The Golden Apple Snail - a Special Report

The Golden Apple Snail

- a Special Report

1. Introduction

The Golden Apple Snail (Pomacea canaliculata), originates from South America, and is widely distributed in swampy habitats in Brazil, Argentina, Bolivia, Paraguay and Uruguay.

It starts life in a cluster of bright pink eggs that are laid along the edges of waterways. Fully grown, it resembles a Golden Delicious apple, hence the name. Since it is a rather pretty snail, people kept them in their fish tanks. In 1980, it was introduced from Argentina to Taiwan through the aquarium trade.

These snails have a high reproductive rate and grow rapidly to quite a large size. In 1981, thinking it might have some potential as a new source of high protein food, government programs in Taiwan and the Philippines encouraged people to set up farms to rear them for food.

However, it soon became apparent that nobody liked the taste of the snails. The projects failed and the farms were soon abandoned. No one expected then that the snails could escape and spread along irrigation networks of rice-growing areas, nor that they would feed voraciously on rice seedlings. Soon, they spread throughout the Philippines and Taiwan, rapidly gaining a reputation as a pest of plague proportions.

In 1989, the Food and Agriculture Organization of the United Nations estimated rice yield loss in the Philippines to be between 1% to 40%. In the Philippines, the total area infested rose from less than 3% in 1982 to as much as 15% by 1991, and steadily increased. Of a total of 3 million hectares of rice, 1.2 to 1.6 million hectares are now infested.

By 1986, 34 of the 47 rice-growing districts in Japan had become infested. Since then, the snail has invaded other rice growing countries, moving from China, Hong Kong, South Korea and Papau New Guinea into south-east Asia, affecting Vietnam, Cambodia, Laos, Thailand, Indonesia and Malaysia. It has also reached the southern parts of the USA and is expected to spread farther.

In early 1997, they reached Sarawak, first brought from Kalimantan by a Long Balai resident in Ba Kelalan, to be bred for food. From there, it was brought into Bario by migrant Indonesian workers. Since then, it has destroyed an estimated 239 hectares of padi at Ba Kelalan, Long Semadoh and Bario.

The total cost of this golden apple snail invasion is very high. While it does not feed on mature rice plants, it causes significant damage to very young rice seedlings. Apart from this direct loss in rice yield, additional costs include that of controlling the snail and replanting lost seedlings. The snail effects humans too. It harbours various parasitic worms that cause skin irritations, and is host to the lungworm (Angiostrongylus cantonensis) which, normally parasitic on rats, can cause a fatal form of meningo-encephalitis in humans.

2. The Life Cycle

They live in ponds, swamps, irrigated fields, canals and water-logged areas, can even survive in polluted water with low oxygen levels, bury themselves in moist soil during the dry season, lie dormant for 6 months and become active again when the soil is flooded. They stay submerged during the day, usually hiding in dense vegetation near the edges or the surface. They are more active during the night, often leaving the water in search of new feeding areas.

They can live for 2 to 6 years. Adults mate any time of the day, usually among crowded plants where there is continuous water supply throughout the year.

A female snail lays 200 to 500 eggs at a time, and between 1000 to 1200 eggs during one month. The eggs, in bright pinkish-red clusters, are laid at night on any vegetation, bunds or objects (e.g. twigs, stakes, stones, etc.) above the water surface.

The eggs slowly turn paler pink and take 7 to 14 days to hatch. The hatchlings, voracious feeders, grow fast and mature in 75 to 90 days. The female snail will be able to start laying when it is just two months old. Its most destructive stage is when the shell is between 10 to 40 mm long.

They feed on almost all types of plants, from algae to water hyacinth, rice seedlings and other leafy plants, preferring the softer young parts since they feed by scraping plant surface with the rough tongue. They also feed on decomposing organic matter.

In padi fields, newly-transplanted seedlings, up to 15 days after transplanting, are most vulnerable. The snails eat the base of young seedlings first, before moving on to the stems and leaves. They can even consume the young plants in a whole field overnight. Soon, one starts to notice obvious signs of their activity, missing plants and cut leaves floating on the water surface.

3. Control Measures

In the early 1990's, ten years after the snail was introduced for farming, nearly all Filipino rice farmers saw the snail as their greatest enemy. There are many ways of dealing with the problem, from hand-picking snails to transplanting older seedlings that are less vulnerable to the snail, or raising ducks in the rice fields.

