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In wetland rice the first symptom of damage by P. canaliculata is a reduced plant stand where the snails have severed the plant stalks below the water level. The tillers are cut first and then the leaves and stems are consumed under water. The crop is highly vulnerable at the early seedling stage. In taro, damage to the corms is readily visible, and active snails are easily seen feeding on both corms and leaves that have drooped so that their tips break the water surface.
Most of the literature on management of apple snail pests undoubtedly refers to P. canaliculata, which is the most widespread introduced species of Pomacea in Asia. However, because of the past confusion regarding the identities of the species introduced to Asia, some of the information purportedly relating to P. canaliculata may relate to either or both P. canaliculata and P. maculata.
Economic Threshold Levels
Litsinger and Estaño (1993) suggested that two or more snails per square metre represented a high damage risk, whereas fewer than two snails per square metre represented a low risk. However, the damage potential of P. canaliculata in rice depends on water depth, seedling age and pest density, in decreasing order of importance (Teo, 2003). Most farmers can judge the amount of control effort needed from their past experience.
Sanitary and Phytosanitary Measures
Many countries and other administrative regions have developed quarantine restrictions related to Pomacea spp. In some cases these restrictions apply to all or most species of Pomacea, because of the difficulties of distinguishing them and because little is known of the pest potential of species other than P. canaliculata and P. insularum.
Eradication of invasive apple snails is only likely to be possible in the very early stages of invasion when the new infestation is highly localized. For example, when P. maculata (and perhaps P. canaliculata) were first reported in Cambodia in August 1995 (Preap et al., 2006), recommendations were made by November 2005 to eradicate them immediately, as they were only known from a suburban concrete pond and concrete back yard tanks in Phnom Penh, a clay jar in Svay Rieng and from a few small ponds in Prey Veng (Cowie, 1995a; Preap et al., 2006). They could have been eradicated entirely at that time but other than destroying the snails in the clay pot, no action was taken and they subsequently spread to at least ten provinces (Preap et al., 2006). In Vietnam it took only five years from initial introduction for Pomacea spp. (probably both P. canaliculata and P. maculata; Hayes et al., 2008) to be present in every rice-growing province of the country (Naylor, 1996). Rapid response is therefore crucial.
In the early days of invasion of Asia by Pomacea spp. there was little awareness of the problems the snails could cause and therefore considerable efforts were made to promote their aquaculture, for example in the Philippines (Naylor, 1996). There was little awareness in some uninfested countries of the problems caused in countries with serious infestations. For example, when apple snails were first discovered in Cambodia (Cowie, 1995a) there was apparently no public awareness of the major problems they were already causing in neighbouring Vietnam. Raising public awareness is not only important to prevent the further spread of Pomacea species in Asia and elsewhere (e.g. USA, Europe), but also to warn people in regions in which parasites such as Angiostrongylus cantonensis occur of the dangers of infection.
Eradication of invasive snails is in general extremely difficult (Cowie, 2011). Eradication of a new infestation of the native Asian apple snail Pila conica was accomplished in Palau by manually collecting the snails from the infested pond, which was then covered with a layer of oil; the pond probably also was infested with a species of Pomacea, probably P. canaliculata, as pink egg masses were reported (Cowie, 2002). This is the only report of eradication of any apple snail species and was only possible because a very small area (a single pond) was infested.
Although many of the following measures reduce snail numbers, at least to some extent, their impacts on yield losses are much less rigorously documented.
Cultural and physical/mechanical control and sanitary measures
Several cultural methods are very effective at minimizing snail damage. The most often used in rice cultivation are methods of crop establishment, seeding rate and water management. The following details are derived primarily from FAO (1989), Litsinger and Estaño (1993), Halwart (1994a), Cowie (2002) and Joshi and Sebastian (2006) unless otherwise indicated. Levin (2006) detailed the various methods used in taro farming in Hawaii.
Ploughing and harrowing during the off-season increase the mortality of dormant snails in the soil. Therefore land preparation for a non-rice crop in the off-season decreases the snail population, particularly if the community carries it out. Flooding the land before planting revives dormant snails, which are then crushed by mechanized land preparation carried out by hydrotiller. Other control methods include levelling the field to facilitate drainage and to remove small refuges used by the snails. Planting crops at high densities, burning straw and planting on ridges above the water line also control the numbers of snails.
