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:
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:
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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