COS 109-1 - Effects of insect herbivory on herbicidal control of Pistia stratiotes haplotypes

Abstract: Biological invasions are one of the most serious threats to the structure, integrity, and function of ecosystems worldwide, including aquatic ecosystems. Developing integrated pest management practices that are environmentally and economically sound are required to reduce the impacts of these invasions. The invasion by Pistia stratiotes is of particular importance to freshwater ecology because it threatens the ability of natural resource managers to maintain open navigable waters. Pistia stratiotes also negatively impacts freshwater ecology and has been the focus of various management strategies, including using herbicides and classical biological control agents. Herbicides are used as part of a maintenance control strategy for the treatment of this and other invasive plants, which has led to an herbicide-centric approach that has reduced the impact of this weed in Florida. The goal of this study is to determine whether herbivores influence the efficacy of herbicides against two P. stratiotes haplotypes most commonly found in Florida. Here we employ a 3 x 2 x 2 factorial design with five replications. Factor 1 (herbicide [Diquat]) contains 3 levels (control [water], quarter of the label rate, and half of the label rate). Factor 2 (herbivory) contains 2 levels (restricted herbivory [insecticide = control] and unrestricted herbivory [no insecticide]). Factor 3 (haplotype) contains two levels (A and E). Treatments were evaluated one week prior and two weeks post herbicide application by sampling five plants per treatment to measure insect densities, percent defoliation, and biomass. The two principal herbivores found throughout this study were two native insects: Samea multiplicalis (Lepidoptera: Crambidae) and Elophila obliteralis (Lepidoptera: Crambidae). Both P. stratiotes haplotypes in the unrestricted herbivore treatment weighed less and covered significantly less surface area, compared to the control (restricted herbivory), across both evaluation periods. When herbivores were present, biomass and coverage of P. stratiotes was significantly and similarly reduced regardless of herbicide rate. In conclusion, this study suggests that integrating insect herbivores into the larger herbicide-centric management strategy for P. stratiotes could reduce herbicide usage and the associated cost.