Submitted to: Journal of Aquatic Plant Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 16, 2006
Publication Date: October 1, 2006
Citation: Moran, P.J. 2006. Water nutrients, plant nutrients, and indicators of biological control on waterhyacinth at Texas field sites. Journal of Aquatic Plant Management. 44:109-115. Interpretive Summary: Waterhyacinth plants float on the surface of canals, reservoirs, and lakes throughout the Southeast. They make water pumping difficult, and make boating in waters choked by the weed impossible or dangerous. Waterhyacinth was introduced to the U.S over 100 years ago and so there are almost no large animals, insects, or diseases that can kill it. The USDA-Agricultural Research Service tried to level the playing field 30 years ago by introducing two beetles from South America that feed and reproduce exclusively on waterhyacinth. Sometimes, though, the plants grow so fast that the beetles can’t keep up. Waterhyacinth takes up its nutrients from the water. We sampled waterhyacinth plants and water at 15 field sites over one year and found that the amount of nitrogen and phosphorous in the leaves depended on the concentration of nitrogen in the water. Water nutrients varied a lot over four seasons, with the highest levels occurring in winter. In the Rio Grande in deep South Texas, waterhyacinth was chopped up and sprayed in the summer of 2003, and this control event pushed water nutrients to levels that were higher than the levels at sites off of the river that still had plants. Just looking at off-river sites, life for the waterhyacinth beetles may be tough at sites with stagnant water. Plants at these sites accumulated lots of weevil damage and disease symptoms, but had low plant nitrogen content and were small compared to plants at sites with flowing water. The beetles themselves reduce the nitrogen content of the leaves as they feed. The results demonstrate the importance of water and plant nutrients in determining the success of biological control of waterhyacinth.
Technical Abstract: Waterhyacinth (Eichhornia crassipes (Mart.) Solms) can take up water nutrients sufficient for growth under highly variable conditions. Few studies have linked water and plant nutrients to measures of biological control in field populations. Fifteen sites in coastal Texas were sampled in 2003 and 2004, some repeatedly. Water samples were analyzed for inorganic nitrogen and reactive phosphorous. Plant biomass, damage by adult waterhyacinth weevils (Neochetina spp.), necrosis caused by a fungal plant pathogen (Cercospora piaropi), and leaf nitrogen (N) and phosphorous (P) contents were determined. Water inorganic nitrogen concentration was positively correlated to leaf N and P. Water and plant nutrients were not linked to plant biomass. Plant N content was negatively associated with leaf damage by waterhyacinth weevils. Water collected over one year from sites on the Rio Grande before, during, and after a mechanical and chemical control event had up to 3.5-fold higher nitrogen and phosphorous levels than did samples from off-river sites not subjected to control. In 2004, sites with flowing water tended to have higher water nitrogen, plant N and shoot biomass than did sites with stagnant water. Damage by weevils and coverage by C. piaropi were 3.1-fold and 1.4-fold higher, respectively, at sites with stagnant water. The roles of waterhyacinth weevils and C. piaropi in biological control were likely affected by complex interactions between water nutrients, water flow, plant growth, the stability of weed populations, and the effects of damage on nutrients.