H.02.01: Life Cycles & Biology of Snails & Slugs?

Snails and slugs are a kind of mollusk and they are related to some of the shellfish. They move by sliding along a mucus or slime trail secreted by their single, muscular foot. They feed mostly on living and decaying dead plant material. They are often the most destructive pests and the most difficult to manage in the home garden. They are particularly attracted to seedlings, leafy foliage and bulbs. The symptoms of their feedings are ragged holes in the foliage, stripped stems, and slime trails. They feed mainly at night, on cloudy or foggy days or after rain.

The require a damp environment and high humidity. They seek out shady hiding places or burrow into the soil in the day time to escape the heat and sun. They are moderately tolerant of mildly icy conditions and adult snails have the capacity to go into extended dormancy if conditions turn dry.

The catalog for Gardens Alive says, "Only 5% of the slug population is above ground at any one time. The other 95% is underground digesting your seedlings, laying eggs, and feeding on roots and seed sprouts".

They are hermaphrodites and while cross-fertilization is common, cases of self-fertilization have been reported. Snails lay up to 100 eggs per batch while slugs lay closer to 50. The eggs can remain viable over extended periods if the conditions are dry and then hatch when conditions turn moist. Adult slugs may live as long as a year while adult snails may live for several seasons. There are hundreds of different species of land snails with 40 to 50 different species of slugs depending on location.

Insert in H.03: Prevention and Treatment of Garden Pests using Cultural Practices

H.03.01: Snails & Slugs; Management using Cultural Practices?

There are a number of cultural practices that are effective in the control of snails and slugs in the garden. In practice, it is the combination of these various cultural practices that is the most effective. Hand-picking + the use of barriers + better plant selection + better garden sanitation is usually the best overall answer.

See "Common-Sense Pest Control" by William Olkowski, Sheila Daar and Helga Olkowski. 1991. Newton, Conn.: Taunton Press (Listed in Appendix B)

The cultural practices include:

eliminate hiding places: Snails and slugs are partial to moist shady areas where they rest and lay their eggs. A good first step in snail and slug maintenance is to reduce or eliminate those resting or birthing sites that are close to the garden. Specifically, boards, stones, debris, weedy areas around tree trunks, leafy branches close to the ground and dense ground covers. Slugs particularly like to hide under flat surfaces.

Dense ground covers can be made less appealing to slugs and snails by thinning. This creates a drier environment as it lets in more sunlight and more moving air.

hand picking: Hand-picking is best done at night when the slugs and snails have left their hiding places and are actively feeding. The best time is two hours after sunset. Use a flashlight, look for slime trails, and put the captured snails and/or slugs in a solution containing 3% to 5% rubbing alcohol or a 1:10 mix of household ammonia and water. Chop sticks, broad-pointed tweezers, or plastic gloves are suggested for the squeamish. Olkowski suggests flushing the catch down the toilet.

use of mulch: There are conflicting views as to whether a mulch of compost or composted manure creates or corrects snail and/or slug problems. Some see compost as providing snails & slugs with their much loved moist environment as well as a good place to hide. Personal experience and others, however, is that the generous use of compost or composted manure reduces snail & slug populations. The speculation is that the additional organic material helps to build an increased population of natural predators. There are a number of ground beetles, rove beetles and other garden creatures that actively feed on the snails and slugs.

Olkowski mentions that some of the city parks in San Francisco have gone to the use of cedar bark chips as a mulch because it reduces snail and slug populations. There are reports that a mulch of pea gravel is effective as well.

use of companion plants: Problems with snails and slugs can be minimized by avoiding the use of their favorite cuisine except as trap crops and by using the plants they don't like as barriers. The following snail and slug FAQs lists a number of plants by snail and slug preference:

tillage: Commercial plots are not usually plagued by snails or slugs because they are far enough removed from sheltered areas and because they are disked regularly. This is confirmation of he basic need for garden sanitation:

  1. Remove trash;
  2. Remove weedy growth;
  3. Remove as much daytime shelter as possible and
  4. Keep a broad, clean and dry section between crops and problem sources.

Insert in H.04: Prevention and Treatment of Garden Pests using Beneficial Organisms:

H.04.01: Snails & Slugs; Management using Beneficial Organisms?

"Pests of the Garden and Small Farm" by Mary Louise Flint (See Appendix B) says, "Snails and slugs have many natural enemies, including ground beetles, pathogens, snakes and birds, but they are rarely effective enough [by themselves] to provide satisfactory control in the garden"`. An other predator worthwile mentioning is the turtle. The more important natural predators include:

ducks, geese & chickens: Olkowski says, "...first-hand experience with ducks convinces us that in certain settings they can contribute to easy, safe snail control. Because ducks eat seedlings, newly planted areas should be protected with chicken wire. Also, if they are kept too long in one spot the ducks mash down low vegetation. Rotation is the answer."

