Golden Apple Snails in Cambodia

Chanthy Pol

Cambodian Agricultural Research and Development Institute (CADRI)
Plant Protection Program, Phom, Penh, Cambodia
E-mail: plap@bigpond.com.kh

Golden apple snails (Pomacea sp.) were discovered CIAP in Svay Rieng in August 1995. A rice farmer was raising them in a clay jar. He was unaware that these snails are rice pests. Mr. Theav Chhay, the Chief of the DOA Toul Koktrap Research Station held a meeting to explain to the farmers that the snails are pests and should be not bred. The farmer destroyed the snails and eggs after the meeting. The farmer purchased the snails in Phnom Penh. CIAP and the Plant Protection and Phytosanitary Inspection Office searched the Phnom Penh area and found several places where people were breeding large numbers of the snails. In additional to Phnom Penh, the snails are being raised in at least 9 provinces: Kampong chhnang, Kampong Cham, Kampong Speu, Kandal, Prey Veng, Pursat, Siem Reap, Svay Rieng, and Takeo.

CIAP experiments indicated that the newly discovered snails ate rice seedlings but not tillering rice. A typical snail consumed 7 rice seedlings per day in laboratory experiments.

In October 1995, Dr. Robert Cowie, a snail taxonomist at the Bishop Museum in Hawaii, was brought to Cambodia to identify the newly introduced snails. He concluded that Cambodia has two species of golden apple snail: Pomacea canaliculata and Pomacea insularum. Based on the rate of spread of golden apple snails in other countries, and taking into account the poorly developed Cambodian irrigation system, Dr. Cowie estimated that the snail would becomes a rice pest within 2 years, and would be a major rice pest, perhaps the major Cambodian rice pest, in 5 years.

In Nov. 1995, farmers in Takeo were found intentionally placing golden apple snails in their rice fields to rise as food; just as they would normally place the native Farmers were not aware that the golden apple snail is a rice pest.

Golden apple snails spread through Cambodia because people thought that they would make money by culturing these snails and selling them as food. This same pattern has been observed throughout Asia, beginning with Taiwan in 1979 when the snail was introduced from South America, probably Argentina. Since then the snail was deliberately introduced to Japan in 1981, the Philippines in 1982, China in 1985, Korea and Malaysia in 1987, and to Indonesia, Thailand, Vietnam, and Laos in1989. In each of these countries the snails escaped from breeding sites, and became widely established in rice fields.

Golden apple snails produce bright pink or red egg masses around grass, sticks, rice, or anything extended above water in paddies. If the eggs are broken a blood-red liquid is released. Snails begin life quite small, with shells less than 1 cm long, but eventually their shells grow to 6 or 7 cm in length. The shells range in color from amber to dark brown. The native Cambodian apple snails have black or dark green shells. However, a layer of algae often covers the shells of older golden apple snails, making them appear green. The algae are easily brushed off to reveal the golden color of the shell.

The differences between the golden apple snail and the native apple snail are presented in Table 4.3.

Table 4.3. Differences between the exotic and native apple snails

For golden apple snail management guidelines see Section 6, Technology Recommendations.

6.3.1 Golden Apple Snail Management Guidelines

The golden apple snails in Cambodia are potential rice pests and should be eliminated where possible. Given the history of golden apple snails in neighboring countries, it is only a matter of time before this snail becomes a serious pest in Cambodia. A description of the golden apple snail, and how to distinguish it from the native apple snail is give in Section 4.3.1


Snail Management in Rice (Navarro 1993)

1. Transplant older seedlings, or increase-seeding rate in direct seeded rice.
2. Install screens on water inlets.
3. Remove snails from rice paddies anytime before final harrowing.
4. Collect and crush egg masses.
5. Place bamboo stakes around the field to give the snails a place to lay egg where they are easily collected and destroyed.
6. Hand-pick snails in the morning and afternoon when they are active. Collected snails can be eaten by ducks and pigs. If completely cooked, the snails can be eaten by humans.
7. Herd ducks through the rice paddies immediately after harvest and 30 to 35 days after transplanting early maturing rice, or 40 to 50 days after transplanting late maturing rice.

Chemical Control of the Golden Apple Snail

Presently, chemical control of the golden apple snail cannot be recommended in Cambodia. There are no molluscicides available in Cambodia. Of the pesticides available in Cambodia, CIAP research indicates that the insecticide Endosulfan (Thiodan) effectively eradicates snails, even heavy infestations. Endosulfan, however, is not recommended for snail control because it has long residual activity; kills fish, frogs, and other desirable organisms; and accumulates in the food chain.

