Abiotic and biotic influences on the performance of two biological control agents, Neochetina bruchi and N. eichhorniae, in the Sacramento-San Joaquin River Delta, California (USA)

[Display omitted] •Water hyacinth and weevils declined significantly during high river flow periods.•Weevils exhibited a 3 month lagged numerical response to water temperature.•Weevil densities increased with higher nitrogen content in water hyacinth roots.•We found the presence of microsporidia in the weevils, with low overall abundance.•Metapopulations of weevils in the Delta exhibited low population genetic structure. Variability in the effectiveness of biological control programs can be due to a myriad of abiotic and biotic interactions. Here, we explore these interactions in relation to the suboptimal biological control on the invasive water hyacinth, Pontederia (=Eichhornia) crassipes, by two weevils, Neochetina bruchi and N. eichhorniae, in the Sacramento-San Joaquin River Delta in California, USA (Delta). We conducted monthly surveys from 2015 to 2017 throughout 1667 km2 to analyze the impacts of temperature, nutrients, river flow, pathogens and genetic variation in relation to the performance of these biological control agents. Weevils (predominantly N. bruchi) demonstrated a positive numerical response to increased nitrogen content in water hyacinth crown and roots. Weevil densities and water hyacinth biomass declined significantly during periods of high river flow. We found negative impacts of cooler temperatures on weevil performance, with a significant lagged-response of weevil densities in the field to water temperature from three months prior and a lack of population growth under laboratory conditions simulating winter air temperatures in the Delta (3.9–13.9 °C). We confirmed the presence of a microsporidian pathogen, with 10.7% average prevalence of infection across four seasons and low infection loads. In addition, we found low genetic diversity of N. bruchi in the Delta compared to native populations, low population genetic structure and a lack of genetic isolation by distance among metapopulations. Overall we conclude that cooler temperatures and periods of high river flow are likely the main factors limiting the current biological control program in the Delta.