Replanting was often the only answer but successful field trials in the Philippines showed that growing fish in the rice fields could keep snail populations down. Besides controlling the snails, it also supplemented the farmer's diet and generated extra income.

About 40% of Filipino farmers use pesticides to control the snail. There are, currently, no known chemicals which are safe, effective, and cheap enough for farmers to use. The existing chemicals are mostly persistent, dose-cumulative organotins that create health effects such as nails falling out, skin problems, blurring vision, and blindness. There are also significant and long-lasting downstream effects on natural marine ecosystems. The chemicals are not very effective without good water control, and many of them are now banned in Japan, Taiwan and the Philippines.

A 1996 FAO field experiment in Viet Nam, which faced a population explosion of the fast-breeding snail after extensive floods swept the country, explored the use of biological controls and natural predators to fight pests. They discovered that raising fish in the rice fields was one of the best ways to control the snail. The common carp was found to reduce snail populations, particularly young snails with a shell height of less than 1 cm, by up to 90% over a three-month period.

The aquaculture research institute in Viet Nam ran training courses to teach farmers how to breed black carp, common carp, Nile tilapia and catfish species. The common carp appeared to be the most effective agent since it has a better survival rate than black carp and eats more snails.

4. The Integrated Approach

It is now quite evident that no single method is likely to bring about the total elimination of snail populations. Hand-picking, while effective, is labour intensive. Ducks only feed on young snails. And water control can be especially difficult during the monsoon season. A more holistic aproach is called for.

Many of our farmers live in remote areas, beyond the reach of normal media, and many may still remember hearing, and still believe, that these snails make good eating! A key requirement would be a public information and poster campaign targetted at the more remote longhouses.

5. Controlling Golden Apple Snail Populations

a. Land Preparation

When the water level is critically low, the snails will seek shelter in damp areas. Before draining a nearly dry field, a few deep trenches should be made in them, so that they will retain some water. Snails congregating in these areas can be captured easily.

In the Philippines, the leaves of the following plants, Blumea balsamifera, Citrus microcarpa, Tinospora rumphii, Monochoria vaginalis and Nicotiana tabacum, found to be toxic to snails, were placed in these trenches to kill the snails. Some of these plants, or related species, may be found to occur naturally in Bario too.

All vegetation and obstacles around a field, and on the bunds, should be cleared much as possible. The snails need these to deposit their eggs on. With no suitable laying sites available, they are forced to lay their eggs on bare ground where the eggs are more exposed and vulnerable.

It is not possible to completely prevent a field being infested by snails from adjacent fields. Limit migration along water inlets and outlets by putting wire or woven bamboo screens over these openings to reduce the entry of hatchlings and adults. The screen should be cleaned regularly to clear obstructions and trapped snails should be removed.

b. Transplanting

Young transplanted seedlings are most vulnerable during the first ten days. Planting older seedlings will help minimise damage.

Snails prefer areas where there is standing water. Flash flood newly-transplanted fields for first fifteen days to limit their activity.

Before transplanting, clear both the field and its edges of all bushes and tall grasses. Then, place thin bamboo stakes in water-logged fields or along its edges to attract egg-laying adults. Any eggs laid on these stakes can be quickly seen and destroyed.

c. Hand Picking

Hand picking successfully controls snail populations without harming the environment. But it is a labour-intensive method that works best when done on regular basis.

Good results can be obtained if this activity is seen as a gotong-royong effort, collecting snails in the mornings and afternoons when they are most active and easy to find. Traps baited with banana, papaya and lettuce, or trumpet flower, cassava and taro leaves, even old newspapers, can be used to attract the snails and facilitate collection. Eggs could also be collected at this time.

d. Predators and Natural Enemies

The introduction of any predator species should only be carried out after considerable research, and with great caution. The horror stories from Australia and New Zealand, where predators introduced to curb one problem resulted in the creation of another and larger problem, are too many to ignore. Such unexpected side effects that could make matters worse.

Ducks can be used to control snail populations - they do eat substantial amounts of snails but only the smaller ones. However, it might be worth cautioning villagers on the risks of subsequently eating these ducks - they can often serve as an intermediate host for trematode parasites!

Another method being considered is the use of botanical pesticides that are biodegradable and less likely to leave harmful residues. It might prevent the use of pesticides that cannot differentiate between the Golden Apple Snail and harmless native snails!

Additional information available from:

1. The Philippine Rice Research Institute.
2. Applesnail Net
3. Food and Agriculture Organization