The death of the hyacinth – biological control or herbicides?

During the past month the hyacinth has been dying on the Hartbeespoort Dam and there has been much speculation and clashing opinions regarding the cause, some citing biological control and others herbicide spraying.
Currently a group of scientists from the Centre for Biological Control of the Rhodes University is in Hartbeespoort to investigate the mass die-off of the plants.


Biological control, in the form of weevils was instituted on the dam in 2018 by the Rhodes University in conjunction with the Department of Environmental Affairs and preliminary indications are that the hyacinth die-off might be ascribed to the presence of the ‘bugs’.
According to Prof Julie Coetzee of the Centre of Biological Control the team’s sampling on the dam in November showed thousands of Megamelus weevils per sampling unit.
“Based on our lab work, we were convinced that it wouldn’t be long before mass die-off of the plants started to occur. What’s interesting is the number of winged adults – this is a sign that the plant quality is deteriorating and that the insects are getting overcrowded, so the winged adults are going to disperse in search of healthier plants. We have found dispersed populations at least 70 km away from release sites in the Eastern Cape, maybe the Hartbeespoort bugs will make their way to Roodeplaat, and further down the Crocodile River to Roodekoppies!”
So far it could not be established whether herbicides were used at all by the environmental or water affairs departments. Kormorant is waiting for a response from both.
More information on the biological control bugs and the progress will be reported on in next week’s Kormorant.