Snails and slugs are a constant problem for outdoor and greenhouse growers of orchids. They love the open medium of bark used for orchids and can cause great damage to new growths of both roots and plant tops. Most of the time the first sign that there is trouble will be an almost ruined plant, they will invade that fast. It is important to get rid of these pests before bringing in plants for the winter.
The first suggestion, as always, is that prevention is the best solution to the problem. There are as many ideas on preventing the critters as there are people who battle them. One ex-military expert suggests a “perimeter defense, local defense and all out war” approach. Others advocate diatomaceous earth (DE), others various slug and snail pellets. My personal prevention regimen involves a monthly application of approximately a tablespoon DE to each pot; scattering a snail and insecticide inside on the floor of the greenhouse and outside the perimeter; and using the “beer in a container” catch approach. I personally can’t stand having to mess with the dead pests in the beer approach, but periodically I will do so just to get those that have escaped the other ways. Please note, never put slug or snail bait in your orchid pots, always put it around them on the ground.
Other recommended approaches are to use a .5 to 1 inch (3 centimeter) strip of copper around the bottom of all benches or approaches to the orchid location. This can also be used around the bottom of pots. The theory is that the snails and slugs will not cross the copper because it gives them a very slight shock which they dislike.
A recent test confirmed that spraying with a 1% caffeine solution (strong brewed coffee has about .5% caffeine) will kill 65-90% of snails present in potting medium without harming orchids. This can be done by spraying or by pour-through. Caffeine can also be purchased in purified form for agricultural use and mixed with water for this purpose.
In almost all instances a combined approach always works better than relying on just one preventative or poison. There will be some of the pests that will get past your defenses, but few will make it past two or three.
Snail and slug poisons vary by state, so you should consult with your local nursery to determine what is available in your area. As noted above, never put any of these products in your pots — many of the products can be toxic to orchids. Most of the products are granular and will attract the pests from the medium where they hide during the day. Be sure that you read all directions in relation to children and pets. If you chose to use the DE method, be sure to use a mask when applying so that you do not inhale any of the product. Beware of dogs and children who might inhale also.