Management strategies for thrips in strawberries: the importance of the landscape

Abstract: Thrips are an important pest of everbearing strawberries in Switzerland. The
emergence of resistance and the ban on several active ingredients mean that alternatives to
insecticide treatments need to be developed. The introduction of generalist predators, mainly
phytoseiid mites and Orius spp., is common practice and gives good results. However, some
growers, although persisting with using the same predator release practices, have sometimes
found that predator populations are insufficient to keep thrips populations below the economic damage threshold. The population dynamics of thrips and influencing factors need to be understood. We aimed to assess how thrips population dynamics were related to, or varied by, thrips species, grower practice and the location of strawberry plots. In 2022 and 2023, a network of growers in all the main strawberry-growing regions of Switzerland was established. Fortythree plots were sampled at each flowering cycle (up to three). Each sample consisted of fifty flowers taken from the entire plot. Thrips populations were counted, and adults were identified morphologically. At the same time, the management practices of the growers as well as the crops, the topography and the temperatures of the plots were recorded. In total, 7285 adult thrips were sampled. Fifteen species were observed, the main ones being Frankliniella occidentalis (68 % of total samples), Thrips tabaci (13 %), F. intonsa (12 %), and T. fuscipennis (5 %) in varying proportions from one plot to another. Of all the parameters observed, only the environmental factors: elevation, temperature and distance from a meadow explain a sufficient proportion of the variability in thrips populations on plots. We also observed a negative relationship between the distance to forest and the extent of damage done by thrips to strawberries. The lack of variability in outcome between the control strategies used suggests that the strategies currently in place in Switzerland (insecticides or/and natural predators) are similarly effective. These results underscore the importance of considering the landscape in thrips management strategies.