Fitness costs and reduced feeding in spinosad resistant Frankliniella occidentalis

Frankliniella occidentalis (Pergande) is considered to be one of the most economically important pests of agricultural crops worldwide. High levels of resistance to spinosad have been detected in field populations of F. occidentalis. In the present study, the life history traits, body size and feeding behavior were compared between spinosad susceptible (Spin-S) and resistant strains of F. occidentalis (Spin-R).   The stability of resistance was also tested. Results showed that the developmental period of Spin-R individuals was prolonged during the pre-adult stage, the adult longevity was shortened and the fecundity decreased significantly. Compared to Spin-S individuals, the relative fitness of Spin-R insects was only 0.47, indicating an obvious fitness cost. The resistance to spinosad in Spin-R thrips is unstable, showing an obvious decrease after about two generations without exposure to spinosad and a sharp decline to lower values after seven generations. The body length and width of Spin-R Individuals were relatively smaller than those of Spin-S insects, especially in the adult stage. The feeding behavior was recorded using the electrical penetration graph (EPG) technique. Most measured feeding behavior differed significantly with Spin-R Individuals having fewer or shorter probing periods than Spin-S individuals, indicating that the Spin-R strain has an overall reduced feeding capacity. The concurrent detection of instability of resistance in the absence of selection and deleterious effects on fitness components and feeding behavior suggest that a fitness trade-off is associated with spinosad resistance in F. occidentalis.  This is crucial Information for implementing a successful rotation strategy to delay resistance development in F. occidentalis control programs.