All resistance alleles are not equal: the high fitness cost of super-kdr in the absence of insecticide.

BACKGROUND: Mutations in the voltage-sensitive sodium channel are an important mechanism of resistance to pyrethroid insecticides. In Musca domestica, common resistance alleles are kdr, super-kdr and kdr-his. The levels of resistance that these alleles confer is known, but the fitness of these alleles relative to each other and to susceptible alleles is unknown. We used crosses from congenic strains of M. domestica to establish populations with known allele frequencies and then examined the changes in allele and genotype frequencies over 25 generations under laboratory conditions. RESULTS: There was a significant fitness cost for the super-kdr allele, which decreased from the starting frequency of 0.25 to 0.05 after 25 generations. The fitness of the kdr, kdr-his and susceptible alleles were similar. The greatest change in genotype frequency was seen for the super-kdr/super-kdr genotype, which was no longer detected after 25 generations. CONCLUSION: The fitness cost associated with the super-kdr allele is consistent with previous reports and appears to be a factor in helping to restrain high levels of resistance in field populations (the super-kdr allele confers higher levels of resistance than kdr or kdr-his). It is known that the relative costs of different alleles are environmentally dependent, but our results also demonstrate that the relative fitness of given alleles depends on which alleles are present in a given population, as previous pairwise comparisons of allele fitness do not exactly match (except for super-kdr) the results obtained using this four allele study. © 2020 Society of Chemical Industry.

Signatures of Insecticide Selection in the Genome of Drosophila melanogaster.

Resistance to insecticides has evolved in multiple insect species, leading to increased application rates and even control failures. Understanding the genetic basis of insecticide resistance is fundamental for mitigating its impact on crop production and disease control. We performed a GWAS approach with the Drosophila Genetic Reference Panel (DGRP) to identify the mutations involved in resistance to two widely used classes of insecticides: organophosphates (OPs, parathion) and pyrethroids (deltamethrin). Most variation in parathion resistance was associated with mutations in the target gene Ace, while most variation in deltamethrin resistance was associated with mutations in Cyp6a23, a gene encoding a detoxification enzyme never previously associated with resistance. A "nested GWAS" further revealed the contribution of other loci: Dscam1 and trpl were implicated in resistance to parathion, but only in lines lacking WolbachiaCyp6a17, the paralogous gene of Cyp6a23, and CG7627, an ATP-binding cassette transporter, were implicated in deltamethrin resistance. We observed signatures of recent selective sweeps at all of these resistance loci and confirmed that the soft sweep at Ace is indeed driven by the identified resistance mutations. Analysis of allele frequencies in additional population samples revealed that most resistance mutations are segregating across the globe, but that frequencies can vary substantially among populations. Altogether, our data reveal that the widely used OP and pyrethroid insecticides imposed a strong selection pressure on natural insect populations. However, it remains unclear why, in Drosophila, resistance evolved due to changes in the target site for OPs, but due to a detoxification enzyme for pyrethroids.

The next generation of insecticides: dsRNA is stable as a foliar-applied insecticide.

BACKGROUND: RNAi is a powerful tool used to study gene function. It also has been hypothesized to be a promising new method for control of insect pests on crops, although the perceived instability of dsRNA in the environment has constrained thinking about the options for this new type of pest control. RESULTS: We confirmed that foliar application of Colorado potato beetle dsRNA actin is highly effective for control, demonstrated that treatment with actin-dsRNA protects potato plants for at least 28 days under greenhouse conditions and found that the dsRNA is not readily removed by water once dried on the leaves. CONCLUSION: These new results suggest that foliar application of dsRNA could be a valuable control strategy for some pests. Technological aspects of spraying dsRNA that need to be considered in the future are discussed.