Abstract
The melon thrips, Thrips palmi, is an increasingly important pest of vegetables in northern China. Some populations have developed resistance in the field to the insecticide spinetoram. Understanding the origin and dispersal of insecticide‐resistant populations can shed light on resistance management strategies. In this study, we tested susceptibility of seven greenhouse populations of T. palmi to spinetoram collected from a small area of about 300 km2 in Shandong Province and examined population genetic structure across the area based on a segment of mitochondrial cox1 gene and 22 microsatellite loci to infer the possible origin and dispersal of insecticide resistance. Levels of resistance to spinetoram differed among seven populations, which included one population with high resistance (LC50 = 759.34 mg/L), three populations with medium resistance (LC50 ranged from 28.69 to 34.79 mg/L), and three populations with low resistance (LC50 ranged from 7.61 to 8.97 mg/L). The populations were genetically differentiated into two groups unrelated to both levels of resistance and geographic distance. The molecular data indicated high levels of gene flow between populations with different levels of resistance to spinetoram and low gene flow among populations with the same level of resistance, pointing to a likely separate history of resistance evolution. Resistance levels of two tested populations to spinetoram decreased 23 and 4.6 times after five generations without any exposure to the pesticide. We therefore suspect that resistance of T. palmi most likely evolved in response to local applications of the insecticide. Our study suggests that the development of resistance could be avoided or resistance even reversed by reducing usage of spinetoram.
Highlights
Applications of insecticide have led to the evolution of resistance in many insect pests (Nauen, Slater, Sparks, Elbert, & Mccaffery, 2019)
Population genetic approaches provide a useful approach to testing these scenarios, because they can trace the dispersal of individuals and possible spread and selection for insecticide resistance and test how this spread coincides with resistance (Crossley, Chen, Groves, & Schoville, 2017; Fu, Epstein, et al, 2017; Pélissié, Crossley, Cohen, & Schoville, 2018; Shi et al, 2019; Yang et al, 2019)
We found varying levels of spinetoram resistance among populations collected from a small area
Summary
Applications of insecticide have led to the evolution of resistance in many insect pests (Nauen, Slater, Sparks, Elbert, & Mccaffery, 2019). In cases where resistance has independent origins, developing in geographically distant populations, some populations may remain susceptible to an insecticide while others have varying levels of resistance, and these resistance patterns may be unconnected to geographic proximity (Shi et al, 2019). Originating from tropical countries of Asia, T. palmi was introduced and became established across South-East Asia, South America, the Caribbean, Florida, Australia, and West Africa (Cannon, Matthews, & Collins, 2007) In recent years, this species spread to northern China and became a serious pest in greenhouse vegetables (Cao et al, 2019). A recent study showed that some populations of T. palmi have developed varying levels of resistance to spinetoram in northern China, most populations remain susceptible (Gao, Gong, Cao, et al, 2019). Based on our knowledge of the biology and resistance status of T. palmi, we hypothesized that resistance was more likely to develop independently in multiple populations rather than being solely a consequence of gene flow
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