Keeping Slugs and Snails Out of Grapes

The damp, cool conditions we have experienced so far this season have been ideal for the development of slug and snail populations. These molluscs need moist soil in which to lay their small circular eggs - and they've had plenty of opportunity to lay eggs this year. Slugs and snails may cause some minimal damage from leaf feeding in grapes, but the real issue is with snails that may be in the canopy when grapes are harvested. Mature snails are often close enough in size and weight to grapes, making them difficult to separate at harvest. It's easy to tell if there are snails and slugs in your vineyard; the glistening slime trails they leave behind are easily visible on soil in the morning.

As the weather gets hotter and drier this summer, slug populations may level off but don't ignore snails later in the year. When it is hot and dry, snails will find protected spots, pull themselves into their shells, secrete waterproof barriers at their shell entrances, and wait to become active when conditions are cool and moist again. This summer habit is equivalent to hibernation in the winter and is called "aestivation".

Since snails and slugs like moist, cool environments, one method of reducing their numbers in vineyards is merely to keep headlands plowed. These strips will dry out faster than weedy or grass areas and the molluscs will be less eager to cross them in search of food. In 2002, a good product for slug and snail control was registered in Ontario. "Sluggo" is a degradable bait containing Ferric (iron) phosphate which is toxic to snails and slugs. As far as product safety goes, Sluggo is a vast improvement over older baits because it is not harmful to pets, birds or wildlife. Application of Sluggo is easy as well - it should be spread at a rate of 25 to 50 kg per hectare (for smaller areas or strips, that's equivalent to 2.5 g to 5 g per square meter). Do not put out Sluggo in piles - spread it as directed on the label for best results. Reapplication may be necessary after two weeks if all the Sluggo bait is consumed or if slugs and snails continue to be a problem.

Figure 1 Brown Garden Snail

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