Less Herbicide does more when Biological Control is Present in Pontederia Crassipes

Authors: DEANGELIS, DONALD - Us Geological Survey (USGS); Tipping, Philip; GOODE, ASHLEY - University Of Florida; Smith, Melissa; GETTYS, LYN - University Of Florida; KNOWLES, BRITTANY - St Johns River Water Management; Pokorny, Eileen; SALINAS, LUZ - Los Alamos National Research Laboratory; XU, LINHAO - University Of Miami

Submitted to: Ecological Modelling
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/8/2023
Publication Date: 11/17/2023
Citation: Deangelis, D., Tipping, P.W., Goode, A., Smith, M., Gettys, L., Knowles, B., Pokorny, E.N., Salinas, L., Xu, L. 2023. Less herbicide does more when biological control is present in Pontederia Crassipes. Ecological Modelling. https://doi.org/10.1016/j.ecolmodel.2023.110566.

Interpretive Summary: Biological control of invasive waterhyacinth (Pontederia crassipes) reduces biomass and flowering, but not coverage, necessitating the use of synthetic herbicides to attain mandated coverage levels. In many cases, spraying has transpired without thought to the presence or impact of biological control, but increasingly, data from mesocosms shows that the presence of biological means less herbicide is needed to attain control. This study uses data from a mesocosm study to model how different spray habits and insect damage will interact in the waterhyacinth mat. The model suggests that a mat sprayed 80% with herbicide will have 86% less biomass when biological control is present than without. This suggests that management should include the positive impact from biological control when planning treatments.

Technical Abstract: To explore an integrative invasive plant control approach combining herbicide treatments and biological control by insects to Pontederia crassipes (formerly Eichhorniae crassipes) we performed an experimental study and simulation modeling. The objective was to determine the combinations of herbicide treatment and biological control that limit invasive P. crassipes in Florida freshwater aquatic systems. The experiment consisted of 14 different treatments of P. crassipes in 1.67 m2 outdoor tank mesocosms, seven herbicide treatments with and seven without biological control agents including the insects Neochetina eichhorniae, Neochetina bruchi, and Megamelus scutellaris. Seven treatments included one no-spray treatment and six different combinations of a one-time application of the herbicide Diquat®. The six spray treatments consisted of half- and full- strength herbicide, each sprayed over 40%, 80% or 100% of a given tank, with five replicates of each treatment. An overarching hypothesis was that leaving part of a tank unsprayed would provide sufficient habitat for the biological control agents that would allow them to colonize plant regrowth and more effectively lower biomass accumulation. P. crassipes biomass was measured on five days over the 167-day period. Data from the no-biocontrol treatments were used to calibrate the model of P. crassipes growth. The experimental results showed that the lowest P. crassipes biomass resulted from an initial one-time treatment of half-strength herbicide treatment to 80% of the area in combination with biological control. This treatment reduced P. crassipes biomass by 68% compared to the control over 167 days. Because the system did not reach a steady state by the end of the experiment, the model was used to project the dynamics of the system. Based on model output, after 800 days, P. crassipes under the 80% herbicide treatment with insects present was about 82% lower than P. crassipes under treatments without biological control. These results will help guide the design of control strategies in field trials.