Insecticide resistance in diamondback moth Plutella xylostella (L.) in Canterbury, New Zealand

Several local populations and one Australian strain of diamondback moth, Plutella xylostella (L) were compared with regard to their susceptibility to five insecticides commonly used to control this pest on brassica crops throughout New Zealand. The local strains were sampled from vegetable brassica fields in and around Christchurch, while the Australian population was reared from insects obtained from a carton of broccoli acquired at the Brisbane markets. This broccoli was said to have originated from a broccoli growing area in Gatton, 200 km west of Brisbane. This region exports broccoli to New Zealand. Traditional laboratory bioassay methods reflected a level of resistance in the Gatton population that emulated the high levels of resistance already documented in major brassica growing areas in Australia. However, some local populations tested also showed high resistance levels to pyrethroid insecticides. Although these levels were not as high as that determined for the Australian population, they were high enough to cause concern for local growers and heighten the need for a resistance management strategy. The possibility of imported resistant strains arriving on consignments of Australian broccoli or being transported in trans-Tasman airstreams and either establishing as founder events and/or interbreeding with local populations (thereby accelerating the evolution of insecticide resistance locally) was investigated. Local and imported strains were interbred using reciprocal crosses (resistant females by susceptible males and vice versa), and the resulting F1 progeny were tested for evidence of sex linkage of resistance traits and whether one or more gene(s) were responsible for the resistance. The F1 progeny were then crossed with the most phenotypically different parent (the susceptible Harewood strain). A plateau in the dose response data around 50% mortality was not revealed indicating that P. xylostella resistance to pyrethroids was likely to be due to more than one gene. The level of resistance to pyrethroid insecticides observed from bioassays of the Gatton and Gatton-Harewood crosses should be of some concern to local brassica growers whose crops may be exposed to the small populations dispersing from imported Australian consignments. However, this particular threat has been largely, but not entirely diminished, by the discovery of high resistance to pyrethroids in a cabbage field in a relatively isolated site at Halkett, 20 miles west of Christchurch. While the influence of immigrant resistant moths from Australia can not be totally discounted, the most likely cause for this comparatively high level of resistance is over-reliance of pyrethroid insecticides and a relatively stable population. This trend is now becoming more evident from bioassay results taken from other vegetable brassica producing areas in New Zealand. The implications of these findings for insecticide resistance management are discussed.