Author
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Center, Ted |
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Dray, F Allen |
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Submitted to: Journal of Applied Ecology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 12/12/2009 Publication Date: 3/8/2010 Citation: Center, T.D., Dray Jr, F.A. 2010. Bottom-up control of water hyacinth weevil populations: Do the plants regulate the insects?. Journal of Applied Ecology. 47, 329-337. Interpretive Summary: Water hyacinth is one of the world’s weed that affects virtually all aspects of the management of aquatic resources including navigation, irrigation, flood control, recreation, and fisheries. Biological control agents have been released in many parts of the world in an attempt to control this weed with varying levels of success. Two of the most successful controls have been two species of weevils from South America. Results have not been consistent, however, due to the dynamic nature of aquatic habitat which undergo flushing, flooding, drying, nutrient influxes, etc. We suspected that nutrient dynamics in the form of fertilizer pulses and season release of nutrients from decaying plants affected these weevils ability to reproduce and thereby build up in numbers that are sufficient to exert control of the weed. We found that the ovaries of the weevils responded positively to increases in plant quality associated with higher fertilizer inputs which led to greater numbers of offspring. Hence, much of the variation in the effects of these insects is clearly attributable the variable nutritional quality of the plants. Technical Abstract: A key measure of dietary sufficiency relates to an insect’s reproductive ability so oögenesis, a nutrient-limited process, can be subject to bottom-up regulation. We hypothesized that aquatic nutrient flux seasonally affects ovarian development thereby controlling population growth of two specialist weevil species, Neochetina eichhorniae and N. bruchi, on the floating plant Eichhornia crassipes. We hypothesized that the two differed in sensitivities to nutritional quality and interacted differently depending on host quality. To examine this we cultured E. crassipes plants under 20 fertilizer regimens to induce quality differences, measured as %N, in emergent shoots. Reproductively-dormant, over-wintering females were released into these cultures as single or mixed species populations. They were later retrieved and dissected to ascertain ovarian status. A population response was evident as reproductive females increased proportionately with fertilizer rate. A lesser individual response occurred as ovarian status responded to nutritional quality. Numbers of progeny increased exponentially to over 300 weevils/tank in the highest fertilizer treatment. Mixed species colonies produced more individuals under high fertility than single species cultures (236 ± 32 mixed; 155 ±25 N. bruchi and 170 ± 33 N. eichhorniae pure). Neochetina eichhorniae was more productive than N. bruchi in mixed and pure cultures at all fertilizer levels, especially in low and intermediate conditions, but N. bruchi performed nearly as well in high fertilizer tanks. Ratios of the two species remained consistent regardless of their occurrence in single species or mixed species colonies. However, N. eichhorniae seemed more adaptable to a wider range of plant quality and more tolerant of lower nutritional circumstances, which explains its prevalence in the field. The coupling of reproductive status of the parental generation with the population response of the F1 generation indicates that plant quality drives population growth of both species so bottom-up regulation clearly occurs. |