Bayluscide (Chronitralid) is used to control golden apple snails in the Philippines. It does not accumulate in the food chain. However, it is toxic at .5 ppm to fish, and other aquatic organisms. Bayluscide is not available in Cambodia.

5.5 GOLDEN APPLE SNAIL EXPERIMENTS

The Golden Apple Snail (Pomacea) was reported for the first time in Cambodia in August 1995 (CIAP' 1995 Annual Research Report'). A persistent, though unconfirmed, story is that golden apple snails were brought to Cambodia by refugees returning from Thailand. The snails are now being raised in Phnom Penh and at least nine other provinces. The only reports of golden apple snails in rice fields come from Pursat (confirmed by CIAP) and Takeo (unconfirmed).

To study the pest potential and possible control of the golden apple snail we conducted a series of experiments described below.

5.5.1 Experiment 1: Pest Potential of the Golden Apple Snail
When the golden apple snail was first discovered in Cambodia there was some question of its identity. Although the snail produced the distinctive pink and red eggs by which Pomacea are known, the snails themselves differed from the Pomacea of the Philippines in several respects. The Cambodia snails generally lacked stripes on their shells, though the growth rings sometimes discoloured giving a stripped appearance. In contrast, the Pomacea of the Philippines have stripes perpendicular to the growth rings of the shell. The Cambodia snails tend to be much larger and lighter in colour than those in the Philippines. To make certain that the Pomacea found in Cambodea was indeed a potential rice pest we conducted a simple unreplicated experiment described below. We considered that snails, which ordinarily do not eat rice, might do so if the only other option was starvation. To test whether apple snails readily switch diets, we compared the golden apple snail to the native apple snail (Pila sp). The native apple snail inhabits rice field and is not considered a pest.

Materials & methods

Three cement basins of 2.2×2.8m at the CIAP glass house in Phnom Penh were used as experimental plots. Each basin had its own net-cover to prevent snails from escaping. Each net-cover had a zipper used as an entrance for constructing the experiment and collecting data. Fifteen days old seedlings of IR66 were transplanted at single seedling per hill. Spacing between hills was 20×20cm. Five days after transplanting 1kg of golden apple snails (Pomacea sp.) were placed in one basin did not receive any snails.

The native snails used in this experiment was collected from rice fields around Phnom Penh.Golden apple snails were collected from the ponds where people were raising them for food. Both species were starved for 2 days before being placed in the experimental plots. The total number of plants consumed by snails daily was recorded.

Results

Within 7 days the golden apple snails consumed 100% of the rice plants in their basin (Table 5.18). No crop loss occurred in basin with native snails and the basin without snails. During the first two days of the experiment, native snails fed on algae. When the algae was consumed, native snails began to die, presumably of starvation. By the end of this experiment, only two native snails remained alive. Only the snail-free basin had algae covering the water surface. Apparently the snails consumed the algae in the other two basins.

Discussion

The golden apple snail clearly has the potential to become a serious rice pest.
The native apple snail does not eat rice at all, even when no other food is available. Snails apparently reduce algae blooms in rice fields. The farmer' s practice of placing native apple snails in the paddy is therefore an effective form of augmentative biological control of algae. Since most farmers place the native apple snails in the field to harvest as food later, the application of biological control may often be unintentional.

Table 5.18 Plant loss due to golden apple snail feeding. Jan 3-9, 1996. CIAP Glass-house, Phnom Penh.

5.5.2 Experiment 2: Damage Potential of the Golden Apple Snail

To determine the damage potential of the golden apple snail, we divided snails into three groups according to size, reasoning that larger snails would eat more rice. The damage potential was in fact size dependent, but to our surprise it was the medium sized snails that consumed the most rice. This is perhaps because the largest snails stop growing and therefore require less food. The feeding rates of the small, medium, and large snails are 1.8, 8.4, and, 4.7 tillers per day respectively. These feeding rates are quite high compared to other major pests. For example, a stem borer will destroy a single rice tiller over an entire season; and a rice bug will destroy 5-10 grains of rice per day.

If the feeding rates calculated in this study are correct, then in a newly transplanted rice field with 70-100 tillers/m2, an infestation of medium sized golden apple snails at a density of only one snail per square meter could destroy an entire field in 8 to12 days.

