When is biological control likely to succeed?

Posted on March 23, 2015

Blanketing growths of water hyacinth (Eichhornia crassipes), one of the world’s most invasive aquatic plants, pose a serious and widespread threat to freshwater ecosystems. Although biological control agents, notably weevil and moth species, can be used to contain water hyacinth populations, their success depends on the extent to which insect herbivory can outpace nutrient-driven weed growth. To develop a method for assessing the likely success of biological control, water hyacinth plants were grown at Rhodes University, South Africa in 50-litre tubs containing chemical fertiliser, organic fertiliser (cow manure), or unfertilised water. The tubs were exposed to natural light. Freshly-produced leaves were sampled repeatedly over a 64-day period in spring. The carbon : nitrogen ratio, an indicator of plant health, was measured, as was the concentration of the nitrogen isotope δ15N, which reflects the nutrient loading of the surrounding environment. In the fertilised treatments, carbon : nitrogen and δ15N stabilised after about two weeks, showing that the leaves had reached equilibrium with the environmental nitrogen loading. The δ15N concentration of plant leaves became depleted in the chemical fertiliser treatment but became enriched in the manure solution. Within a treatment, there was a very significant correlation between the δ15N concentration and the maximum length of water hyacinth shoots. These results showed that shoot lengths, carbon : nitrogen ratios and δ15N concentrations can be used in combination to predict nutrient-enriched sites where bio-control is likely to fail. For example, in the South African study area, problem sites are those where the maximum shoot length is above the regional average (49.2 cm), where C:N ratios are above 15, and where δ15N levels are either below 0.00 p.p.t. (in the case of high loadings of synthetic fertiliser) or above 10 p.p.t. (where there are high loadings of sewage or manure).

Reference: Jaclyn M. Hill, 2014.
Investigations of growth metrics and δ15N values of water hyacinth (Eichhornia crassipes, (Mart.) Solms-Laub) in relation to biological control. Aquatic Botany 114, 12– 20. http://www.sciencedirect.com/science/article/pii/S0304377013001721#