Using apple snails as a biological control agent for parasites
Apple snails (ampullariids) are usable as biological control agents for other Bulinus and Biomphalaria snails. These pulmonate snails are an intermediate host for trematoda parasites that can affect man and cause swimmers itch and schistomiasis. Although apple snails do get infected, the parasite isn't able to complete its life cycle in most apple snails and no infectious larvea are released from the snails. However, several reports have revealed that apple snails can carry and release non-human schistosome trematodes. In such cases, the cercariae penetrate the skin, where they die and cause a hypersensitive reaction, resulting in eruptions and intense itching. This is better known as swimmers-itch.

The actual control of the parasite bearing snails is reached by two means: Competition and predation.
The apple snails consume most of the available food with their voracious appetite for vegetation. This in combination with the efficient energy - reproduction rate of the apple snails, enables them to displace most other snails from their habitat. Besides the fact that competition element, apple snails also eat the eggs of other snails and can even predate on other snails directly. Small apple snail species as Marisa cornuarietis attack their victim by putting their proboscis (head/mouth) in the shell opening and eating the snail out, while larger apple snail species are known to destroy the shell of the other snail in order to reach the soft tissues.

The first time apple snails are used as biological control agent was in the early 1950's. Marisa cornuarietis was introduced in Puerto Rico in an experiment to control the Biomphalaria glabrata population. In the following years a great reduction in biomphalarid snails was observed and the Marisa snails were observed to feed on the Biomphalaria glabrata and their eggs.

Similar results are reported from the Dominican Republic and Tanzania. On both cases, the schistosome bearing snail population was greatly reduced or even displaced completely by the Marisa snails. Despite these successes, there is some concern that the widespread of Marisa cornuarietis could damage rice and taro production.

Other apple snails that have been reported to be successful in reducing Biomphalaria glabrata, Biomphalaria sudanica and Bulinus globosus are Pila ovata and Lanistes carinatus. Both snails species appeared to consume both biomphalarid eggs masses as well the snails itself, even when lettuce was available ad-libido. In a comparative studie with Pila ovata, Lanistes carinatus and Marisa cornuarietis, Pila ovata was reported to be the most successful predator on biomphalarid snails.

Furhermore Saulea vitrea is known to eat eggs masses and also adults of schistosome-transmitting snails under laboratorium conditions.
In Sierra Leone, Saulea snails, are reported not to co-exist with schistosome-transmitting pulmonates, suggesting a strong competetion, leaving no room for those pulmonates.
Note: temperature can also benefit one species above another. Pulmonates in Kenia, for example, inhabit the colder areas, whereas ampullariids are more successful under higher temperatures.

Pomacea glauca, Pomacea Haustrum and Pomacea canaliculata are other apple snail species that have been successfully used as biological control agents. These snails compete with other snails for food and predate on them as well.

Despite the fact that many apple snail species successful predate and compete with schistosome-transmitting snails, cautions should be taken with introducing exotic ampullariids. There are, unfortunately, enough examples of the devastating effects that apple snails can have on human food production, as is the case with the introduction of Pomacea canaliculata in the rice fields of Asia. Before introducing exotic apple snails species, it should be emphasised that there are often local ampullariid species that are reported to have an impact on schistosome-bearing snail populations and that these native ampullariids should be studied first before exotics are introduced. In case of Africa, Pila, Lanistes and Saulea snails could be as usefull as the South-American Marisa and Pomacea snails. One should, however, realise that many ampullariids often co-exist with schistosome-transmitting pulmonates and that the success of introducing ampullariids could be temporary.

Related: 'Parasites' section, 'Snail eradication' section, 'Bibliography on the golden apple snail' section, 'Pest Alert' section, 'Asian distribution map' section and 'Ecology' section.




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