The Centre for Biological Control (CBC) at Rhodes University sampled plants at various sites on the Hartbeespoort Dam at the end of November and found that the dam has shown a marked increase in water hyacinth coverage.
“The cover at the time of the visit on 25 and 26 November was 36% of the dam’s surface. The plants growing on the dam are healthy, reproducing well, and could be expected to begin flowering within the next few weeks. Due to the combined effect of the cold winter and the massive reduction in plant cover resulting from the previous biocontrol efforts, current biocontrol agent populations are lower than anticipated because the plants on the dam likely germinated from seed,” said the Centre for Biological Control.
“However, there are increasing populations of the weevils Neochetina bruchi and N. eichhorniae, as well as the planthopper Megamelus scutellaris, and the moth Niphograpta albiguttalis. Weekly releases of M. scutellaris are being made in an attempt to bring these numbers up to damaging levels, with 6000 released over the last week. These releases were made at the biocontrol agent rearing station at Redstone Estate, and on an enclosed floating water hyacinth pod, which had a healthy population of the planthopper already established. We can still expect the numbers of insects to increase naturally from now until the end of the growing season when populations are usually greatest between March and April.”
Due to the resurgence of water hyacinth from the seed-bank, soil samples have been collected to determine the extent of the seed bank. Should this analysis prove fruitful, a more comprehensive study will be proposed in order to quantify seed density at more sites around the dam to predict how the water hyacinth will recover in the future, thus guiding future management practices as part of the adaptive management plan for the dam.
“Going forward, the plan is to maintain the cultures of insects at Redstone Estate and in other areas around the dam so that releases can begin in December/January, while Rhodes University will continue to provide insects as often as possible to slow the growth of the plants on the dam. This process, being biological, can take some time and requires patience,” CBC said.