Abstract

Competitive  interactions between species can be mitigated or even reversed in the presence  of anthropogenic influences. The thrips species Frankliniella  occidentalis and  Thrips  tabaci are  highly invasive and damaging agricultural pests throughout the world.  Where the  species co-occur, one species tends to eventually predominate over the other.   Avermectin and beta-cypermethrin are commonly used insecticides to manage  thrips in China, and laboratory bioassays demonstrated that F.  occidentalis is  significantly less susceptible than T.  tabaci to  these insecticides.  In laboratory cage trials in which both species were  exposed to insecticide treated cabbage plants, F.  occidentalis became  the predominant species.  In contrast, T.  tabaci completely  displaced F.  occidentalis on  plants that were not treated with insecticides. In field trials, the species  co-existed on cabbage before insecticide treatments began, but with  T.  tabaci being  the predominant species.  Following application of avermectin or  beta-cypermethrin, F.  occidentalis became  the predominant species, while in plots not treated with insecticides,  T.  tabaci remained  the predominant species. These results indicate that T.  tabaci is  an intrinsically superior competitor to F.  occidentalis, but  its competitive advantage can be counteracted through differential  susceptibilities of the species to insecticides.  These results further  demonstrate the importance of external factors, such as insecticide  applications, in mediating the outcome of interspecific interactions and produce  rapid unanticipated shifts in the demographics of pest complexes.

Keywords: invasive species, interspecific competition, western  flower thrips