Diamondback moth resistance to insecticides and its management in the Sigatoka Valley, Fiji

Abstract

Diamondback moth (DBM), Plutella xylostella L. (Lepidoptera: Plutellidae) is a serious cabbage pest in Fiji. Excessive insecticide use has led to resistance to all commonly used insecticides, including the recently introduced anthranilic diamide, rynaxypyr. Grower surveys and laboratory bioassays show that the limited range of insecticides available in Fiji results in their overuse and contributes to field resistance. In 2009, 95% of cabbage farmers used pyrethroids against DBM (resistance ratio (RR) to deltamethrin = 41-191), 68% used indoxacarb (RR=40-89), 28% used lufenuron (RR<10) and 23% used organophosphates (RR not determined). Use of Bacillus thuringiensis (Bt) (8% of growers) and rynaxypyr (5%) was limited and field populations were highly susceptible to both insecticides. By 2012, insecticide use had changed markedly, rynaxypyr was used by 41% of growers, pyrethroids by 76%, indoxacarb by 46% but Bt was not used at all. This resulted in RRs for pyrethroids and indoxacarb falling to 12-30 and 1-26, respectively, but the RR for rynaxypyr increased to 13. In the absence of selection in the laboratory, resistance to rynaxypyr remained static over 10 generations but increased markedly (RR=52) within just two generations when exposed to the insecticide. This rapid development of stable field resistance to rynaxypyr is a significant threat to its sustainable use. Field trials show that resistant field populations can be effectively managed by strategic applications of Bt. The approach to making Bt readily available to Fijian farmers is discussed and progress on the development and implementation of a national insecticide resistance management strategy presented.