They can be herded from place to place as they tend to stay close to watering pans. They tend to prefer snails over slugs. The problem with using ducks, geese, chickens or other birds as control agents for slugs and snails is that they tend to be opportunistic feeders. While they may eat snails or slugs, they can just as easily feed on something else if they have the mind to do so and the something else can include young, tender foliage.

ground beetles & rove beetles: Beetles tend to be opportunistic and feed on a wide variety of living organisms and decaying organic matter. The larval stage of the ground beetles and, to a lesser extent, the adult stage of the ground beetles more or less feed on slugs and occasional snails depending on what else might be available. Both the larval stage and adult stage of the rove beetles actively feed on snails. A problem, however, is that neither ground beetles nor rove beetles feed on the snail or slug eggs. A fair summary is that neither ground beetles or rove beetles are particularly efficient in controlling snails or slugs.

frogs & toads: Frogs and toads are generally shy creatures that hide in dark, damp places feeding on slugs, grubs spiders, and a variety of insects. The frogs seem the better hunters of garden slugs and it has been estimated that slugs represent about a quarter of their total diet. Frogs and toads are attracted to a garden by ponds and particularly ponds with attached bog gardens but note that as amphibians they are particularly vulnerable to the toxic effects of many pesticides..

Two good references on frogs and toads:

hedgehogs: Hedgehogs feed on a very wide variety of plant and animal materials. They are particularly partial to beetles, caterpillars, earthworms, slugs and snails. They hide in brush during the day and come out at night to hunt for pray. They are considered beneficial to the garden because most of their prey are considered as "pest". They can be attracted to a garden by providing brush as shelter, water, and some extra food. (They love cat food) Hedgehogs are found in Europe, Asia, and Africa.

See London Wildlife Trust's web page on hedgehogs

opossums: Opossums are marsupials that are native to Central America and southeastern United States. They were introduced along the West Coast of the United States in the early 1900's where they have since prospered. They feed on a wide variety of plant and animal materials including snails and slugs. They are generally considered beneficial to the garden because of their predation on snails and slugs; however, they are also predatory of poultry.

Robert Smaus, Gardening Editor for the Los Angeles Times, often speaks of his rapture in listening to the sounds of opossums munching on the snails in his garden. He calls it "music to his ears".

See Nebraska Wildlife's web page on opossums

decollate snails: The decollate snails, Rumina decollata, are natural predators of the brown garden snail. They have been in commercial use in some of the citrus groves in Southern California for several years with great success. They are restricted from use in many areas because they are also predatory of other mollusks as well. They are not generally recommended for use in the home garden because they also feed on seedlings, small plants, and flowers.

nematodes: The beneficial nematode, Phasmarhabditis hermaphrodita, feeds on and kills a variety of snails and slugs. These organisms have been on the market in the UK for several years. There are several brands of this same nematode. The question was posted on several UK newsgroups as to the performance of these organisms in the home garden. The consensus view was that they worked well on slugs, were somewhat less effective for snails, and that applications needed to be repeated about every 4 weeks. They are also a bit pricey but they are half the price they were when first introduced. They are not available, at the present time, outside of the UK.

The use of Methiocarb pellets is considered as the standard control for slugs on high value horticultural crops such as lettuce and strawberries. A number of field tests showed that the nematodes gave equivalent control for slugs as the Methiocarb but without the environmental problems associated with Methiocarb use. The relative long term costs have yet to be determined.

Insert in H.06: Prevention and Treatment of Garden Pests using Traps & Barriers:

H.06.01: Snails & Slugs; Management using Traps & Barriers?

Traps and barriers should be used as supplements to the cultural practices mentioned in Section H.03.01. Counting on the use of traps and barrier as the only control measures is probably not realistic. One of the major problems with barriers is that while they may do a good job of keeping outside slugs and snail from getting into the planting beds, they also do a good job of keeping any snails and slugs that may be present in the planting beds very much in place. The better traps and barriers are:

baited pit traps: The most common slug trap is no more than an empty pet food can or yogurt cup filled with enough beer to drown slugs should they fall in. The traps are spaced throughout the garden at a density of about one trap for every 10 square feet or 10 square meters of garden. The slugs are apparently attracted to the yeast and once in they can't get out. The containers are best partially buried so the top edge is an inch (2-3 cm) or so above grade to keep the ground beetles or the rove beetles from falling in as well. The trap needs to be dumped daily.

Pest Management Guidelines from the University of California at Davis says, that beer-baited traps "attract slugs and snails within an area of only a few feet, must be refilled every few days to keep the level deep enough to drown the mollusks, and are more effective if the beer is fresh rather than flat".

There are two additional problems with the open containers. One, dogs and other large animals will often drink the beer and two, the traps flood out in the rain or when exposed to overhead watering. Gardens Alive (gardener@gardens-alive.com) sell a green plastic slug trap that has a weather tight roof and enough bait to last a month. It is said to be effective but when used at the rate of one per every 10 square feet or 10 square meters, they become expensive.

Garden writer Gene Logsdon uses half-gallon sized coffee cans. He cuts a rectangular hole, just large enough to admit a slug, about a third of the way up the side of the can. The can is then buried to a depth of the opening, beer is added, and he uses the plastic cap to prevent the larger animals from getting at the bait.

collecting stations: Snails and slugs can be trapped under boards or flower pots positioned throughout the garden. They attach themselves to the bottom surface, the traps are inverted daily, the prey are scraped off into a plastic bag, sealed and thrown away or scraped into a bucket containing a 1:10 mix of household ammonia and water.

