Status of Waterhyacinth/Hydrilla Infestations and Associated Biological Control Agents in Lower Rio Grande Valley Cooperating Irrigation Districts

摘要

During September 1999, surveys to quantify plant infestations and insect biological control agents of both Eichhornia crassipes and Hydrilla verticillata were initiated on the Rio Grande from just west of McAllen, TX, and east to Brownsville, TX. Waterhyacinth biomass/cu m differed significantly from site to site and ranged from 3 kg/cu m to about 21 kg/cu m. Both Neochetina eichhorniae and N. bruchi were commonly collected from all waterhyacinth sites. However, agent numbers were highly variable with an almost fourfold difference in numbers of agents/cu m observed at the various sites. Strong relationships between various plant parameters and number of insect agents were observed that serve as an illustration of possible insect herbivory effects. For example, at sites with only 20 total Neochetina spp./cu m, the number of flower stalks/cu m averaged about 14 per cu m. This is contrasted to sites with 80 Neochetina spp./cu m where the number of flower stalks/cu m was reduced over twofold. Hydrilla populations appeared to differ significantly from site to site. While no quantitative measurements of the hydrilla populations were made, visual inspections of the sites revealed hydrilla populations ranging from small, scattered populations to extensive infestations across major portions of the river. In most cases, the hydrilla appeared healthy with no obvious signs of stress. The only insect agent collected from the survey sites was H. pakistanae. It was surprising to collect any H. paktstanae since the closest release site is over 400 km to the north and populations at this site (Choke Canyon Reservoir) have remained low to nonexistent since the early to mid 1990's. Population numbers on the Rio Grande appeared low with only 300 immatures/kg and about 4 percent of the leaves damaged.