Water hyacinth cover is down to 29% on Hartbeespoort Dam, the Centre for Biological Control (CBC) said this week.

“Over the last 3 weeks, we have seen huge fluctuations in cover from 56% at the end of February, down to 40 %, back up to 53%, and now at 29% cover. A lot of this variation is due to wind and water movement which compresses and expands the mats of water hyacinth. Furthermore, as the hoppers start to reduce plant vigour, the plants are able to condense more. This movement is so important in breaking up the mats which then allows sinking of the biocontrol-damaged plants on the leading edges,” said Prof Julie Coetzee of the CBC.
The Megamelus bug populations are also increasing, with an average of 700 insects/m 2 across the dam. This is comparable to insect numbers in December 2020 when the water hyacinth cover was at its maximum. There are also reports of Megamelus swarming at lights at night, which is an indication that the plant quality is decreasing, so the hoppers are leaving in search of better food.
“These swarms won’t last long as we’ve shown in the past, and the benefit of course, is the reduction in water hyacinth.”
She said another interesting phenomenon is the lack of blue-green algal (cyanobacteria) scums over the last four years, and this can be ascribed to no herbicide spraying of the water hyacinth. Spraying of the hyacinth firstly reduces competition to the algae as light and nutrients become available for algal growth and the spraying results in rapid decomposition of the plants which releases additional nutrients that the algae can use. A biological control approach reduces the water hyacinth cover slower than a herbicide application, so the resources for algal growth are less available.
“In addition, water hyacinth produces a chemical, called an allelochemical, that is toxic to the blue-green algae which prevent it from proliferating. The take-home message is “be patient and rely on biocontrol, water hyacinth cover will be minimal towards the end of April, and no blue-green algal outbreak should occur. Be impatient and spray, water hyacinth will disappear quickly, so will the biocontrol agents, and the risk of a blue-green algal bloom is high,” Coetzee said.