| The diamondback moth, Plutella xylostella L. (Lepidoptera:Plutellidae), is one of the most important insect pests of cruciferous vegetables throughout the world, particularly broccoli, cauliflower and cabbage. According to a recent survey that overall management costs were estimated at US$ 4-5 billion annually. The use of insecticides sprays has been the main method for diamondback moth control in most areas of China. Because of the widespread selection pressure in field, P. xylostella has developed resistance to almost all chemical classes of insecticides, and it has become one of the most difficult pests to management. It is essential to monitor of insecticides resistance over wide areas of cruciferous vegetables production, in order to master the level of resistance in the current. And the result will provide a scientific guideline for use of insecticides and resistance management in P. xylostella.With the widespread insecticides resistance to major chemical classes of insecticides, the insecticides can be effective used against P. xylostella were less and less, it is necessary to develop some new insecticides with unique modes of action. Tolfenpyrad is an insecticide which was supposed to possess a different mode of action from other existing insecticides, but there is little information on the toxicity efficacy of tolfenpyrad on diamondback moth and also no IRAC standard bioassay method for Group 21 A. It is important to establish an bioassay standard to evaluate the activity of tolfenpyrad against the diamondback moth in the laboratory conditions, and assess resistance risk in P. xylostella to this new insecticide, it is very meanful to protect this kind of insecticide and formulate resistance management strategy.1. Monitoring resistance of field populations of Plutella xylostella to eight insecticidesIn this study, a leaf dipping method was development for bioassay of insecticide susceptibility of P. xylostella. Thirty-one populations of the diamondback moth were collected from fields of crucifer vegetables in China from 2012 and 2013 for resistance monitoring to chlorantraniliprole, spinosad, chlorfenapyr, abamectin, beta-cypermethrin, diafenthiuron, chlorfluazuron and Bt. Based on resistance levels, chlorantraniliprole exhibited highly effective against P. xylostella in most locations of China, except the Taihe and Wuxi populations where have developed moderate to high resistance (RR=24.8-63.4) and high to very high resistance (RR=61.4-162.0) to chlorantraniliprole, respectively. Approximately 61% of tested populations displayed moderate resistance to spinosad, while other tested populations revealed narrow variation (<3-fold) in their tolerant level to this insecticide. Obvious variation (93-fold) in the response of P. xylostella to chlorfenapyr existed in field populations of which 32% displayed low level resistance, and 36% exhibited moderate resistance. Particularly, the Wuxi population collected in 2013 september had developed very high resistance (RR=260.1) to chlorfenapyr. Almost all of the tested populations had developed high to very high resistance to abamectin and beta-cypermethrin. Overall, diafenthiuron and chlorfluazuron exhibited highly effective against P. xylostella in most locations of China. Decreased susceptibility to moderate resistance to the bio-pesticide Bt was observed. The results in this study provided valuable information for choice of alternative pesticides and for integrated resistance management of P. xylostella.2. Toxic effects of tolfenpyrad on Plutella xylostella in laboratoryIn order to use it more effectively, series assays were conducted to assess the toxicity of the diamondback moth against tolfenpyrad. Comprehensively evaluated the results, the second instar larvae, treating 72h and at the temperature of 25℃ were considered the best suitable characters for the bioassay of tolfenpyrad. Meanwhile, the results revealed significant difference in the LC50 values among the former 3 instars And the LC50 values decreased with the larvae instars and treating time increased. The first instars were the most susceptible with the LC50 values of 7.89,1.85,1.60 and 1.19 mg/L after 24,72,120 and 168 h treatment, while the fourth instars were the most tolerant, with the LC50 values of 1287.15,200.19,119.33 and 100.74 mg/L after 24,72,120 and 168 h treatment, respectively. Furthermore, the LC50 values of 1st and 2nd instars larvae against tolfenpyrad were determined at the temperature of 25,28 and 30℃, the results showed that the toxicity gradually increased with the elevation of temperature, but no significantly difference was observed. Tolfenpyrad would be an effective alternative insecticide for the control of diamondback moth in the field.3. Assessment of resistance risk in P. xylostella to tolfenpyrad and abamectinA field-population of P. xylostella collected from Wuxi in Jiangsu was selected after repeated exposure to tolfenpyrad to determine the risk of resistance evolution. After 10 generations of selection, there was a 1.39-fold increase in LC50 value [initial LC50=1 2.349 mg/L (Fo), final LCso=29.540 mg/L (F10)]. The realized heritability (h2) of resistance was estimated as 0.1023 by using threshold trait analysis. A field-population of P. xylostella collected from Songjiang Shanghai was selected after repeated exposure to abamectin to determine the risk of resistance evolution. After 18 generations of selection, there was a 16.75-fold increase in LC50 value [initial LC50=1.57 mg/L (F0), final LC50=26.305 mg/L (F18)]. The realized heritability (h2) of resistance was estimated as 0.1732 by using threshold trait analysis. These results suggest that the risk of resistance development exists in P. xylostella to tolfenpyrad and high risk of resistance development exists in P. xylostella to abamectin after continuous application. |
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