Monday, 6 April 2009

Getting Control Back From The Aliens

Getting control of Alien vegetation is an important subject for any gardener in South Africa. Our water resources and indigenous plants are under threat, and we all need to learn to identify and remove invasive plants whenever we find them. Fortunately, we have a little help that reminds me a bit of the plot from The War of the Worlds.

The other day, while driving, I spotted a very unhappy Opuntia growing on the side of the road. It looked as if its life had literally been sucked out of it - and looking closer, it had!



Thousands of tiny bugs called Cochineal insects had latched on and were pushing their little beaks into the plant. They were quite rapidly killing the plant by sucking the sap out of it.
This way of dealing with weeds is called biological control, and South Africa is one of the top three countries in the world when it comes to this method of invasive plant control. Since 1914, we have introduced over 80 species of biological agents in order to control or destroy the invasive plants that thrive in our ideal conditions.

The concept behind biological control, is that because invasive plants are in a very strong position in a new environment with no natural enemies, the playing fields need to be levelled. The best way to do this is to introduce their natural enemies from their countries of origin. this either destroys, or helps manage the burgeoning alien populations.

Biological control is an ideal way to get rid of invasive weeds, because:
  1. it causes no pollution and affects only the targeted invasive plant
  2. it is self-sustaining and as a result, permanent
  3. it is very cost-effective
  4. it won't disturb the soil or create barren areas where other invaders could establish, because it kills the targeted plants over time, and allows the natural vegetation of the area to recover gradually in the shelter of the dying weeds.
Obviously these biological agents need to be introduced extremely carefully, and there are very strict controls in place to make sure that there are no unforeseen results. It doesn't work in all situations, but there has been excellent results with Water Lettuce, Port Jackson Acacia, Red Sesbania, and of course Opuntia.

Something else to think about, is that this little Cochineal insect can also be harvested to make a crimson dye which is sometimes used in food colouring. Try not to think of that next time you're eating food thats been dyed red!
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