Insecticide resistance increases the vector competence: a case study in Frankliniella occidentalis

The western flower thrips, Frankliniella occidentalis, is the most efficient vector for Tomato spotted wilt orthotospovirus (TSWV), a devastating plant pathogen. Insecticide resistance is a common issue in pest management. The interactions between insecticide resistance in insect vector and the transmission of plant pathogen, however, are largely unknown. In this study, we assessed the effects of spinosad resistance on TSWV transmission using a pair of near-isogenic lines of susceptible (Ivf03) and resistant (NIL-R) F. occidentalis. NIL-R had a prolonged pre-adult stage, but a shorter adult life span than Ivf03. More importantly, the pupation rate and sex ratio (female/male) in NIL-R were significantly reduced in comparison with Ivf03. When given a shorter acquisition access period (AAP) of 6 h, virus propagation and transmission efficiency in NIL-R were significantly higher than Ivf03. In contrast, a longer AAP of 96 h led to a similar virus acquisition and transmission efficiency for both strains, although TSWV replication was greater in NIL-R. Our combined results suggest that (1) substantial fitness costs (e.g., reduced pupation rate) are associated with spinosad resistance in F. occidentalis; however, (2) insecticide resistance increases vector competence.