for National Geographic News
The purple-blue flowers protruding from mats of green leaves spreading over a tranquil pond make a beautiful sight. But in South Africa, as with many other countries invaded by the South American water hyacinth, the fight to stem its relentless spread has long been a losing one.
Until five tiny bugs were enlisted to solve the problem.
South African scientists have imported the insects from the Amazon Basin, the original habitat of the hyacinth where they are its natural enemies. They are increasingly proving the best way of keeping it in check in Africa as well. The bugs feed on different parts of the plant and in different ways, and can be used in combination or singly as their effectiveness differs with climates and seasons.
The bugs include two beetles (Neochetina eichhorniae and Neochetina bruchi) which tunnel into the leaves and crown of the plant, a mite (Orthogalumna terebrantis) which hollows out feeding galleries between the leaf veins, a moth (Niphograpta albiguttalis) whose larvae feed on the leaf surface and burrow into the crown, and a mirid (Eccritotarsus catarinensis) which causes browning of the leaf through chlorophyll extraction.
The water hyacinth (Eichhornia crassipes) is a prolific grower. In favourable conditions it can double in population every 12 days, quickly clogging waterways and smothering indigenous species by cutting off their sunlight and oxygen. The main reason for its spread around the world is its beauty and its consequent popularity as an ornamental plant.
The havoc it is causing in parts of Africa is underscored in a new booklet, Invasive Alien Species in Africa's Wetlands, in which it is described as the world's worst water weed. The booklet has been released by IUCN (the World Conservation Union), together with the Ramsar Convention on Wetlands of International Importance and the Global Invasive Species Programme (GISP).
"The hyacinth has inflicted immense damage on wetlands and [Africa's] biodiversity, as well as on the economy," said Geoffrey Howard, regional program coordinator for IUCN in eastern Africa.
The plant forms floating mats which obstruct shipping and river flows, and which get into water supply systems, drainage canals, and hydro-power generators. Dense colonies at lake edges smother indigenous species, and they make it hard for rural communities to collect water, fish from shore, or get their fishing boats through.
The water loss the weed causes through transpiration is well above that of open water, causing reservoir and lake levels to drop faster than from evaporation.
"The harm it and other invasive species does to African wetlands alone runs to billions of dollars annually. This, while their impact is only just beginning to be appreciated," said Howard.
The hyacinth is free-floating, with long feathery roots and hollow stems that lend it buoyancy. It grows from seed and propagation. In shallow water it can flower profusely when it gets rooted in soil, leaving many seeds which can last up to 15 years and which germinate as conditions become favourable.
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