces avermitilis, a bacterium that lives in soil. Abamectin causes toxicity in insects by binding to and disrupting glutamate-gated chloride channels, leading to the flow of chloride ions into cells, which disrupts cell function. Why wasn’t there resistance to abamectin? “This is likely due to the resistance mechanisms found in these strains,” Lee says. “Because of the mode of action of abamectin, it was not overcome by these resistance mechanisms. However, a decrease in abamectin susceptibility in field populations of B. germanica, when used for an extended period, had been reported in another study in Indiana.”
This study provided the first comprehensive test for insecticide resistance in German cockroach populations from California in over 30 years. Lee says, “We found that German cockroaches display broad-spectrum resistance to most major classes of insecticides in California populations.”
There are several caveats that Lee and colleagues mention in their paper. In the bait assays, since the cockroaches are contained in the test arena, they might touch or eat the bait multiple times, which would tend to increase the insecticide’s effectiveness. Another caveat is that only adult males were tested in this study, which could overestimate the effectiveness of baits. The nymphs and females are less likely to be killed in these lab tests. If you are unable to kill the males, you are even less likely able to kill the other stages. In terms of potential solutions, the researchers note the need to screen for insecticide resistance in field populations on an ongoing basis as a component of control programs. In addition, they suggest that synergist compounds could be included in insecticide baits to increase their effectiveness.
Because the insecticide baits tested in the study are available in stores, their results are valuable not only to entomologists and pest management professionals but also to the general public. This work provides an important advance for understanding the degree of insecticide resistance in B. germanica in California, but additional research is needed to examine resistance in populations in other regions. These findings from Lee and colleagues can help inform optimized control strategies, with the promise of lessening the huge economic and health burdens of infestations of this resilient species.