A problem plant is any plant, shrub or tree which has a negative environmental impact in a particular locality. The main negative impacts of problem plants are invasiveness i.e. the supplanting of naturally occurring species and subsequent loss of bio-diversity, and excessive water consumption. Most invasive plants are alien to the biomes in which they thrive, but some indigenous species such as Acacia nilotica can become invasive as a consequence of poor grassland management practices. Read ‘Bart the Bugman’ at the end of this page.
These are some of the Invasive Plants which have been encountered, recorded and photographed along the river banks since the third day of the walk.
Black Wattle – Acacia mearnsii
American Bramble –Rubus cunefolius
Black Jacks – Bidens bipinnata
Morning Glory – Ipomoea purpurea
unidentified thorny thing
unidentified, but looks nasty
Camphor Trees – Cinnamomum camphora
Bug Weed – Solanum mauritianum
Parrafin Weed – Chromaleana odorata
amadumbe – Colocasia esculenta
Water Hyacinth – Eichhornia crassipes
Mexican Sunflowers – Tithonia diversilolia
We asked Bart the Bugman to tell us a little more about the problem weeds in Inanda dam.
“The two major problems are water hyacinth and water lettuce (Hyacinth is the biggest aquatic problem weed in the world and both species double their area in ten days) All aquatic plants play a good role because they absorb nutrients. The problem weeds are exacerbated by excess nutrients. Given the high nutrient loads, if we just leave these weeds in the dam, they will take over. We need to remove it out of the system. Unfortunately we have found that harvesting it for compost is not economically viable.
The bugs are extremely successful on hyacinth and water lettuce and we can control it quite quickly. For Hyacinth there is a whole suite of bugs. Eccritotarsus (a Mirid) is a little chlorophyll sucking insect with wings that breeds in the hyacinth.
The success of bio-control agents is the fast rate at which they breed. We need to be able to release them and in three months (which is the weevil life cycle) they must have multiplied a hundred fold. Then they make an impact. Then there is a midge, Orthogalumna – it seems to spread quite well down the rivers. We have had good success down below the N2. For hyacinth there is also a little moth , Niphograpta larvae that does the damage to the stems of the plant. I discovered some frass (the droppings of the larvae) and took pictures and sent it through to SASRI (South African Sugar Research Institute) I was very excited about that as we had not released it here – it could have been released in Hammarsdale where there is a dam where a lot of bio-control work is done as these moths are very mobile. The big ones that you saw on the hyacinth are called Neochetina, a hyacinth specific weevil. The Neohydronomus is a little weavel eats water lettuce. These bio-control agents are tested to ensure that they feed only on the host species. This is to ensure that there is no threat of commercial crops being devoured if the hyacinth dies back. Fortunately the bugs survive the cold and can continue their work over winter.
Other problem weeds include Parrots Feather (Myriophyllum aquaticum) as well Ludwigia and small patches of Azolla which do have bio-control agent present on them. These bugs were not introduced here but they are highly mobile and Azolla will be colonised by the bio-control agents. There is no bio-control agent for Ludwigia.
Other methods of control include spaying and mechanical removal. We spray using an integrated approach to the weeds. There is no way of eliminating these weeds, we can only try to control them. We use mechanical removal if the system is completely blocked up and the water cannot moved, with ropes and labour or with a amphibious machine called a Truxor which has a cutting bar on the front. For spraying we use a glyphosate without any wetting agents. Glyphosate is safe to use in aquatic environments but only a product called Kilomax. Taloamines, the wetting agents (found in Roundup) are what are toxic to aquatic life. Spraying Glyphosate is safe for humans but we must take precautions with not getting it into our eyes.
The bugs are bred at SASRI which has an entomology department where they study the bugs that affect sugar cane. DWA contracts SASRI to breed the bugs for both aquatic and terrestrial weeds. PPRI (Plant protection research institute) based at Cedara also have a number of Entomologists looking at bugs for controlling terrestrial weed such as bug weed, chromalaema and lantana.”
For more information on Invasive Alien Species see: http://invasives.org.za/