Effects of the fungus Sclerotinia sclerotiorum and the scale insect Pulvinariella mesembryanthemi on the ice plant Carpobrotus edulis from native and non-native areas: evaluation of the biocontrol potential.
Carpobrotus edulis is a highly invasive plant in coastal temperate areas worldwide. In a preliminary attempt at biological control, we evaluated the potential use of the fungus Sclerotinia sclerotiorum and the insect Pulvinariella mesembryanthemi as biocontrol agents. We carried out a greenhouse-experiment to evaluate the effects of both agents, separately and together, on short-term (physiological) and long-term (survival, growth, biomass) estimators of plant performance. We compared the susceptibility to both agents in plants originating from native and non-native areas. The fungus had immediate and negative, but short-lasting effects on chlorophyll content and photosynthetic-radiation use efficiency. No significant effects on plant survival, growth and biomass were observed after 1 year. Infestation with the insect increased photosynthetic performance and decreased the root/aerial biomass ratio of the plants, suggesting a counteractive response to insect feeding. After 5 months, the reflectance parameters were negatively affected. Only half of the infested plants survived for a year, and the growth and biomass were lower in the surviving plants than in untreated plants. In half of the surviving plants, the most heavily-infested parts died. The insect-infested plants were usually infected by mould. The number of insects per plant was lower in native populations and when both biocontrol agents were used together. Nevertheless, no long-term synergistic/additive effects of the insect and fungus were observed, and the susceptibility of native and introduced plants was not different. Therefore, use of the insect seems to be the best strategy for controlling C. edulis, as it decreases plant growth and increases mortality and stress susceptibility.