How to Manage Pests
UC Pest Management Guidelines
Floriculture and Ornamental Nurseries
Snails and Slugs
Both snails and slugs are members of the mollusk phylum and are similar in structure and biology, except slugs lack the snail's external spiral shell. Slugs and snails constantly secrete mucus, which later dries to form the silvery "slime trail" that indicates the presence of these pests. Adult brown garden snails lay about 80 spherical, pearly white eggs at a time into a hole in the topsoil. The eggs are initially white, but turn brown as they develop. They may be mistaken for slow-release fertilizer. Eggs may be laid up to six times a year. Brown garden snails mature in about two years, whereas slugs reach maturity in about a year.
Snails and slugs are most active at night and on cloudy or foggy days. On sunny days, they hide in places that are out of the heat and sun. Often the only clues to their presence are their silvery trails and plant damage. In mild winter areas, such as southern California and coastal locations, young snails and slugs are active throughout the year. During cold weather, snails and slugs hibernate in the topsoil. During hot, dry periods, snails seal themselves off with a parchment-like membrane and often attach themselves to tree trunks, fences, or walls.
Snails and slugs feed on a variety of living plants as well as on decaying plant matter and algae. On plants, they chew irregular holes with smooth edges in leaves and can clip succulent plant parts. They can also chew fruit, flowers, and young plant bark. Because they prefer succulent foliage, they are primarily pests of seedlings, herbaceous plants, and ripening fruits such as strawberries, lettuce, artichokes, and tomatoes, that are close to the ground. However, they will also feed on foliage and fruit of some trees.
Snails and slugs can be trapped under boards positioned throughout the nursery. Traps can be made from 12" x 15" boards (or any other convenient size) that are raised off the ground by 1-inch runners. The runners make it easy for the pests to crawl underneath. After counting, scrape off snails and slugs daily and destroy them. Amber snails are more difficult to detect as they tend to live on the surface of the planting medium. Look for these snails on pot surfaces and where wet (planting medium) and dry areas (container) meet, generally around the inner lip or head space of the container.
A good snail and slug management program relies on a combination of methods.
Handpicking can be very effective if done thoroughly on a regular basis. Snails and slugs can be trapped under boards positioned throughout the nursery and destroyed as described above. Crushing is the most common method of destruction. Do not use salt to destroy snails and slugs; it will increase soil salinity.
Several types of barriers will keep snails and slugs out of planting beds. The easiest to maintain are those made with copper flashing and screens. Vertical copper screens can be erected around planting beds. The screen should be 6 inches tall and buried several inches below the soil to prevent slugs from crawling beneath the soil. Copper screens can also be placed between the bench support legs and the bench to prevent snails and slugs under the bench from reaching the top where the pots are located. Copper foil or netting (for example, Snail-Barr) can be wrapped around planting boxes and bench legs to repel snails for several months or longer, depending on the integrity of the copper strips. Copper barriers are thought to be effective through the reaction between the copper and the slime that snails and slugs secrete, causing a flow of electricity.
Instead of copper bands, Bordeaux mixture (a copper sulfate and hydrated lime mixture) can be brushed or sprayed on trunks or bench legs to repel snails. One treatment should last about a year. Adding a commercial spreader may increase the persistence of Bordeaux mixture through two seasons.
Snails and slugs have many natural enemies, including ground beetles, pathogens, snakes, toads, turtles, and birds, but they are rarely effective enough to provide satisfactory control in the nursery. In California, a predaceous snail, the decollate snail (Rumina decollata), is available for use in Fresno, Imperial, Kern, Los Angeles, Madera, Orange, Riverside, Santa Barbara, San Bernardino, San Diego, Ventura, and Tulare county. However, its release for snail control in commercial nurseries is not recommended since all plants must be snail-free (including beneficial and pestiferous species) prior to shipping. Additionally, it feeds only on small snails, not full-sized ones. Because of the potential impact of the decollate snail on certain endangered mollusk species, it cannot be released outside of the above mentioned counties. Decollate snails may feed on seedlings, small plants, and flowers. When shipping, all decollate snails must be removed.
Commercial snail and slug baits can be effective when used properly in conjunction with a cultural management program. (Note: Baits will kill decollate snails if they are present.). The active ingredient of most baits is either metaldehyde, iron phosphate, or sodium ferric EDTA. Growers have reported that under high rainfall or high humidity conditions, iron phosphate appears to be more effective. Placement of the bait in a commercial bait trap reduces hazards to non-target animals such as dogs and can protect baits from moisture, but may also reduce their effectiveness and attractiveness to snails and slugs and is not recommended for commercial nurseries. Bait should be scattered throughout the beds and near areas where snails hide to be most effective. The timing of any baiting is critical; baiting is less effective during very hot, very dry, or cold times of the year, because snails and slugs are less active during these periods. As snails and slugs are more likely to travel when soil is moist, it may be beneficial to irrigate before applying the bait to promote snail activity.
Methiocarb (Mesurol) is a sprayable molluscicide. It is a "Danger" labeled pesticide and can only be used two times per crop per year. This product is very effective and fast acting, but due to its high toxicity and label restrictions should only be used as a "rescue" treatment.
|Common name||Amount per acre||R.E.I.‡||P.H.I.‡|
|0.5:0.5:100||1 lb||See label||See label|
|MODE OF ACTION GROUP NUMBER1: Multi-site contact (M1)|
|COMMENTS: Apply as a dilute spray only. Do not use with antagonistic microorganisms. Check with your certifier to determine which copper products are organically acceptable if you are mixing your own formulation. For information on making a Bordeaux mixture see UC IPM Pest Note: Bordeaux Mixture.|
|(Durham 3.5, 7.5)||Label rates||12||0|
|COMMENTS: The bait has minimal impact on other organisms in the field. Reapply as bait is consumed; minimum re-treatment interval is 21 days and a maximum of 6 treatments per season is allowed. Monitor to determine where the product should be broadcast.|
|(Mesurol 75W)||2 lb/100 gal||24||0|
|MODE OF ACTION GROUP NUMBER1: carbamate (1A)|
|COMMENTS: Danger signal word. Can only be applied twice in a cropping cycle. Should only be used as a rescue treatment.|
|COMMENTS: Reapply as bait is consumed or at least every 2 weeks.|
|E.||SODIUM FERRIC EDTA|
|COMMENTS: For terrestrial uses. Do not apply directly to water or to areas where surface water is present or to intertidal areas below the mean high water mark. Do not contaminate water when disposing of equipment wash water or rinse water. Apply near pots rather than broadcasting the material. May cause damage to succulent leaves.|
|‡||Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|1||Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.|
|#||Acceptable for organically grown produce.|
|*||Permit required from county agricultural commissioner for purchase or use.|
UC IPM Pest Management Guidelines:
Floriculture and Ornamental Nurseries
UC ANR Publication 3392