Imidacloprid‐susceptible Nilaparvata lugens individuals exceeded resistant individuals in a mixture population with density pressure

BACKGROUND: Fitness costs associated with insecticide resistance in pest insects have mainly been studied under optimal laboratory conditions. However, resistant insects face more stressors than just insecticides in the field, and how the resistant population reacts to these stressors is of practical importance for the control of pest insects such as the brown planthopper Nilaparvata lugens. The aim of the present study was to explore the impact of population density on the competitiveness of resistant and susceptible individuals. RESULTS: Two isogenic N. lugens populations, a highly imidacloprid-resistant population (HZ-R) with a resistance ratio (RR) of 227.10 and a relatively susceptible population (HZ-S) with an RR of 2.99, were created from a field-resistant population (HZ; RR 62.51). The high resistance levels of HZ-R and HZ were mainly attributable to the overexpression of multiple cytochrome P450 (CYP) genes such as CYP6ER1, CYP6AY1, CYP6CW1 and CYP4CE1 compared with HZ-S, this being supported by piperonyl butoxide synergism. HZ-R was observed to be more resistant to thiacloprid and etofenprox compared with HZ and HZ-S. Most interestingly, in high population density treatments, HZ-S individuals were much more competitive than HZ-R individuals. CONCLUSION: Imidacloprid-resistant individuals of N. lugens are less competitive than their susceptible counterparts under density pressure. © 2017 Society of Chemical Industry.