Harms, N. E. and J. T. Cronin. 2020. Biological control agent attack timing and population variability, but not density, best explain host density across a latitudinal gradient. Scientific Reports https://doi.org/10.1038/s41598-020-68108-w.


Spatial variation in plant-herbivore interactions can be important in pest systems, particularly when insect herbivores are used as biological control agents to manage invasive plants. The geographic ranges of the invasive plant alligatorweed (Alternanthera philoxeroides) and its biological control agent the alligatorweed flea beetle (Agasicles hygrophila) do not completely overlap in the southeastern US, producing spatial heterogeneity in interaction strength that may be related to environmental gradients. We studied this system across a latitudinal distribution of the alligatorweed flea beetle to test whether latitudinal variation in alligatorweed density was best explained by agent mean or maximum density, variability in agent density, agent attack timing, or a combination of biological control and environmental (i.e., weather) variables. Latitude was negatively and positively related to mean agent and host densities, respectively. Variability in agent density increased with latitude and was positively correlated with host density. Agent attack timing was negatively correlated with winter and spring temperatures and spring and summer precipitation, and positively correlated with seasonal temperature extremes, which was then directly influential on agent density and variability in density, and indirectly on host density. This study demonstrates that, contrary to common wisdom, weather-related timing of agent activity and population variability, but not agent mean density, contributes to the spatial heterogeneity observed in target weed populations.

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