Abstract:
The integrated control of water hyacinth, Eichhornia crassipes (Martius) Solms-
Laubach (Pontederiaceae) has become necessary in South Africa, as biological control
alone is perceived to be too slow in controlling the weed. In total, seven insect
biological control agents have been released on water hyacinth in South Africa. At the
same time, herbicides are applied by the water authorities in areas where the weed
continues to be troublesome. This study investigated the assumption that the two
control methods are compatible by testing the direct toxicity of a range of herbicide
formulations and surfactants on two of the biological control agents released against
water hyacinth, the weevil, Neochetina eichhorniae Warner (Coleoptera:
Curculionidae) and the water hyacinth mirid, Eccritotarsus catarinensis (Carvahlo)
(Hemiptera: Miridae). A number of the formulations used resulted in significant
mortality of the mirid and the weevil. Products containing 2,4-D amine and diquat as
active ingredients caused higher mortality of both agents (up to 80% for the mirid) than
formulations containing glyphosate. Furthermore, when surfactants were added to
enhance herbicide efficiency, it resulted in increased toxicity to the insects. We
recommend that glyphosate formulations should be used in integrated control
programmes, and that surfactants be avoided in order to reduce the toxic nature of spray
formulations to the insect biological control agents released against water hyacinth.
