Impact of the biocontrol agent Eccritotarsus catarinensis, a sap-feeding mirid, on the competitive performance of waterhyacinth, Eichhornia crassipes

The mirid, Eccritotarsus catarinensis, was released in South Africa to aid in the biological control of waterhyacinth (Eichhornia crassipes). Post-release evaluations are needed to quantify the mirid's impact on the weed in South Africa. The subtle feeding damage that it causes is not easily measured, but studies have shown that interactions with other plant stresses, e.g., plant competition, can often magnify impacts of plant-feeding insects. The impact of the mirid was therefore evaluated using an additive series analysis of competition between waterhyacinth and waterlettuce (Pistia stratiotes), as influenced by mirid herbivory. Competitive abilities of waterlettuce and waterhyacinth were determined using an inverse linear model with plant weight as the yield variable. In the absence of herbivory, waterhyacinth was 23 times more competitive than waterlettuce, but only 10 times more competitive when exposed to mirid feeding. Waterlettuce was only 0.9 times as aggressive as waterhyacinth that was free of herbivory, but 1.5 times as competitive when mirids were impacting waterhyacinth. Most importantly, in the presence of herbivory on waterhyacinth, interspecific competition coefficients from waterhyacinth on waterlettuce were no longer statistically significant. These results show that the mirid destabilizes waterhyacinth's competitive interactions between these two floating plant species, although impacts were subtle. This insect is unlikely to be an effective agent by itself, but it will be a useful complement to the existing biological control agents in South Africa.