| dc.description.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 | en_US |
