Reduced susceptibility to permethrin in diamondback moth populations from vegetable and non-vegetable hosts in southern Australia

Diamondback moth (DBM), Plutella xylostella (L.), has attained major pest status in Brassica vegetable crops around the world. In many cases, use of synthetic pyrethroid insecticides for control of other pests, such as Pieris rapae (L.), has disrupted natural enemies and selected for insecticide resistance in DBM, changing the pest status of the moth from minor to major. We estimated levels of resistance to the synthetic pyrethroid, permethrin, using a leaf dip bioassay, for 28 DBM populations collected from brassicaceous weeds, canola, forage turnips and seed turnips and for five DBM populations from Brassica vegetables. Populations were collected in Victoria, Tasmania, southern New South Wales, ACT, South Australia and Western Australia between September 1999 and January 2000. Nineteen of 28 populations from nonvegetable hosts were significantly more tolerant than a susceptible laboratory population (resistance ratios ranged from 2.1 to 6.9). All five populations from vegetable hosts were significantly tolerant (resistance ratios from 3.6 to 13.0). These results indicate that populations of DBM with reduced susceptibility to permethrin may be found in areas that are remote from intensive vegetable growing districts. Brassica vegetables account for only a small proportion of the host plants available for DBM in southern Australia. Large areas of non-vegetable hosts have, in the past, received few applications of insecticides. Reports that growers of forage brassicas and canola are finding it necessary to apply synthetic pyrethroids to their crops with increasing frequency (1–4 applications per crop) suggest that resistant DBM populations are being generated. Alternatively, DBM populations may be moving from the more intensively sprayed vegetable crops onto non-vegetable hosts. Further studies on the insecticide resistance status of DBM populations from a range of host plants in different locations in conjunction with use of molecular markers to study population structure of DBM, may provide evidence of isolation or mixing of populations which will have important consequences for insecticide resistance management.