Cockroaches Are Becoming Immune To Insecticides

Insecticide resistance has been estimated to cost Americans $133 million annually and perhaps $1 billion worldwide. The German cockroach has developed resistance to all of the traditional insecticides and cross resistance has been reported among many new insecticides. One group of insecticides, the pyrethroids, has low mammalian toxicity and is very effective at killing insects. Unfortunately, insecticide resistance has rendered many pyrethroid insecticides ineffective. One form of insecticide resistance is knockdown resistance or kdr. Kdr insects possess a mutated form of a nervous system protein that prevents pyrethroid insecticides from binding to it. Scientists at the Center for Medical, Agricultural and Veterinary Entomology, Gainesville, Florida have developed a detection method capable of discriminating mutated (insecticide-resistant) nervous system proteins from normal ones (insecticide-susceptible) in the German cockroach by examining the genes that encode for them. As part of an IPM approach to pest control, detection of insecticide resistance before treatments are chosen and dispensed would improve control and eliminate ineffective insecticide applications.

Extensive use of pyrethroid insecticides for urban pest control has led to widespread pyrethroid resistance in the German cockroach. A mutation at nucleotide position 2979 (G to C) causing a leucine to phenylalanine change in the S6 transmembrane segment of domain II of the para-homologous voltage-gated sodium channel has been previously identified in knockdown resistant cockroaches and demonstrated to reduce pyrethroid sensitivity in wildtype voltage-gated sodium channels modified with this mutation by site-directed mutagenesis. In a recent survey, it was found that 83% of pyrethroid-resistant German cockroach populations possessed this mutation. Reliable methods to detect resistance, including the kdr-type mutation, are critical components of an effective resistance management plan. A German cockroach strain with a low incidence of the L993F mutation was subjected to intense selection pressure with cypermethrin and subsequently evaluated over several generations for the knockdown resistance phenotype. Correspondingly, we determined the genotype of individual cockroaches of each population at the 2979 position of the para-homologous gene. Genotype was discerned by development of a PCR method that employed a mismatched primer-template set. A direct relationship was observed between mean knockdown time and the presence of the kdr mutation. Further, individuals homozygous for the kdr mutation exhibited a significantly higher mean knockdown time than heterozygotes or wildtype cockroaches. This diagnostic test should significantly improve the capability of pyrethroid resistance monitoring in the German cockroach.