Insecticide Application Practices Among Lowland and Highland Cruciferous Vegetable Farmers and Their Relationship Upon Plutella Xylostella L. Resistance Development

This study aimed to evaluate the insecticides resistance development of diamondback moth (DBM), Plutella xylostella L. in relation to insecticide application practices. A study was initiated with a survey to assess the current insecticides application practices for controlling DBM in the highland and lowland areas in Malaysia. Survey data were analyzed with SPSS statistical software using Independent-samples T-test and Pearson correlation where appropriate. The survey showed complete reliance on insecticide mixtures to control DBM. A total of 11 and 14 active ingredients of insecticides were found to be commonly used in the lowland and highland respectively. The percentage of highland farmers used indoxacarb, spinosad, pyridalyl and fipronil was higher than lowland farmers, while lowland farmers recorded higher usage of chlorfenapyr than highland farmers. Besides that, a comparative usage for emamectin benzoate was reported, with slight greater use in the lowland (53%) than highland (41%). Highland farmers were found to apply significantly higher spray volume at 3.8-fold greater than lowland farmers (t = 7.361; P<0.05). Majority of insecticides were applied at tank concentrations below recommended doses. Highland farmers were found to apply significantly lower doses of emamectin benzoate compared to lowland farmers. A leaf-dip bioassay was conducted to investigate the susceptibilities of two DBM field strains to the six selected commonly used insecticides among the cruciferous vegetable farmers namely emamectin benzoate, spinosad, indoxacarb, fipronil, pyridalyl and chlorfenapyr. Results from the bioassays showed inter-population genetic variation in susceptibility between the two field strains of DBM. Higher LC values for all the insecticides except chlorfenapyr were reported on the Highland strain indicating that the Lowland strain is more susceptible to most of the insecticides tested. Comparison of LC50 values between the two strains showed that the Highland strain is significantly (P<0.05) less susceptible to spinosad and pyridalyl than lowland strain. On the other hand, Lowland strain was significantly (P<0.05) less susceptible to chlorfenapyr. The reported higher LC50 values were consistent with greater uses of the insecticides in respective location and the inter-population variation in susceptibility of DBM is due to local insecticide selection. With a lower usage but significantly higher LC90 for emamectin benzoate, the results suggested that reduced doses of emamectin benzoate practiced by the highland farmers accelerated the rate of resistance development in DBM.

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