Zandvlei Trust

Salinity, Insects and Weeds.

Zandvlei has an advantage over most of the other water bodies in the Cape Town area in the ceaseless battle against exotic waterweeds. This secret weapon is the saline nature of our water. Both the Kariba weed and the water Hyacinth succumb in salinities in excess of 10 ppt (parts per thousand). This value can be put into perspective if one considers that the seawater is 35 ppt. This is why the main water body of Zandvlei is generally free of these exotic weeds. The Westlake Wetlands however has salinity levels, which are too low to retard the growth of these weeds. Of the several control methods used, manual removal is usually the most successful. Unfortunately this method is not very effective in the centre of the wetlands, as you have no dry land on which to stack the plants.

The ideal solution to this is the use of biological control agents. Such biological control agents are usually insects, which prey on plants in their natural habitat. Water Hyacinth for example has several natural predators in Central America. These predators are tested to ensure that they are specific to the targeted species. in other words, before these exotic insects are released everyone concerned must be confident that they will not become agricultural or ecological pests. This unenviable task is the responsibility of the Plant Protection Institute in Pretoria. It is thanks to their intensive research that we can release these control agents without being paranoid about causing even more ecological damage.

The main reason for relying on biological control agents is it is inexpensive and is effective in inaccessible areas. I feel that Kariba weed is the most serious exotic plant in our area at the moment. Due to the very small nature of the plants it is unlikely that manual removal will be totally effective. A weevil, which is very successful in the summer rainfall areas of the country, has been released in the Westlake Wetlands. The water hyacinth has however been targeted using a barrage of five different control agents. These are three species of weevil a sucking bug and a mite. The main obstacle to these insects’ success is the Cape's notoriously cold winters. My feeling is that the winters have been getting progressively milder and that these insects will survive. So keep your fingers crossed till spring!

 Clifford Dorse.

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