Do biological control agents actually work in a natural environment?

Do biological control agents actually work in a natural environment?
Roy W. Jones
Ezemvelo KZN Wildlife; Rhodes University

Water hyacinth, Eichhornia crassipes, was first recorded on the Nseleni River in KwaZulu-Natal in 1978 and has been shown to have a significant negative impact on the biodiversity of the Nseleni River. An effective and sustainable control intervention was therefore urgently required. Although biological control using the two weevil species Neochetina eichhornia and N. bruchi has been credited with affecting a good level of control in South Africa in terms of pre- and post-release evaluation, the lack of a post-release evaluation data has undermined this statement. In a manipulated post-release evaluation experiment on the Nseleni River five experimental plots of water hyacinth of 20 m2  were sprayed with an insecticide (Actara SC) to control weevils. After ten months the plants in the sprayed plots were significantly bigger and heavier than those in the unsprayed control plots that had natural populations of the weevil biological control agents. This study has shown unequivocally that biological control has contributed significantly to the control of water hyacinth on the Nseleni River.