Effects of insect herbivory on herbicidal control of waterhyacinth (Eichhornia crassipes)

Waterhyacinth (Eichhornia crassipes) is the most intensively managed floating aquatic weed in Florida, with $3.4 million spent in Florida during FY 2011-2012 for maintenance control of waterhyacinth and waterlettuce (Pistia stratiotes) in public waters. Thus, even a small reduction in herbicides used to control waterhyacinth could represent significant savings while reducing negative environmental impacts. Biological control of waterhyacinth using insects has been used in many countries and the damage inflicted on the plants likely renders them more susceptible to stressors like herbicides. Although herbicides and biological control have been evaluated separately on waterhyacinth, there is little information regarding their combined effects as part of an active integrated pest management program. A two-year experiment evaluated the additive impact of herbivory by biological control agents and the application of different rates of 2,4-D on waterhyacinth growth and development. We evaluated two levels of herbivory: 1) control (no biological control agents) and 2) biological control (unrestricted attack by Neochetina eichhorniae and Megamelus scutellaris) with three 2,4-D rates: 1) control (no herbicide), 2) low rate (2.1 kg/ha a.i.), and 3) operational rate (4.3 kg/ha a.i.).  The experiment used an insecticide check approach arranged in a completely randomized design with five replicates and was conducted in concrete mesocosms (1.6 m² surface area).  Plant populations not subjected to herbivory quickly re-grew post-spray regardless of herbicide rate, while those subjected to both tactics rarely re-grew during the experiment.  The final biomass of waterhyacinth populations was reduced by 24.7% by biological control only, 21% by the low rate of herbicide alone, 42.2% by the high rate, and 96.6% and 99.9% by the combination of biological control and the low and high rates of herbicides, respectively.  Other plant parameters were also negatively affected by the combination of management tactics.  These results quantify the value of actively integrating the management of waterhyacinth and suggest that further reductions in the rates of 2,4-D could be achieved without sacrificing efficacy.