Baseline Susceptibility of Plutella xylostella (Lepidoptera: Plutellidae) to the Novel Insecticide Spinetoram in China

Spinetoram is a spinosyn, which is a unique class of natural insecticide. Because of its novel mode of action, spinetoram is more potent and faster acting than other insecticides, even the older spinosyn product, spinosad. On account of being efficient on insect order Lepidoptera, spinetoram provides a new alternative for control of Plutella xylostella (L.) (Lepidoptera: Plutellidae), which are resistant to other chemicals. To determine the current situation of resistance of P. xylostella to spinetoram, the susceptibility of 16 P. xylostella populations from different regions of China or different time in addition to the population from laboratory was assessed using a leaf dip bioassay. The variation in spinetoram susceptibility among the 16 field populations was narrow, with median lethal concentrations (LC50 values) ranging from 0.131 to 1.001 mg/liter. Toxicity ratios (TRs) ranged from 1.5 to 7.6 and were 5.6 and 7.6 for populations SY-2 and FX-1, respectively, indicating some low level of tolerance in these populations. A discriminating concentration (a concentration that can detect the occurrence of resistance in a population) of 10 mg/liter, which was identified based on the pooled toxicological data, caused 100% mortality in all nine tested populations. The baseline susceptibility data reflect the natural variation of the P. xylostella populations to spinetoram rather than variation caused by previous exposure.