PS 40-20 - Integrated management of invasive waterhyacinth (Pontederia crassipes) and its effect on fish populations

Background/Question/Methods

Waterhyacinth (Pontederia crassipes) is one of the most globally invasive plants, creating significant economic and ecological problems. In Florida, waterhyacinth alters water quality and chemistry, clogs waterways and costs millions of dollars annually to manage. Current management strategies involve mechanical and chemical removal, and biological control. Questions about the effects of these management strategies have emerged, including which combined strategy is most efficient and the response of members of the ecological community to management. In early 2020, large, outdoor, aquatic mesocosms were stocked with waterhyacinth, native aquatic plants, and small fish (mostly eastern mosquitofish, Gambusia holbrooki, Cyprinodontiformes, Poeciliidae). The mesocosms were managed with herbicides, biological control agents (Neochetina eichorniae and N. bruchi, Coleoptera: Curculionidae and Megamelus scutellaris, Hemiptera: Delphacidae), and a combination of the two. Control treatments were maintained with systemic insecticide and included unmanaged treatments and treatments with only native plants. The mesocosms were sampled throughout 2021 to estimate plant cover and fish population fluctuations.

Results/Conclusions

In the treatment without management, waterhyacinth consistently covered over 50% of the mesocosm surface. Biological control treatments averaged about 10% waterhyacinth cover, herbicide management about 7.5% and the combination of biological control and herbicide about 5%. Eastern Mosquitofish populations grew similarly before management in 2020, but differences appeared as treatments were applied. There was a strong correlation between waterhyacinth cover and Eastern Mosquitofish counts (R² = 0.4563). Waterhyacinth-free control treatments held the most fish, followed by the herbicide management. Biological control and the combination of herbicide and biological control held very similar amounts of fish, only slightly less than the herbicide-only treatment. Finally, the unmanaged treatment averaged less than half the fish than the other treatments. Broadly, chemical and biological control or the combination thereof reduce the coverage of waterhyacinth and allow fish populations to thrive. Management effort should be targeted towards strategies that minimize herbicide applications and save money. We recommend an integrated management strategy including broad application of insects and only minimal, targeted application of herbicide.