A few weeks ago we blogged about taking action against the ‘alien carp’ in the dams – now the fight has turned to ‘alien snails’. Theba pisana, also known as the white garden snail, dune snail, the white Italian snail or simply the Mediterranean snail, is a problem. As you might have guessed from the name it originally comes from the Mediterranean but it is a widespread invasive species in many countries, and a well-known agricultural pest as well as being an enemy of natural ecosystems like the precious fynbos biome. It is thought to have been introduced to South Africa, probably by accident in a shipment from Europe, sometime prior to 1881 and it has thrived, particularly in the Western Cape, which has a climate similar to that of the Mediterranean, and along the coast or in sandy/ limestone rich habitats.
Theba Pisana can be recognized by its white – yellow brown shell colour with light brown spiral markings. The variation in shells is genetically maintained by natural selection and in open habitats (such as the vines at Excelsior) snails with a pale shell are at a selective advantage due to the greater reflectivity of the shell.
Now you might think that such a small snail couldn’t possibly merit draconian action. But en masse these slimy mollusks can have quite an impact by chomping through the fresh green shoots and leaves of the vines and defoliating citrus trees (which are also grown on Excelsior). They show the same voracious appetite for fynbos and have a negative impact on indigenous snail species by out competing them for available food; scientists have deemed them a ‘serious’ pest.
They are active mostly at night and during periods of high humidity, irrespective of temperature. Given the very high densities that Theba pisana can attain at some sites (up to 3,000 per tree), plus their ability to eat a wide variety of plants, their potential impact on the vegetation can be considerable. The snails must maintain an average body temperature of well below 44°C during the hottest times of the day. Since temperatures at ground level frequently exceed this, the snails climb upwards and aestivate (similar to hibernation) above the ground where temperatures are lower.
What to do about this plague? We would really like to have an army of ducks to pick the vines clean of these pesky snails but sadly it’s simply not practical (and farmers must be practical!); ducks are delicious so many would ‘find’ their way to a local dinner tables or be snatched by predators so, in order to keep the duck army intact, it would be necessary to shut them up every night – no mean feat with hundreds of ducks! And snails are seasonal, breeding in autumn to winter and growing to adult size (c.14 mm diameter) by the end of the following summer, so at points in the year there would be hundreds of ducks with very little to eat.
We have tried a homemade porridge of flour, bran, cooking oil, methylated sprits (which attracts the snails) and iron sulphate (totally fine for humans, not so much for snails). The mixture is spread on vine poles, or anything the snail might climb up, and the iron sulphate knocks out the digestive system of the snail, killing it within a week. But obviously all that porridge painting is very labour intensive.
We are currently trying an ecologically friendly fungus used in organic farming (Chitter – Paenibacillus) which is sprayed onto the vines in the late afternoon when strong UV light can be avoided. When the snails come into contact with the fungus it ‘eats’ them from the inside out (including the shell!) and they die of desiccation within a fortnight.
We prefer not to use chemical poisons so if the fungus doesn’t do the job we are planning to trial another product used by organic farmers. It contains bacteria (microbial spores of metarhizium and beauveria) which are feed to the vines through the irrigation system in winter, killing the snail eggs in the ground.
If all else fails perhaps we could start a trend for snail massage…any takers?!
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