Materials and Methods

Golden apple snails, collected from ponds in Phnom Penh, were divided into 3 weight groups: 10-12g, 23-32g, and 55-63g. Ten plastic tubs (52cm top diameter, 36 cm bottom diameter, 19.5cm deep) comprised the experimental units in this study conducted at Stung Meanchey Crop Protection Station. Each tub had its own net-cover to prevent the golden apple snails from escaping. Twenty-five day old seedlings of IR66 were transplanted at 10 hills per tub, 4 seedlings per hill. One day after transplanting, 1 golden apple snail was placed in each of 9 tubs; 3 snails from each size group. One tub did not receive any snails. Snails were starved for 2 days before the 7 day experiment. The total number of rice plant consumed by golden apple snails was recorded daily.
Data Analysis. To compact the data over individuals and display it across time, the average percentage of tillers consumed each day was calculated for each of the weight groups and plotted on a graph. To compact the data over time, the average feeding rate was calculated for each snail for the first 4 days of feeding. Only the first 4 days were used because one snail consumed all 40 tillers by the fifth day. These average feeding rates were then averaged for each weight group and analyzed by ANOVA, DMRT, and LSD with IRRI-STAT software to test for significant differences in mean feeding rates. Weight group sum of squares was partitioned into linear and quadratic components. The individual weights and feeding rates of each snail were plotted on a scatter graph and analyzed by regression analysis with Excel software.

Results

Medium sized snails consumed the most rice per day and small snails consumed the least rice per day (Figure 5.9). The large golden apple snails destroyed an average of 29 tillers within 7 days, consuming a minimum of 2 tillers per day with an average consumption rate of 4.7 tillers per day for the first 4 days. Medium snails completely consumed all 40 tillers within 6 days, consuming at least 3 tillers per day, with an average consumption rate of 8.4 tillers per day for the first 4 days. Small snails consumed and average of 20 tillers within 7days, consuming at least 1 tiller per day, with an average consumption rate of 1.8 tillers per day for the first four days. The most tillers consumed by a single snail on a single day were 16 tillers by a 30.3g snail. No crop loss occurred in the tub without snails, which served as a check.
The feeding rates of medium and small snails were significantly different at the5% level, but the feeding rates of large snails were not significantly different from medium or small snails. Trend analysis finds a quadratic description of the relationship between average weighty and average feeding rate significant at the 1% level. This is entirely consistent with the regression analysis, which indicates that the feeding rate of snails increases as snail weight increases, until snails reach about 40g; then, feeding rate decreases with size (Figure 5.10).

5.5.3 Experiment 3: Effect of the Dragon Bone Plant on Golden Apple Snail

An experiment indicated that dragon bone plant can be used to reduce crab damage to rice (Section 5.6). We therefore decided to test whether the plant could also be used for snail control.

Materials &Methods

Three cement basins measuring 2.8×2.2m and 70cm deep served as the experimental plots. Each cement basin had it's own net-cover which prevented the snails from escaping. Each net-cover had a zipper door used as an entrance for transplanting, photography and data recording. One kg of snails were introduced to each of these basins on 5 June 1996 and fed rice seedlings until 14 July 1996. On 16 July we drained all 3 basins, placing a bag constructed from mosquito netting over the drain to prevent snails from escaping. Then, seedlings (24 days old) of Cambodian Rice 1 (CAR 1) were transplanted at a single seedling per hill, for a total of 168 rice seedlings in each basin. Spacing between hills was 20×20cm. We put 10cm of water in each basin after transplanting and immediately placed 1kg of chopped dragon bone plant into each of 2 basins.

The golden apple snails were collected from pond culture at Po Cheantong in Dang Kor District. Dragon bone plants were collected near the Kap Srau station, Phnom Penh. The total number of plants consumed by snails was recorded daily by visual count, then adjusted to percentage of crop loss.

Results & Discussion

Snails consumed all the rice in the untreated pond within 2 days. In the treated ponds snails did not eat for the first 2 says after dragon bone treatment was applied. Instead, they pulled tightly into their shells and floated on the surface. By the third day snails in treated basins began eating the seedlings. these snails consumed all the rice by the fourth and fifth day (Figure 5.11).

The dragon bone plant apparently contains toxins which the snails attempt to avoid by hiding in their shells. Snails exhibit the same behaviour when Endosulfan or copper is applied to the water. In this experiment the snails could not leave the basin. Under field conditions it is possible that snails would leave the area. When the snails re-emerged from their shells and began feeding, the toxin had probably degraded to tolerable levels.
Although the tested concentration of dragon bone plant did not kill snails, it still has potential as an aid to snail management. When the snails retract into their shells and float on the surface in response to dragon bone, the snails are especially vulnerable to collecting by net.

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