Do tissue carbon and nitrogen limit population growth of weevils introduced to control waterhyacinth at a site in the Sacramento-San Joaquin Delta, California?

Waterhyacinth (Eichhornia crassipes (Mart.) Solms), is a serious problem in the Sacramento Delta. Two weevil species (Neochetina bruchi Hustache and N. eichhorniae Warner) have been introduced as biological control agents. Factors such as weather, disease, predators, and plant quality affect growth and reproduction of insect herbivores. The purpose of this study was to test the hypothesis that nitrogen (N) in the tissue of waterhyacinth was not sufficient to support weevil growth and reproduction. Waterhyacinth at a site in the Delta (Whiskey Slough) were sampled at 2- to 3-week intervals in 1995, 1996, and 1997. Lamina samples were analyzed for tissue C and N. Tissue C varied less than either tissue N or the C:N ratio. Tissue N was greatest in the leaf lamina, followed by stem bases, and leaf petioles. Lamina tissue N was higher in spring and somewhat reduced in late summer and winter. The lamina C:N ratio was generally <15 after mid-May. Comparing tissue N levels for Delta waterhyacinth with a previous study relating weevil growth to tissue N indicates that tissue N should not limit growth and reproduction of either weevil species during spring and summer. Because it grows better on plants with high N content and because it has a greater impact on the growth of high N plants, N. bruchi may be a more effective biological control agent in the Sacramento Delta. Key words: aquatic weeds, Eichhornia crassipes, tissue nitrogen, biological control, weed biology.