It grows particularly well in warm and humid conditions and in water with high nutrient levels, such as from human and animal waste and agricultural fertilisers. It also represents one of the drawbacks of dam-building, for it flourishes in still water, particularly in the shallows, which is where indigenous species mostly breed. In open rivers, collections tend to get flushed away by floods.
The hyacinth has become a problem in Africa particularly on Lake Kariba, Zimbabwe, the great lakes of East Africa, including Victoria, and on slow-moving rivers like the Nile and the Congo, said Helmut Zimmermann, divisional manager of weeds research at South Africa's Plant Protection Research Institute. In South Africa it is a problem in rivers and lakes on the more humid eastern side of the country.
South Africa is among the countries which have taken tough measures to stop people from spreading the hyacinth. Heavy penalties can be imposed for importing, selling, or transporting it. Boat owners can get into serious trouble for leaving it stuck to propellers or hulls when boats are in transit.
Neighbouring Botswana is even stricter. There, careless people could go straight to jail for similar offenses. The country is still free of the weed but is profoundly concerned about the danger of it infesting particularly the spectacular Okavango swamps in its northern reaches, the setting for many of National Geographic's wildlife documentaries.
Another reason for the water hyacinth getting so easily out of control is that animals do not really feed on it, said Zimmerman. Cattle will eat it, but its nutritional value is low and it causes diarrhoea.
The hyacinth can be controlled by mechanical means, using manpower and machines, but this is mostly unsuccessful as it grows faster than it can be cleared. Various herbicides are effective but have considerable risks for other wetlands plants.
Bring in the Bugs
The most effective and sustainable method has proved to be biological control, with the help in particular of the five insects brought in from the Amazon River Basin, said Linda Mabulu, the Plant Protection Research Institute's hyacinth researcher.
But this kind of control takes time, and its biggest drawback is people's impatience, Mabula said. "They want to see quick results and when they don't, they revert to chemical methods which kill the insects and other life without necessarily eradicating the weed.
"Sometimes I think it is not the plant that is the problem as much as it is the people. You ask them not to spray, but they do so, or they spray at the wrong time when the biological agents are most vulnerable."
Great care was taken with the introduction of the Amazon insects to South Africa, Mabula said. They were exhaustively tested and kept in quarantine until it was absolutely certain they would not turn to indigenous plants and become a new problem. The testing is repeated when the insects are provided to other countries in Africa which might have different plants they could attack.
Biological control does not eradicate invasive weeds but merely reduces them to manageable proportions, Mabula said. As with most insects, they start slowing their own reproduction when they are at risk of destroying the plants completely.
The insects have been distributed to most parts of South Africa, and Mabula is confident that, however slowly, the battle is being won. "People are becoming more comfortable with relying on the bugs to do the job. We would prefer them not to use chemicals, but if they insist, we teach them how and at which time of year so that they do not kill off the insects.
"We take more time now to explain to them how it works, and we find that they even become enthusiastic, to the point of some conducting their own experiments with the insects we give them to see which work best for them," she said.
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