An inverted grapefruit rind also makes a good collecting station. Snails and slugs do like citrus and the rind along with the catch are sealed in a plastic bag and put in the trash.

textual barriers: Snails and slugs have a difficult time crawling on certain surfaces. Olkowski says that he uses a 3 to 6 inch layer (7.6 cm - 15 cm) of fine hardwood sawdust on the paths around their garden beds and that it is an effective barrier. Many also use crushed egg shells and claim good results. Maybe the slime simply gets caught up in the loose particles.

desiccating barriers: Testing by researchers at the University of California showed that bands of fireplace ash or diatomaceous earth provided an effective barrier against snails and slugs but only IF the material is dry. Also, the barrier band should be at least an inch (2-3 cm) in height and three inches (8 cm) in width. ( The slugs and snails passed over wet material without significant hesitation.) The speculation is that snails and slugs are reluctant to cross over any material that is drying. "Slug ban", a new product marketed in the UK as a slug barrier is a kind of clay that is extremely drying. The problem is that snails and slugs are usually only a pest when conditions are wet.

Gardens Alive (e-mail) market a snail and slug barrier called "Slug-Out" that is based on coconut oil soaps. They claim that it is not toxic to snails or slugs, bio-degradable and has a service life of about three weeks. This writer has no direct experience with this product.

toxic barriers: Copper is an effective barrier against snails and slugs. Apparently, the copper reacts with the slime that the snails and the slugs secrete causing a toxic reaction - similar to an electric shock According to the University of California, vertical copper barriers work better than horizontal. The vertical barriers should be buried to a depth of 3" (8 cm) to prevent slugs from crawling beneath grade. Going to a height of 6" (15cm) above grade is considered realistic.

The problem with the use of copper is that it is so very expensive. The wholesale price of "thin" (21 mil) sheet copper is about $6.00 per square foot. Copper screen or copper foil is available for less money but it is more difficult to use and has a shorter service life.

physical barriers: A clear, 2 or 3 liter plastic bottle makes a good physical barrier for seedlings. Cut off the bottom, place the bottle over the seedling and push the bottle down into the soil about 2" (5 cm) to keep the slugs from crawling under. A stiff wire anchor can be constructed to help hold the bottle in place. The bottle acts as a snail and slug proof cloche. Water through the top of the bottle and provide the seedlings with some shade to keep them from cooking. Be sure that there are no snails or slugs locked in with the seedling or they will think the shelter is for them.

Insert in H.07: Prevention and Treatment of Garden Pests using Pesticides:

H.07.01: Snails & Slugs; Management using Pesticides?

Metaldehyde and methiocarb are the active ingredients in most of the formulations designed to kill snails and slugs. They are relatively toxic materials to a wide variety of organisms and they are environmentally destructive. The purpose of this section is to address their characteristics - NOT to recommend their use.

metaldehyde: Metaldehyde is a molluscicide used to attract and kill slugs and snails. It is a synthetic aldehyde and it is prohibited for use on certified organic crops by the OCIA. It is classified by the U.S. Environmental Protection Agency as a Restricted Use Pesticide and must carry the signal word "Caution" or "Warning" along with the statement that, "This pesticide may be fatal to dogs or other pets if eaten. Keep pets out of treated areas". It is of moderate to low acute toxicity to mammals, relatively non- toxic to fish and insects, but significantly toxic to birds and molluscs. It breaks down readily when exposed to sunlight.

The University of California Pest Management Guidelines says, "Metaldehyde does not kill snails and slugs directly; rather, it paralyzes them and causes them to dehydrate. When it is sunny or hot, they die from desiccation. If it is cool and wet, they may recover".

Snail and slug pellets are formulated with materials like apple pumice to make them more attractive to snails and slugs. Mounds of these pellets can also attract pets and children. There are a number of incident reports showing metaldehyde causing the death of children and dogs. Birds feeding on poisoned snails and slugs are also at risk.

See Cornell's Pest Management site

methiocarb: (Meserol) Methiocarb is a contact poison that is toxic to a wide range of organisms. It is toxic to mammals, birds, fish, earthworms, a variety of garden insects including bees as well as being toxic to snails and slugs. It should not be used around fruit or vegetables and it is prohibited from use on certified organic crops by the OCIA. It is a synthetic carbamate that works primarily as a contact poison. It has a LD50 of 100 mg/kg and it is residual with a service life of about three weeks. It bio-degrades slower than metaldehyde.

See Cornell's Pest management site

iron phosphate: Gardens Alive market a snail and slug control product called, "Escar-Go" that they claim is a granular mix of Iron Phosphate and a bait. They claim that it gives as good or better control of snails and slugs than "standard chemical poisons". Iron phosphate is considered as an environmentally friendly material and would probably be allowed for use on certified organic crops.

Gardens Alive claim the product has been field tested but this writer has never used it.

Insert in H.08: Characteristics of Pesticides:

add to H.08.03; LD50 figures of 227 for Metaldehyde and LD50 figure of 100 for Methiocarb