Location: Invasive Plant Research LaboratoryTitle: Current levels of suppression of waterhyacinth in Florida by classical biological control agents) Author
Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2014
Publication Date: 1/31/2014
Citation: Tipping, P.W., Martin, M.R., Pokorny, E.N., Nimmo, K.R., Fitzgerald, D.L., Dray Jr, F.A. 2014. Current levels of suppression of waterhyacinth in Florida by classical biological control agents. Biological Control. 71:65-69. Interpretive Summary: Waterhyacinth is the world's worst floating weed and causes millions of dollars in damage to Florida waterways. In most cases, herbicides are used to keep weed populations at a maintenance level but this requires perpetual applications of these chemicals to fresh water. Four insects have been intentionally introduced to attack the plant and all are now present and feeding only on waterhyacinth in Florida. The degree to which they damage and weaken the plant was measured in the field using insecticides to create a true control treatment, thereby allowing side-by-side comparisons of the impact of the insects. Although the insects reduced the biomass of the plants by half and virtually eliminated seed production, the coverage of the plant on the water was relatively unaffected. Most managers use coverage as their measure of effectiveness and, thus, biological control is often viewed as ineffective. As a result, there is little to no effort directed at actively integrating biological control with chemical control, despite the opportunities for cost savings.
Technical Abstract: Waterhyacinth, Eichhornia crassipes, has been a global target for classical biological control efforts for decades. In Florida, herbicides are the primary tactic employed, usually without regard for the activities of the three biological control agents introduced intentionally during the 1970's, namely Neochetina eichhorniae, N. bruchi, and Niphograpta albiguttalis. A series of field experiments from 2008-2010 conducted at four Florida sites used an insecticide-check approach to quantify the current levels of suppression provided by these agents. Niphograpta albiguttalis was rarely found while more than 99% of all Neochetina sp. adults were N. eichhorniae. Although it was not possible to disentangle the relative impacts of Neochetina sp. adults and larvae on individual plant variables, the larvae played a major role in reducing plant biomass (r = - 0.22, n = 162, P = 0.004) and the number of inflorescences (r = - 0.27, n = 157, P = 0.0006). Plots exposed to unrestricted herbivory contained 58.2% less biomass and produced 97.3% fewer inflorescences at the end of the experiments. Despite these large reductions, coverage was relatively unaffected averaging 71.1 + 7.8 % in plots with biological control versus 85.5 + 5.6 % in plots without, a decrease of 16.8%. Most of this difference was attributed to a low-nutrient site where coverage was reduced disproportionately; otherwise coverage trended upward during the course of the experiments and was always close to 100% when the plots were harvested. Although coverage is a somewhat arbitrary metric, especially for floating plants subject to compression and dispersion, it influences the perception of biological control efficacy which, in turn, directly influences herbicide management decisions in Florida. Despite waterhyacinth populations that now produce less than half as much biomass and up to 98% less seed than before the deployment of biological control agents, the overall approach used to achieve maintenance control of the plant in Florida will probably not change unless new agents like Megamelus scutellaris can reduce coverage significantly.