Nutrient dynamics of Salvinia molesta in shaded and open areas: how does plant quality affect success of biological control by Cyrtobagous salviniae?

Nutrient dynamics of Salvinia molesta in shaded and open areas: how does plant quality affect success of biological control by Cyrtobagous salviniae?
Zolile Maseko, Julie A. Coetzee, Martin P. Hill
Zoology and Entomology Department, Rhodes University, Grahamstown

Cyrtobagous salviniae (Coleoptera: Curculionidae) has proved to be a very effective and deleterious grazer on the ubiquitous aquatic invader Salvinia molesta, which has been a global menace of freshwater systems. Despite the many successes of the weevil, it has however failed to record success in areas where salvinia grows as an understory species. Shallow waters which are still, or have low velocities and occur in the shade provide a haven for salvinia to establish. Previous studies have also highlighted the fact that there is failure to achieve biological control of salvinia under shade conditions. Therefore, the focus of this research is on the effects of shade on the nutrient dynamics of salvinia and how in turn this affects the success of biological control by C. salviniae. The study includes both field and controlled greenhouse studies. Variations in microclimates within the field setting will be investigated by comparing the shaded and open sites where variations in plant quality (tissue nitrogen concentration) could be attributed to the prevailing microclimates. The glasshouse controlled experiments will investigate the nutrient content of salvinia plants and control achieved by C. salviniae under three different levels of shade. The parameters to be investigated include total plant nitrogen, water nitrogen content, wet and dry biomass, plant damage, weevil population sizes, air and water temperatures, shade percentages and solar lux.