Resistance risk assessment for novel reduced-risk insecticides in obliquebanded leafroller, Choristoneura rosaceana (Lepidoptera: Tortricidae)

The obliquebanded leafroller (OBLR), Choristoneura rosaceana (Harris), is one of the most destructive pests of tree fruits in Washington. The use of broad-spectrum insecticides against OBLR for decades has led to the development of insecticide resistance in this pest. In this situation, the recently developed chemicals with novel modes of action including rynaxypyr and spinetoram, show promise for controlling OBLR, but resistance remains a threat. The risk assessment for resistance to a particular insecticide before its occurrence in the field could be valuable in developing strategies to manage susceptibility. The rate of resistance development is proportional to the population’s narrow-sense heritability of resistance (h2), the proportion of total phenotypic variation attributable to additive genetic variation, an important factor in evaluating the sustainability of an insecticide on a particular pest population. Studies were initiated to select OBLR for resistance against rynaxypyr and spinetoram to determine the risk of resistance evolution. We treated 2000 neonates from laboratory population at LC70-LC90 for 96 hours using diet-incorporation bioassay at each generation. After four generations of selection, 2.5- and 3.5-fold increases in LC50 values were observed for rynaxypyr and spinetoram, respectively. The realized heritability values were estimated as 0.21 for rynaxypyr and 0.23 for spinetoram. The response quotient (Q) was used to compare the risk of resistance development in OBLR against these insecticides. The Q values were 0.138 and 0.086 for rynaxypyr and spinetoram, respectively. These results indicate that the rate of resistance development in OBLR would be slower against spinetoram than that against rynaxypyr.