Resistance risk assessment and strategies for managing resistance against novel reduced risk insecticides in obliquebanded leafroller, Choristoneura rosaceana (Lepidoptera: Tortricidae)

Obliquebanded leafroller (OBLR), Choristoneura rosaceana (Harris), is one of the major pests of tree fruits in Washington. Use of broad-spectrum insecticides against OBLR for decades has led to the development of insecticide resistance in this pest. Recently registered insecticides, chlorantraniliprole and spinetoram, show promise for controlling OBLR, but resistance evolution is a concern. Risk assessment for resistance to a particular insecticide before its occurrence in the field could be valuable in developing strategies to manage susceptibility. Studies were initiated to screen field-collected populations and select a laboratory population for resistance against chlorantraniliprole and spinetoram using a diet incorporation bioassay. After six generations of selection, 7- and 4-fold increases in LC₅₀ values were observed for chlorantraniliprole and spinetoram, respectively. Realized heritability values were estimated as 0.17 for chlorantraniliprole and 0.18 for spinetoram. Based on the response quotient values, the rate of resistance development in OBLR was slower against spinetoram than that against chlorantraniliprole. In the absence of insecticide selection, the chlorantraniliprole- and spinetoram-resistant populations reverted to susceptibility in five and six generations, respectively. Furthermore, detoxification enzyme assays indicated that the activity of esterases was significantly higher in chlorantraniliprole-selected population whereas levels of oxidases were significantly increased in the spinetoram-selected population. These results indicate that the risk of resistance evolution exists in OBLR against chlorantraniliprole and spinetoram. However, instability of resistance and apparent involvement of different classes of detoxification enzymes in resistance against chlorantraniliprole and spinetoram should make this pest amenable to resistance management strategies involving rotation of these two chemicals.