The Killer Weed

Over 50 tropical and subtropical areas (20, only in Africa) are infested today by the beautiful water hyacinth (Eichornia crassipes), originating in the Amazon basin. The conquered areas go from Africa (except Sahara and Namib deserts and southern tip of South Africa) to India, Indochina, eastern China, Japonia, New Guinea, Indonesia, northern Australia, Central America and southern US.

In Africa, Eichornia first emerged in 1890, in the Nile River, as an ornamental species. With the same aim, the plant was introduced in 1908 in Natal (South Africa). Gradually, along several decades, the plant penetrated Zimbabwe and from this area it reached the Congo basin. In the '70s, the weed boomed in the whole basin of the Congo.

The greatest disaster was provoked by the "green pest" when it entered Lake Victoria. The penetration occurred along the Kagera River, in Rwanda, during 1989. Lake Victoria ensures the subsistence for 30 million people in Kenya, Tanzania and Uganda, being the largest tropical lake, with a surface of 68,000 square km (27,000 square mi).

The invasive power of Eichornia is delivered by its capacity of doubling the occupied area in just 15 days. In one year, Eichornia can grow from 400 tonnes to 1,700 tonnes per hectare, in the conditions of the Victoria Lake. Made of water in a proportion of 90 %, the plant develops grassy piles weighing several tonnes. Its stems and leaves sift everywhere, including the turbines of the hydrocentrals, degrading and even causing the rumbling (under their weight) of bridges, dams, tapping irrigation canals or water pumps. The plants reproduce continuously, skipping sexual breeding, and turning into an infection hotbed in which all kinds of tropical parasites and germs proliferate, causing malaria, cholera, bilharziosis, encephalitis and other diseases.

The water hyacinth speeds up the evaporation of waters of the rivers and lakes, and under its shadow, fish suffocate because of the low oxygen levels.

Mechanical methods of removing the plant are useless. Herbicides do not work. Chemicals that are too strong will destroy all the fauna and flora of the invaded environment, so they cannot be used. Biologists came with a more viable solution against the water hyacinth: a weevil called Neochetina eichorniae, native to South America. The weevil feeds exclusively on Eichornia, making it parch, especially after its larvae carve canals in the stems of the plant. In India, Thailand, Zambia and Benin, these weevils managed to control the expansion of the water hyacinth. But the beetle must be introduced at the beginning of the infestation. And the effect appears on long term, while the plant continues its aggressive advance.

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