TARGET-SITE MUTATIONS, BASELINE SUSCEPTIBILITY AND CROSSRESISTANCE EVALUATION OF FALL ARMYWORM INFESTING MAIZE IN KENYA

Abstract

The fall armyworm, Spodoptera frugiperda, is a pest of gramineous crops known worldwide. It is resistant to various insecticides presenting a serious challenge in its wake. Sodium channels that are voltage-gated, ryanodine receptors and acetylcholinesterase’s active site transmit electrical signals in excitable cells. The sites are targets of various synthetic insecticides. The current study sought to establish baseline susceptibility and investigate the cross-resistance of fall armyworm to a range of nine insecticides,determine its molecular tolerance mechanisms via gene expression, and model target-site mutations in sodium channels. A complete randomized design was used in selection of test insect samples and leaf discs subjected to different insecticides. Spodoptera frugiperda larvae, fourth to the sixth instar were collected from different geographical sites in infested maize plantations. They were then reared in the laboratory under ambient conditions allowing them to mass mate. F2 third larva instar was subjected to leaf-dip bioassay to determine baseline susceptibility. A software named Polo Plus was utilized to determine the median lethal concentrations of the pest populations. The resistance ratio value was determined via division of the median lethal concentration (LC50) value of each field population by the corresponding LC50 value of the susceptible strain. Cross-resistance pattern was determined through Pearson’s correlation analysis. Quantitative PCR were conducted to determine gene expression in the 3 target sites to validate their involvement in molecular tolerance. In silico approach was conducted to locate the existing docking sites on pest’s sodium channels along with the interactions between study toxicants. The results demonstrated a low presence of resistance (1 to 4-folds) to the insecticides tested. Abamectin was the least potent with a ratio of 1 while spinosyns were the most potent (spinetoram, 11188, spinosad, 7079). Lambda cyhalothrin showed weak correlations to the 8 insecticides tested hence a lack of cross-resistance to them. Secondly, voltage-gated sodium channel, acetylcholinesterase’s active site, ryanodine receptors had 13.59, 34.93 and 4.90- fold higher expression than the untreated samples, respectively. The genes in these regions were up regulated in the wild type than in the knock down genes due to the positive fold changes. Thirdly, residue Serine1873 exhibited the most frequent interactions with the 6 insecticides used forming close binding contacts (<4 Å) with the insecticides. This hasn’t been implicated previously in mutations that cause knockdown resistance in this pest. Cartap exhibited the highest number of binding sites. Its binding capability to this site has not been reported previously. Indoxacarb had 3 different binding amino acids namely Serine 1873, Tyrosine 1927, and Asparagine 1045, different from mutations that have been previously attributed to its resistance. In conclusion, spinosyns, and lufenuron exhibited high toxicity to FAW while imidacloprid and abamectin were the least potent. Lower quantities of relative transcripts and the positive fold changes in expression validates molecular tolerance. Residue Ser1873 had the most interactions hence should be considered in making of more efficacious insecticides