Pest continues to threaten rice crops

Originally the golden apple snail (Pomacea canaliculata) was brought to Asia in 1980 to be cultured in ponds for human consumption. But what was originally imported as a source of food has proven to be a major threat to the region's vital rice crops.

Having a voracious appetite for rice seedlings the snail has spread through rice fields, irrigation channels and wetlands, becoming one of the biggest known pests to rice growers and producers there. In the Philippines alone, accumulative crop losses since the snail introduction is estimated to be €1 billion.

Although still spreading in southeast Asia, where the climate has made it a stronghold, the snail is has recently invaded North Australia, Hawaii and southern USA where it has also devastated rice crops. Recent research, however, shows that the invasive snail is a serious threat to natural environments as well.

This, says the report, is because the snail consumes the aquatic plants in natural wetlands. These wetlands harbour great biodiversity, and many of the animals depend on aquatic plants for their survival at some life stage. As well as providing an ideal environment for rice crops, these wetlands are also an important resource in the 'unofficial economy' since the rural poor harvest plants, catch fish and collect drinking water in the wetlands.

The ongoing invasion is nothing but an ecological disaster says Nils O L Carlsson at Lund University, who has quantified the effects of the invasive snail on natural wetlands in Laos and Thailand.

Entrepreneurs brought this South American snail to Taiwan in 1980. The intention was to grow these large snails in ponds to sell them at local food markets and export them to Europe. The project backfired. The Asian markets turned cold to the taste of the snail and all export to Europe was banned when it was found that the snail may be an intermediate host for a parasite that destroy the central nervous system in man.

At this time, however, the explosive invasion had already started. The snail is originally from Southern Brazil and Argentina and there is no feeding, growth or reproduction in the winter. At the higher mean temperatures in South East Asia growth and reproduction is greatly enhanced.

"The female snails lay around 300 eggs weekly and year around and flooded bushes in the wetlands often contain up to 100 000 eggs," said Carlsson.​ "When the snails attacked rice seedlings the invasion received a lot of attention. The farmers panicked and used both registered and non-registered pesticides. The invasion has therefore led to unsustainable use of chemicals and large negative non-target effects on many organisms, including man. Ironically, the snails effect on natural systems has been almost totally neglected, as the measurable economical damage from the snail was thought to be smaller in these systems.

"The wetlands are crucial for the diversity of both in-water, amphibious and land organisms, and rural people harvest plants, catch fish and collect drinking water in these systems. When the invasive snails consume all the plants, these productive systems turn into stagnant ponds with alga blooms."

Carlsson is the first to quantify the effects of the snail in natural wetlands. He found that snail densities in these are as high as in infested rice fields and that the snails consume almost all plants. When the plants are consumed, the snails recycle large amounts of nutrients and these nutrients enhance phytoplankton growth instead. This is therefore a great threat to all organisms that depend on aquatic plants as food, refuge or spawning substrate. The values of the ecosystem services the wetlands provide rural people also drop dramatically after a snail invasion.

This snail invasion is thus not only a continuing threat to rice production but in turn a serious ecological threat to all invaded wetlands. This leads to a vicious circle, in which rice paddy fields are rendered infertile due to the invasion.

Solutions to the continued outbreaks in Asia vary from harsh chemicals to the use of ducks and rats which are known to eat the snails. However, it is clear that despite nearly 25 years of being ravaged by the pest, Asian rice producers are continuing to be beset by the problem, which now has even wider implications for the environment.