Evaluating the impact of herbivory by a grasshopper, Cornops aquaticum (Orthoptera: Acrididae), on the competitive performance and biomass accumulation of water hyacinth, Eichhornia crassipes (Pontederiaceae)

The water hyacinth grasshopper, Cornops aquaticum is being considered for release in South Africa as a biocontrol agent for water hyacinth, Eichhornia crassipes. The release decision was to be based on C. aquaticum’s potential efficacy, in addition to its host specificity. An additive series analysis of competition between E. crassipes and a similar free-floating macrophyte, water lettuce (Pistia stratiotes), was used to assess the impact of C. aquaticum on E. crassipes competitive ability and potential for biomass accumulation. The data was analyzed using an inverse linear model where the competitive performance of both plant species was estimated using multiple linear regressions of the inverse of biomass yield. Eichhornia crassipes was 24 times more competitive than P. stratiotes without C. aquaticum herbivory but this was reduced to 12 in the presence of herbivory which equated to a 50% reduction in its competitive performance. Pistia stratiotes competitive ability increased from 3.8 to 4.8 (i.e. 21%) when E. crassipes was damaged by C. aquaticum. The interspecific competition coefficients from E. crassipes on P. stratiotes were also no longer statistically significant in the presence of C. aquaticum herbivory. Biomass accumulation of E. crassipes was significantly reduced by C. aquaticum herbivory at both planting densities tested. By comparison with similar studies on two of the most abundant, widespread and damaging biocontrol agents already released in South Africa, C. aquaticum has a greater impact on E. crassipes, suggesting it will be a valuable addition to the South African biological control program.