Choosing a suitable planting method is very important to minimize damage. The objective is to prevent older snails feeding on young plants. Transplanting is therefore preferable to direct seeding because the seedlings are older and more resistant to the snails. Using a well-drained location can protect the seedbed. The traditional method is to sow seedlings in a wetbed seedling nursery and then transplant them when they are 3-4 weeks old. Farmers in Laguna, Philippines, have adopted the dapog method, in which the seeds are sown on banana leaves rather than on soil; this reduces the labour involved in pulling the seedlings. The seedlings are 9-14 days old when they are transplanted in hills (a handful of seedlings). Farmers are now adopting direct sowing of pre-germinated seedlings because no seedbed is used and the method is inexpensive.
A direct-seeded crop is vulnerable for 4 weeks after establishment; a transplanted dapog crop is vulnerable for 3 weeks, and wetbed seedlings are vulnerable for 2 weeks. A transplanted crop should be established with seedlings that are 4-5 weeks old to reduce the time in the field (Mochida, 1991; Litsinger and Estaño, 1993; Halwart, 1994a; Schnorbach, 1995; Naylor, 1996). Older seedlings reduce the time in the field during the most vulnerable stage. The crop is vulnerable until the tillers stop elongating, because during this phase of active growth, little silica is deposited and the tissues offer little resistance to the rasping feeding action of a snail’s radula. Four weeks after emergence, the plants are difficult for the snails to rasp because silica has hardened the culms. Snails 1.5 cm in diameter can feed on young plants up to 4 weeks of age and 6.5 cm diameter snails can feed on 9-week-old plants.
Higher seeding rates provide greater tolerance to damage because missing patches can be filled in (Halwart, 1994a; Cowie, 2002). Extra seedlings, maintained along the borders of the field, can be used to replant voids.
Lowering the water level or draining the paddy will not kill the snails because they are able to survive long periods without water. However, snails move only in standing water and are immobile if the water depth is less than half of their shell height. Periodic draining of the fields to a depth of 1 cm is a very effective control practice because it prevents the snails moving and feeding (Yamanaka et al., 1988; Wada, 1997, 2004). The field should be well levelled and maintained at saturation, minimizing the time it contains standing water. Farmers with their own pumps can manage water levels better than those served by large irrigation systems.
In some areas of the Philippines, farmers' traditional practice of crop husbandry is to apply basal soil complete fertilizer (60:40:40 kg/ha of N, P and K, respectively) combined with urea at the final harrowing and levelling. This practice resulted in the apple snails becoming inactive and half of them died. The dead apple snails in fertilizer-treated plots had open opercula whereas those in molluscicide-treated fields had closed opercula. However, in a single element and commercial organic fertilizer trial, no apple snail mortality was observed. It was concluded that the combination of three elements (N, P and K) caused the mortality (Cruz et al., 2001).
The feeding preferences of P. canaliculata for different plants can be explored to divert them from feeding on young rice seedlings. The snails show higher preferences for certain fruits and vegetables, such as melons, watermelons, lettuce, aubergines and tomatoes, than for rice seedlings (Fukushima et al., 2001), although provision of additional food sources may serve to enhance the snail populations. Likewise, these plants can be used to collect the snails and facilitate easy hand picking (Cagauan and Joshi, 2003). In areas where plant attractant materials are scarce, old newspapers can be used to attract apple snails in rice fields before crop establishment (direct sowing or transplanting), and in fields where rice crops have already been established, taro and papaya leaves are the best attractants (Joshi and Cruz, 2001).
Hand picking of snails and removal of egg masses is a widespread control method and is relatively effective, especially on a small scale, but extremely time consuming (FAO, 1989; Cowie, 2002; Levin, 2006; Levin et al., 2006; Hendarsih-Suharto et al. 2006). Hand collection is best done in the morning or late afternoon when the snails are most active. A mechanical device called an egg clapper has been developed to enable farmers to crush egg masses without stooping over (Awadhwal and Quick, 1991). Destruction of eggs can be facilitated by placing stakes in the paddy on which the snails oviposit; stakes with eggs are then readily removed (Cowie, 2002). Wire or bamboo screens can be placed across field irrigation inlets to trap snails moving between fields (Cowie, 2002). This procedure has been widely used in taro farms in Hawaii (Levin et al., 2006). Farmers can facilitate collection by making shallow canals around the edges of their fields, for example by dragging a large rock behind a draft animal; the snails collect in the canals and are easily removed, or can be treated more effectively with localized pesticides, should this be considered appropriate (Cowie, 2002; Levin, 2006; Levin et al., 2006). The use of baits has been suggested as a means of getting the snails to congregate, thereby making them easier to collect. Lettuce, cassava leaves, sweet potato leaves, taro leaves and papaya leaves have been suggested, but baits have to be significantly more attractive to the snails than the crop is, and it is possible that providing additional food as baits would enhance snail numbers (Cowie, 2002). Collected snails can be crushed and fed to ducks; indeed, where duck farming is popular there is a market for the snails as duck food, and collecting them provides employment for landless labourers.
