The common name ‘snail’ is used for most members of the molluscan class Gastropoda, which have a coiled shell that is large enough for the animal to retract completely into. Both snails and slugs are included in the phylum Mollusca and are also referred to as molluscs. There are more than 1,000 species of land snails native to Australia which are not considered to be a problem in agriculture.
Snail biology is very different to that of insects. snails survive as juveniles and adults by slowing their metabolism, which is a form of torpor, so not all individual snails within a population will be actively feeding, mating or breeding at the same time. Thus, to manage snails in vineyards, action may be needed at multiple times throughout the year, with timing of methods dependent on the species present.
There are four types of snail that may need to be managed in Australian vineyards:
[photos: Michael A Nash, University of SA]
Managing snails
We do not recommend the use of snail baits as these are know to kill off-target species the astute farmer will want to keep to help prevent imbalances and to help prevent additional pest pressure on the vineyard.
Possible methods to manage snails include:
Cultural control methods
Cultural control provides an important component of an integrated approach to snail control. Cultivation between vine rows will limit moist refuges and food resources, as will the removal of broadleaf weeds. Thus, removal of food resources at ground level over the winter will help limit populations.
However, it is important to keep bare soil covered and a mix of native grasses including Rytidosperma spp., wallaby grasses and forbs can provide multiple soil health and ecosystem services benefits without providing favourable habitat for vineyard snail pest species.
Sheep
Sheep grazing in the vineyard over the winter is an important management tactic for some species of snails.The grazing alters microclimatic conditions between vines, which reduces snail activity and egg laying over winter. The preferential grazing of brassica weeds will also reduce favourable food for snails.
In grazed vineyard blocks, numbers of Italian snails are consistently half those of blocks not grazed.
Rolling
Physical crushing of snails with modern heavy metal rollers as used in broadacre to crush rocks has been shown to reduce numbers by up to 80%, especially in late summer when snails have climbed up stubble.
This management strategy relies on snails dehydrating in combination with cracking shells, hence environmental conditions must be favourable; that is > 35 °C and < 50% relative humidity.
In vineyards, rolling will not achieve the same results as broadacre due to snails being in vine canopies, so rolling may not occur when conditions are optimal, and for mortality to occur rolling will be more reliant on actual crushing of individuals when they are active (spring) when then are less likely to dehydrate as conditions are still moist.
Because rolling should occur when snails are inactive, hence not breeding, eggs are not released when snails are crushed.
Snail roller used at Guichen Bay and Wrights Bay Vineyards, Mount Benson, SA [Photo: Dean Barker]
Plant management
Growing of plants that accumulate nitrates and calcium (brassica weeds such as wild mustard) favour snail breeding. Growing of cover crops that limit brassica plants will limit snail food resources, as will cover crop species that limit mollusc growth, such as linseed and chickpeas.
Biocontrol
Encouragement of generalist predators, such as flatworms and nematodes within the vineyards will help limit snail populations. Native species of nematode have been found to burrow into snails and release bacteria with mortality >90 % observed in laboratory trials within 10 days of the nematodes being placed with snails.
Two parasitoid fly natural enemies with some potential to limit snail populations are being used in other agricultural systems and are discussed further below.
Ducks, chickens and geese
Snails are preyed upon by variety birds such as starlings, crows and all forms of poultry. Ducks have been suggested for the control of aquatic Pomacea canaliculate , golden apple snail in Asian rice paddies stocked at a rate of 5 to10 per ha, with a cross between Khaki Campbell and Mallard showing the greatest preference for feeding on snails reducing densities from 5 to less than 1 m².
Ducks feeding on snails under vines is well suited as a biological control in organic viticulture. A suggested stocking rate would be 2 to 5 per ha.
Indian runner ducks are a breed of Anas platyrhynchos domesticus, the domestic duck and are commonly used in vineyards to contribute to biocontrol of snails and other molluscs.
Indian runner ducks in the Angove Vineyard McLaren Vale.
Lizards
Anecdotal evidence suggests shingleback lizards enjoy eating garden snails and Italian snails on the edge of crops. Providing habitat for lizards is another tool to reduce snail numbers.
Parasitoids
Family Sciomyzidae (commonly known as marsh flies) have been released as a classical biological control agents in Australia. Sarcophaga villeneuveana, a flesh fly has been used due to its host specificity on pest snails, in particular Cochlicella acuta, the pointed snail, but has also been found to parasitise C. barbara now known as Prietocella barbara, a sister species of European small snail with rates up to 25% in areas near flowering vegetation. These have been released in broadacre areas of SA and WA, but not specifically into vineyards.
Summary
Managers need to shift their thinking from ‘how do I control’ snails in vineyards to thinking about how to limit snail impacts on grape quality. Maintaining a healthy vineyard environment that provides natural biocontrol services is key.
The integration of cover crops between vines, provision of resources for a stable invertebrate community, including natural enemies and biocontrol agents, as well as understanding environmental conditions favourable for snail breeding will lead to snails not causing economic impact, even though some, including may native snails, will be present within the vineyard.
