Evaluating the impact of the biological control agent, Megamelus scutellaris Berg (Hemiptera: Delphacidae), on Pontederia crassipes Mart. (Pontederiaceae) (water hyacinth) senescence and detritus formation at Hartbeespoort Dam

Abstract

This study evaluated the biological control of Pontederia crassipes (water hyacinth) at Hartbeespoort Dam, South Africa, focusing on the impacts of plant senescence on water chemistry and detritus formation. Two studies were conducted, the first under laboratory conditions where the amount of detritus formed from senescing water hyacinth plants under biological and chemical control, as well as changes in water chemistry could be compared. This study found that water hyacinth produced increased amounts of detritus over time, compared to healthy plants across all treatments, indicating that senescing plants add to the sediment load of the water. Additionally, pH was found to decrease over time in the presence of senescing plants, restoring the acidity of water compared to alkaline water when water hyacinth plants are healthy. The second study was conducted in situ at Hartbeespoort Dam and monitored the effects of density of the biological control agent, Megamelus scutellaris, water hyacinth cover and eutrophic water on detritus formation. It was found that biological control effectively decreases water hyacinth abundance leading to the senescence of the plants, ultimately increasing the amount of detritus produced. Detritus production was significantly correlated to water hyacinth root length, plant abundance and root biomass. Findings indicate that while biological control effectively decreases plant abundance, it may influence nutrient concentrations in the eutrophic dam waters. This research suggests that the senescence of water hyacinth mats impacts sediment loading and nutrient water chemistry, these impacts are no different to water hyacinth under biological, chemical or no control at all.