Susceptibility of Diamondback Moth to cartap and abamectin, from five areas with different histories of insecticide spray, was compared using a leaf-dip bioassay. Six concentrations of each insecticide plus water control were evaluated with three to seven replications per concentration. Cabbage leaf disks (2 cm diameter) from 35-day old plants were immersed in test solutions for 10 s. The disks were allowed to air dry for 2 h and then placed in Petri dishes. Ten second-instar larvae were then transferred to each disk. After the treatments insects were maintained in a chamber at temperatures of 20 ± 1ºC and a photoperiod of 13L:11D. Larval mortality was assessed after 48 h of cartap treatment and after 72 h of abamectin treatment. Bioassay results for each insecticide were analysed using the probit method. Resistance ratios were calculated dividing the LC50 of each strain by the LC50 of the most susceptible one, which was collected in a field with no insecticide spray. The resistance ratio to cartap ranged from 2.8 to 7.1, with the most resistant strain coming from a field sprayed several times with the insecticide. To abamectin, the resistance ratios ranged from 1.1 to 12.1. The most resistant strain was from a field without abamectin spray. One cause of this result could be the migration of resistant insects into the field.
Brassica oleracea var. capitata; cabbage; Plutella xylostella; Diamondback Moth; insecticide resistance
