Diamondback moth resistance to commonly used insecticides and its management in Fiji

Abstract

The diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae) is the most economically important insect pest of Brassica crops worldwide and a serious pest of cabbage crops in Fiji where bi-weekly applications of synthetic insecticides to control it are commonplace. In 2008 a population of P. xylostella was collected from cabbage crops in the Sigatoka valley, the major production area on the main Island of Viti Levu, and its susceptibility to a range of insecticides was compared to that of a standard laboratory insecticide susceptible population (Waite population). The field population exhibited significant resistance to the pyrethroid deltamethrin (resistance ratio (RR) = 487) and indoxacarb (RR=56) but was susceptible to both the acylurea lufenuron and Bacillus thuringiensis subsp. kurstaki (Bt-k). Farmer surveys conducted in 2009, showed that 95% of cabbage farmers in the Sigatoka valley used synthetic pyrethroids against P. xylostella, 67% used indoxacarb, 28% used lufenuron, 23% used organophosphates but only 8% used Bt. In late 2009 six separate populations of P. xylostella were established by collecting larvae and pupae from cabbage fields in the upper, mid and lower regions of the Sigatoka valley, in Koronivia on the east coast of Viti Levu and near Labassa on the neighboring island of Vanua Levu. All three of the Sigatoka valley populations were resistant to deltamethrin (RR ranged 41- 191) and indoxacarb (RR ranged 40-89) but they were susceptible to lufenuron and Bt-k. The Labassa population was resistant to deltamethrin (RR= 150), moderately resistant to indoxacarb (RR=15) but susceptible to lufenuron and Bt-k while the Koronivia population was resistant to lufenuron (RR=29), moderately resistant to indoxacarb (RR=12) but susceptible to deltamethrin and Bt. In the absence of selection in the laboratory, resistance to deltamethrin and indoxacarb in the Sigatoka populations declined and disappeared completely within 10- 15 generations, indicating that resistance to these insecticides confers a fitness cost on diamondback moth. However, laboratory selection of these populations with either deltamethrin or indoxacarb resulted in marked and rapid increases in resistance to these insecticides (deltamethrin RR=466 and indoxacarb RR=892) following selection for three generations with deltamethrin and indoxacarb respectively. Further, there was significant cross-resistance between deltamethrin and indoxacarb. However, increased resistance to deltamethrin and indoxacarb did not affect the susceptibility of insects to lufenuron, Bt-k or the anthranilic diamide chlorantraniliprole, which was uniformly effective against all field populations and the laboratory susceptible population. Bionomic studies in the laboratory showed that resistant field populations of diamondback moth completed development more quickly, had smaller (lower mass) pupae and suffered greater levels of immature mortality than populations with greater susceptibility to insecticides. However, in all populations, irrespective of insecticide resistance status, adult diamondback moth sex ratios were female biased. In the field populations, patterns of insecticide resistance can be correlated with local insecticide use. In order to prolong the useful life of the limited number of insecticide available to cabbage farmers in Fiji an insecticide resistance management (IRM) strategy designed to minimize the selection pressure on target insects was developed and implemented on in the major cabbage growing areas of both Viti Levu and Vanua Levu. The strategy is based on the strategic application of selective insecticides only when densities of the target pest complex exceed experimentally determined threshold levels. Four or five different active ingredients are alternated in a “window strategy” that ensures that successive generations are not exposed to the same insecticide, thereby reducing selection pressures. The results are discussed in the context of the historic use of insecticides in Fiji and the development and implementation of the IRM strategy is considered.