07/01/2020
Update on the Hartbeespoort Dam
During the past two to three weeks a remarkable transformation took place on the water hyacinth infestation of the Hartbeespoort Dam. We noticed a gradual browning of hyacinth leaves. (See attached HyaMatla Harvester on 22 Oct 2019 and again on 6 Jan 2020).
After closer investigation I noticed that there is a tremendous increase in biological control activities. Attached is 3 photos taken at De Rust on 6 Jan 2020. The images of hyacinth plants close-up indicate damage caused by weevils.
Two types of weevils Neochetina eichorniae, Neochetina bruchi and Megamelus scutellaris can be seen. The phenomenon is spread widely across the Dam, but is predominant in the western basin. The image at the dam wall indicates that the suspicion about herbicide spraying cannot be linked to the current situation since the effect of herbicide spraying would cause the most broad leave plants to die, and become a black bio-mass that will sink gradually within 2 to 3 weeks. In comparison, biological control only attacks water hyacinth which is their natural food. (Note biological control originates from South America where they were observed as the natural enemy of water hyacinth. Extensive testing was done by Dr Carina Cilliers in the 1970/80 period to proof that weevils were safe to apply in South Africa, since they only feed on water hyacinth).
Flowering of water hyacinth started in November 2019. Currently very few flowers are observed.
The growth of water hyacinth has slowed remarkably. Compare sentinel image for the last two months as reported by Linda Kleyn:
The percentage cover of the surface has dropped significantly, especially in the western basin of the Dam. The dam level was about the same at 95% refer the website for Surface Water Storage-NIWIS: http://niwis.dws.gov.za/niwis2/SurfaceWaterStorage. During the recent flood of 9 to 11 December when sluice gates released considerable water, only limited water hyacinths were released since the sluice gates are designed to release through underflow and will only overflow if dam level exceeds 100%.
Harvesting by Hyamatla composting operations, was closed over the holiday period and will resume on 6 January. Their mechanical removal contribution together with that of estates, cleaning their shoreline of water hyacinths, is strongly supported.
Below is the response of Prof. Julie Coetzee of Rhodes University after I advised them about the observed activities with regard to hyacinth infestation.
The following letter is from:
Julie Coetzee
Deputy Director: Centre for Biological Control
Associate Professor of Botany
Rhodes University
This is phenomenal! I have seen the social media posts about herbicides on the dam, and obviously had my doubts. Your pictures confirm this. When we sampled in November, there were thousands of Megamelus per sampling unit (a black bin with the base cut out of it), and based on our lab work, we were convinced that it wouldn’t belong 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 Harties bugs will make their way to Roodeplaat, and further down the Crocodile River to Roodekoppies!
I will be at Harties on Friday for the start of fieldwork, so it will be great to see this for myself.
Thank you to the authors of the letters
Frederick Botha and
Julie Coetzee from Rhodes University
