Ladies and Gentlemen, friends and folks and my fellow, paranoid conspiracy theorists. It is with a heavy heart and constant dread, that I bring you this latest piece of news. We’ve been invaded.
They’ve spread far and wide and multiplied in numbers beyond belief.
Even as you read this, know that they’re out there, drawing ever close, armed with viscous slime and a hundred teeth, single minded in their purpose.
Feed. Ravage. Multiply. Take over.
Aliens I am referring are not from Mars or any outer planet. They are from earth itself. They are the Giant African Snails which have invaded our land in recent years. Giant African Snail (Achatina fulica) is one of the world’s largest and most damaging land snail pests. The Global Invasive Species Database has included this snail among the “100 World’s Worst” invaders.
It is non-host specific and can consume at least 500 different types of plants. Cereals, most vegetables – including tomato, chilis, beans, amaranthus, eggplant, okra, cucumber, French beans, beetroot, yam, cauliflower, cabbage, sweet potato, drumstick, peas, turmeric and ginger, fruits and ornamental plants – are all vulnerable for attack by these foraging mollusks. Vegetables are easier to nibble on as they are softer. Papaya and cabbage are snail favorites. They also damage plantation crops such as coffee, areca nut, rubber, vanilla, and pepper, even coconut by feeding on seedlings or buds. These snails impact habitats by devouring native plants and competing with indigenous snail species.
The snail is native to coastal East Africa (Kenya and Tanzania), but is now widespread on all continents except Antarctica. It is highly adaptive to a wide range of environmental conditions and is capable of modifying its life cycle to suit local conditions. Giant African Snail is a threat to the sustainability of crop systems and native ecosystems, has a negative impact on native fauna, and acts as a vector of human diseases.
The introduction of Giant African Snail outside its native range dates back to the early 1800s, when it spread to Ethiopia, Somalia, Mozambique and Madagascar. The first occurrence outside Africa was in West Bengal (India) through Mauritius in 1847. Two individuals of the Giant African Snail were brought to India from Mauritius by the pioneer of malacology in India, William Benson, in 1847. Benson left India soon after his return from Mauritius and handed the snails to a friend and neighbour in Chowringee, Kolkata, and it was the neighbour who subsequently released them in his garden. From these two individual snails the species spread throughout much of South Asia. In the Asia-Pacific region, the snail is recorded from Bangladesh, China, Fiji, India, Indonesia, Japan, Kiribati, Malaysia, New Zealand, Palau, Papua New Guinea, Philippines, Samoa, Solomon Islands, Sri Lanka, Vanuatu and Vietnam and its range is still expanding.
The fully grown Giant African Snail consists of 7 to 9 (very exceptionally 10) whorls, with a moderately swollen body-whorl and a sharply conical spire, which is distinctly narrowed but scarcely drawn out at the apex. The snail can reach up to 20 cm in length and up to 12 cm in diameter. The normal life span of the Giant African Snail is 3-5 years, but some may live as long as 9 years. The snail reaches sexual maturity in less than a year.
It is a simultaneous hermaphrodite, which means that each individual is capable of producing both sperm and ova. Instances of self-fertilization are rare and may occur in low populations. Reciprocal copulation, typically lasting 6-8 hours, produces viable eggs. It can lay eggs within 8-20 days after copulation, usually in nests excavated in the soil or among leaves and stones on the ground surface. The snails lay up to 100 eggs in their first year and up to 500 in their second year; the fecundity declines after the second year, but the snails may live up to five years with a total egg clutch of up to 1,000. In tropical humid conditions, eggs can hatch after 11 days. Adult size is reached in about 6 months; after that the growth slows, but does not ever cease. Giant African Snail is capable of aestivating for up to 3 years in times of extreme drought.
Giant African Snail is commonly found in countries with a tropical warm, humid climate. The species occurs in agricultural areas, coastal areas and wetlands, disturbed areas, natural and planted forests, riparian zones, scrublands, shrub lands and urban areas. The snail also thrives in forest edges, modified forest, and plantation habitats. The snail prefers hot lowlands and the warm temperate lower slopes of the mountains. It needs temperatures well above freezing year-round, and high humidity at least during part of the year; the drier months are spent in dormant aestivation. It is killed by sunshine. Giant African Snail remains active at a temperature range of 9 to 29°C, and survives temperatures of 2°C by hibernation and 30°C by aestivation. It is active at night and in most cases spends the day buried underground. Giant African Snail is highly adaptable to dry and cooler climates and hibernates 10 to 15 cm deep in soft soil during less favorable conditions for up to one year.
Giant African Snail is a macrophytophagous herbivore and it also eats sand, very small stones, bones from carcasses and concrete as sources of calcium. It is a threat to native snails and affects native ecosystems by altering the food chain by providing alternative food sources for predators. This voracious snail feeds on a variety of vegetables and is considered to be a major agricultural pest; it also attacks plantations of teak, rubber, coffee and tea. It causes severe damage to horticultural and medicinal plants. In most parts of the world, the damage is greatest when the species is first established. Giant African Snail also acts as an intermediate vector of the Rat Lungworm Angiostrongylus cantonensis, causing eosinophilic meningoencephalitis in humans, as well as a gram negative bacterium, Aeromonas hydrophila, causing a wide variety of symptoms, especially in persons with compromised immune systems. The parasites carried by the snail are usually passed to humans through the consumption of raw or improperly cooked snails. It also enhances the spread of plant diseases like black pod disease of cacao caused by Phytophthora palmivora, which it spreads through its faeces. It is a general nuisance in human habitations since their decaying bodies release a bad stench and the calcium carbonate in the shell neutralizes the acid soils, altering soil properties and also the types of plants that can grow in the soil.
The natural spread is slower than intentional spread. The main pathways of spread are through trade, transport and tourism and smuggling the snail for ornamental purposes. The snail is also imported as a food source. Other pathways of spread include moving snail infested soil, plants and agricultural products. The snail is consumed as food and also used for medicinal and research purposes. It contributes to the degradation of animal matter.
Poor quarantine regulations and the animal’s high reproductive capacity are the main reasons for the rapid dispersal of this snail. Preventing its introduction is the most cost effective option. Because of the huge risk that Giant African Snail poses and also its multiple methods of dispersal, strict quarantine and surveillance activities are necessary to control its spread. Creating awareness about the various negative impacts of the snail can help stop the illegal import of Giant African Snail for trade and its international spread.
The locations of hiding places and snails are to be destroyed. Hand collection and destruction will be effective during early phase of infestation. Cut pieces of papaya stems may be placed for attracting and trapping the snails. Use wet gunny bags and papaya leaves as bait to collect and destroy them.
Marigold can be raised as trap crop around vegetable fields. Lime or bleaching powder may be sprinkled in the infested area. Common salt may also be spread on the snail infested area. Predatory snails, hermit crabs, birds like Coucals and millipedes are found to feed on this snail and check the increasing population.
Research done in Kerala found a method of first attracting the snails with vegetable waste and then a point application of TDCS (Tobacco Decoction and Copper Sulphate) mixture to destroy the snails pretty effective.
If a concentrated effort is not taken across affected Indian states, the invasion of this alien Giant African snails will continue and it will spell disaster for both agriculture as well as local biodiversity.
If you spot any infestation of African snail kindly add location information in http://indiabiodiversity.org/group/spotting_alien_invasive_species/show so as to help us compile the distribution of invasion.
My huge thanks to Abhijith APC & Vivekanand Bhaktha for providing pictures from Mysore & Mumbai.