The edges, dikes or bunds that surround the rice paddies, taro patches, etc. should be neatly maintained. This reduces egg-laying sites and allows snails to be more easily seen and destroyed. It may also decrease the chances of snails moving between paddies (Cowie, 2002).
In Japan, physical control of P. canaliculata by rotary cultivator is efficient as it decreases their density (Takahashi et al., 2002a). In submerged direct sowing, 48.1% of the area was damaged by P. canaliculata, while in a rotary cultivation field it was 2.3% (Takahashi et al., 2002b). See also Wada (2004).
Crop rotation between rice and a dryland crop has been investigated in Japan with some success, the usual crop being soybean (Wada, 2004; Wada et al., 2004). In Hawaii, leaving taro fields fallow and dry for at least a year is an effective control measure but results in economic losses because the land is not productive (Levin, 2006).
None of the predators of apple snails in their native ranges have been shown to play a significant role in snail population regulation, although snail kites may be important in this regard (R.H. Cowie, personal observations). In South-east Asia, various fish, birds, rats, lizards, frogs, toads, beetles and ants are known to feed on introduced apple snails or their eggs (Halwart, 1994a). Some of these, especially rats, also cause serious damage to rice, and introduction or promotion of others as biocontrol agents may have unknown environmental consequences. Only ducks and fish have attracted any serious consideration as potential control agents.
Rice farmers often breed ducks and herd them into rice fields to eat the snails in the period before transplanting (Cowie, 2002; Wada, 2004). A similar approach has been taken for taro in Hawaii (Levin, 2006; Levin et al., 2006). Various duck varieties have been used (Teo, 2001; Levin, 2006; Levin et al., 2006). Two to four ducks per 100 m² were effective in controlling young snails (Vega 1991; Pantua et al., 1992; Rosales and Sagun, 1997; Cagauan, 1999), but some farmers reject this practice because duck faeces contain fluke cercariae that penetrate the skin, which results in itchiness or paddy-field dermatitis (Cagauan and Joshi, 2003). A density of 5-10 ducks per ha in continuous grazing for a period of 1-2 months significantly reduces the pest density from 5 snails per m² to < 1 snail per m² (Cagauan, 1999). As ducks graze on and otherwise damage young rice seedlings, it is appropriate to release the ducks when the transplanted seedlings are 4 weeks old. For direct-sown rice, a longer waiting period of 6 weeks is necessary. Using ducks for control may be more effective against P. canaliculata than using chemical molluscicides because the chemicals become ineffective either due to poor drainage in the plots or because snails are still buried in the soil (Cruz and Joshi, 2001).
Fish have also been suggested as biological control (Rondon and Sumangil, 1989; Morallo-Rejesus et al., 1990), but few quantified studies have been undertaken (Cagauan and Joshi, 2003). Cyprinus carpio (common carp) and Oreochromis niloticus (Nile tilapia) are popular species for controlling P. canaliculata, with the former more effective than the latter in removing snails (Halwart, 1994b). C. carpio crack the snail's shell, ingest the soft tissue and spit out the broken shell; thus they can feed on snails up to 12 mm high. In contrast, O. niloticus ingests the whole shell, and can therefore only feed on snails smaller than 3 mm. In Japan, black or Chinese carp (Mylopharyagodon piceus) and C. carpio fingerlings have been released to feed on newly hatched snails (Mochida et al., 1991). Models predicting predation rates are provided by Yusa et al. (2001), Ichinose and Tochihara (2001) and Ichinose et al. (2002). One of the problems with using fish is that the water must be kept deep enough for them, which may not be compatible with other methods (Wada, 2004).
Little is known of microorganisms associated with ampullariids that might be useful in control, nor of parasitoids that attack either the snails or their eggs. In the Philippines, twelve bacterial isolates were tested, seven of which were effective against P. canaliculata (Cowie, 2002).
Halwart (1994a) recommended that specific natural enemies for P. canaliculata, such as the predatory Sciomyzidae, should be sought in its native home in South America.
All deliberate introductions of non-indigenous species, including as biological control agents, should be carefully evaluated prior to introduction in terms of both their positive and negative potential impacts, and monitored after introduction.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
In its natural range, P. canaliculata has been considered ‘harmless and useless’, as it is neither an important crop pest nor human health hazard and it is not used as a human food or for any other purpose (Cazzaniga, 2006). However, where it has been introduced, it has caused serious economic harm, has become a human health problem in some regions, and has the potential to have serious environmental and biodiversity impacts.