Water Nutrients, Plant Nutrients, and Indicators of Biological Control on Waterhyacinth at Texas Field Sites

摘要

The impact of biological control agents on waterhyacinth (Eichhornia crassipes (Mart.) Solms) may depend on water nutrient levels, via their effects on plant nutrients and biomass. This study examined associations between water and plant nutrients, and between nutrients, plant biomass and damage, and placed these associations in the context of variable field site disturbance related to chemical and mechanical control and natural factors. Fifteen sites in coastal Texas were sampled, some repeatedly. Water samples were analyzed for dissolved inorganic nitrogen (DIN) and soluble reactive phosphorous (SRP). Plant biomass, damage to leaf laminae by adult waterhyacinth weevils (Neochetina spp.), necrosis caused by a fungal plant pathogen (Cercospora piaropi), and leaf nitrogen (N) and phosphorous (P) content were determined. DIN was positively correlated with leaf N and P content. Water and plant nutrients were not linked to plant biomass. Plant N content was negatively associated with leaf damage by waterhyacinth weevils. In December 2003, DIN and SRP values were 2.6- and 2.2-fold higher, respectively, at five sites on the Rio Grande that had been subjected to mechanical and chemical control than at four sites off of the river. In 2004, sites at whichnatural disturbance and/or plant removal were frequent did not vary from low-disturbance sites in water or plant nutrient levels or in plant biomass. Damage by weevils and coverage by C. piaropi were 3.1-fold and 1.4-fold higher, respectively, at sites with low disturbance. The role of biological control agents in limiting waterhyacinth growth and invasion depends in part on interactions between water and plant nutrients, plant nutrients and weevil damage, and disturbance factors acting on weed populations.