Integrating herbicide rates, coverage, and classical biological control insects (Megamelus scutellaris, Neochetina eichhorniae,and Neochetina bruchi) to manage Pontederia (Eichhornia) crassipes

Abstract

The invasive floating aquatic plant Pontederia crassipes once monopolized large areas of open water in Florida, USA. Following the implementation of wide-scale herbicide applications in the mid-1950s, coverage declined but the plant was not eliminated. Herbicides are costly, requiring regular re-application and sustained funding to maintain suppression. Although classical biological control using host-specific insects can augment the impact of herbicides on weed populations, current herbicidal management of P. crassipes in Florida does not intentionally utilize the biological control insects already present (Megamelus scutellaris, Neochetina eichhorniae, and Neochetina bruchi) despite strong evidence they can improve outcomes. A completely randomized mesocosm experiment was conducted to examine the outcomes of an integrated plant management design using combinations of herbicide rates, spray coverage, and biological control agents to control P. crassipes. The analysis indicated that treatments with both moderate levels of herbicidal control (treating 40–80% of the mat) and biological control agents provided similar levels of control to those that treated 100% of the mat with herbicides. Additionally, leaving untreated refuges increased the local density of some species of biological control insects. This experiment shows that biological control insects can reduce the rate and frequency of herbicide applications, an outcome that would likely garner increased public support for continued operations.