Controlling slugs and snails in the greenhouse

Slugs and snails are unlikely to make it onto the top of the pest priority list for most greenhouse growers, especially when compared to major pests such as thrips, whitefly, aphids and mites. However, they remain as one of those irritating, minor pests that most growers at some point have had to deal with, not only for the damage they can cause, but also because they can be so difficult to get rid of.

These pests, collectively known as gastropods (gastro = stomach; pod = foot), have their mouth located on the top end of the foot; hence the name gastropod. They are nocturnal and as such, seldom seen. Their night time activities however, can be easily seen in the light of day, by the holes they chew in leaves and the slime trails that decorate soil and plant surfaces. Actually, they don't chew holes in leaves; their mouth is more like a file with hundreds of tiny teeth that rasps a hole as it is eating.

Slugs like it cool and damp, so slug problems are most likely to be found where there is poor drainage or in high moisture areas such as rooting benches. The damage they cause is most severe on young seedlings which are often completely destroyed. Larger crops can suffer damage to leaves and flowers, and the unsightly slime trails on plants can devalue the crop.

Chemical control options are limited to two different types of baits; one using the active ingredient metaldehyde (Slug-Em or Deadline M-Ps) and the other using iron phosphate (Sluggo). Both of these have their limitations:

  • Metaldehyde is a specific molluscicide and does not affect beneficial insects or natural enemies. The baits should be applied evenly and thinly across the soil surface, not in piles which may attract animals and will not give adequate protection. Slugs when poisoned by metaldehyde cease feeding immediately and produce large amounts of mucous, leading to dehydration and death. Metaldehyde is toxic to animals, so care needs to be taken to ensure that household pets do not have access to baited areas especially during application when they may think they are being fed. The bait pellets become ineffective when mouldy, so reapplication may be necessary. In recent years many commercial products have increased in quality and do not degrade as quickly.
  • Sluggo has the major advantage of being much less toxic to animals. It works as a stomach poison and is slow acting so the slug often crawls away and dies unseen. Baits are usually spread at a much higher rate than metaldehyde baits and can therefore be more expensive. At very high slug and snail pressures there is a question of efficacy in controlling damage with these baits.

It is very important to identify the species of snail or slug which is causing the damage. Not all slugs are the same in their behaviour and feeding habits. Some slugs live and feed underground while others don`t feed at all on green leaves preferring the mould that grows on the soil, but they can still leave nasty slime trails and transmit plant diseases. Once the pest species is known, the most appropriate control method can be set out. Some pest species are easily identifiable, such as the grey garden slug (Deroceras reticulatum) which are greyish and produce a milky slime. Look at the colour of slime produced by slugs after running a finger over them several times. These are by far the most common species found in Canada and are usually controlled by baiting. However other species such as the large dark slugs are sometime difficult to identify and some expert identification may be needed. In the case of greenhouse slugs most pests fall into the latter category.

There are a number of natural enemies that attack and eat slugs and snails, although none that are available commercially. However, growers who are using IPM and biocontrol for other pests are creating an environment without pesticide residues that will encourage the establishment of these beneficials. Ground dwelling organisms such centipedes and various beetles commonly feed on slugs or their eggs, and a group of predatory flies known as marsh flies make slugs and snails their food of choice.

Apart from chemical control options, there are various cultural and sanitation measures that growers can take to reduce problems caused by slugs and snails:

  • Improving drainage to eliminate wet floors beneath benches, has a number of crop production benefits, not least of which is the removal of conditions ideal for slugs and snails
  • Wooden benches that retain moisture, especially if they are starting to rot, can provide refuges for slugs and snails either in cracks or beneath pots. Plastic or metal benches are less likely to provide a suitable habitat.
  • Legs of benches can be treated with things such as black soap or copper tape which are readily available from many organic farm supply shops and act as a barrier to slugs and snails.
  • Slugs and snails often seek refuge under boards or debris. These should be removed and weeds controlled to limit the suitable habitat. Alternatively, growers could make use of this knowledge, by strategically placing boards on the ground in areas of high slug pressure, checking underneath them daily, and removing any slugs found. The effectiveness of this strategy can be increased by baiting under the boards with a food such as a premix chicken feed (which has been found to be very effective) Make sure they are checked regularly, otherwise they just become another piece of usable habitat.
  • Many home owners are probably familiar with the use of beer traps, and they can be quite effective in small areas. The container should be dug into the soil (in a ground bed crop or under benches) or use a shallow dish on the bench. The beer should be changed every 2-3 days. In small areas of very intensive production (e.g. rooting benches or areas with localized damage), a nighttime visit to handpick the offending animals from plants can prove very effective.
  • Substances such as diatomaceous earth can be used as a barrier over which slugs and snails are reluctant to move. Apply it around sensitive crops or areas.

Slugs and snails are unlikely to be the most damaging of pests for most growers, but their persistence and the difficulty involved in controlling them, can make them a real nuisance. The best advice is to go back to basics - look at the situation where the problems are occurring and eliminate the cool and damp environments preferred by slugs. As with many pest problems, good sanitation and growing practices are half the battle.


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