Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 8, 2003
Publication Date: April 1, 2004
Citation: Moran, P.J. 2004. Feeding by waterhyacinth weevils (Neochetina spp.) (Coleoptera: Curculionidae) in relation to site characteristics, plant size and biochemical factors. Environmental Entomology. 33:346-355. Interpretive Summary: Waterhyacinth is a weedy plant that floats on the water surface and was introduced into the U.S. from South America in the 19th century because it has odd-shaped leaves and pretty flowers. Because few insects and diseases attack waterhyacinth, it can grow rapidly and clog entire rivers, canals, and reservoirs. In the Lower Rio Grande Valley of Texas, waterhyacinth appeared 10-15 years ago and is now reducing the flow of the Rio Grande, disrupting water intake pumps on the river, and spreading into irrigation canals and ponds. Costly mechanical removal or chemical treatment of infested waters can temporarily solve the problem, but new plants arrive and the problems return. Biological control of weeds involves the use of insects and diseases to weaken and kill weeds. Two beetles were introduced by USDA-ARS scientists in the 1970s to control waterhyacinth and have recently spread into the Rio Grande Valley. This study found that the amount of damage done by the beetles varies greatly among field sites. Differences in damage were related to the size of plants. Small plants sustained more damage and were growing under conditions of low nutrient supply in the water. At sites where waterhyacinth was mechanically removed, plants that grew back into the sites were healthier and tolerated high levels of beetle damage. Plant protein content was lower in plants with heavy beetle damage, reflecting the negative impact of the beetles on plant health. The results of this study suggest that biological control will work best in the Lower Rio Grande Valley when plants are unhealthy due to poor growth, and are left alone so that beetles have time to exert maximum effects.
Technical Abstract: Biological control of waterhyacinth (Eichhornia crassipes (Mart.) Solms.) by waterhyacinth weevils (Neochetina bruchi (Hustache)) and (Neochetina eichhorniae (Warner)) is influenced by variation in field site characteristics, season, and the induction of plant stress, when these factors influence plant growth and physiology. Field sites in the Lower Rio Grande Valley were sampled for four seasons to determine if levels of leaf scarring caused by adult weevil feeding were associated with altered plant size, biomass, insect density, and biochemical measures. Scar densities were highest on plants from two sites at which root and dead plant material biomass were high at site-specific times. At one of these sites, weevil feeding accumulated on small leaves on plants growing slowly under the influence of multiple abiotic and biotic stress factors. At the other site with high scarring, leaf area was not reduced, and mechanical control and regrowth may have allowed plants to compensate for weevil feeding. Scarring was not associated with variation in low combined densities of egg, larval and adult weevils or larval gallery density. Soluble protein concentration was lower in plants from the two sites with high scarring than at two other sites in Spring 2002, and peroxidase activities were lower in Fall 2001. Trends in protein and peroxidase measures related to leaf age partially explain tissue choices made by adult weevils. The results illustrate the importance of examining dynamic plant physical and physiological traits, in addition to plant damage, in evaluating biological control of waterhyacinth.