Snails and Slugs

Both snails and slugs are similar in structure and biology, except slugs lack the snail's external spiral shell. Snails and slugs move by gliding along on a muscular "foot." This muscle constantly secretes mucus, which later dries to form the silvery slime trail that signals the presence of these pests. They are most active during the night and early morning when it is damp. In southern California, particularly along the coast, young snails and slugs are active throughout the year.

Adult brown garden snails lay about 80 spherical, pearly white eggs a month in shallow depressions in the topsoil. They may lay eggs up to six times a year.

Slugs and snails are of major concern on perennial artichokes especially in winter. The juveniles and adults feed on all parts of the plant. In heavily infested fields, slug feeding on foliage causes shot holes on the leaf lamina. Slugs are particularly injurious to the buds when they scrape off soft tissues from the artichoke bracts. This injury later turns black, and the quality and marketability of the affected produce is greatly reduced. Mollusks do not pose any threat to annual artichokes throughout California.

Controlling weeds and applying bait are important practices in keeping snail and slug populations under control.

Cutting back artichokes annually helps reduce the slug population. Regions where the artichokes are cut shallower or not cut at all usually sustain higher infestations of both slugs and snails. Keeping weeds under control reduces the plant cover under which the slugs and snails take refuge during the day. Similarly, artichokes planted in heavier soils are more prone to slug and snail damage than in lighter soils. Frequent cultivation and tillage generally keep the infestation under control by killing the slugs and exposing their eggs to natural predators like ground beetles and birds. However, slugs staying very close to the base of the plants or under the crown remain unaffected by this operation, and in due course their populations rebound.

In fields that have yearly problems with these pests, start baiting immediately after the annual cutback of the perennial artichokes in May and June by broadcasting the bait on the ground around the base of the plant. Avoid applying the bait directly to plant foliage or as a heap on the ground near the base of the plant. Continue baiting at 2- to 3-week intervals through November, preferably after irrigating the field. In the past, control with baits has been only marginal, in part because slugs and snails have always been dealt with on a crisis basis. Research indicates that peak egg laying in slugs occurs from late September through early November. Most eggs deposited before late October hatch during fall; those deposited in November hatch from late February through spring. Therefore, slugs are best controlled during October, when they are more mobile on the ground surface in search of food and mate.

Several new formulations of metaldehyde bait have been introduced in recent years. Granular baits and micropellets have given better control, as the individual particles remain intact in the rain and irrigation water. Also, when the sand-based granular formulations are used during the production phase of artichokes, individual granules can get lodged in the flower buds and become a health hazard to the consumers. Slug control with metaldehyde is dependent on many factors, including weather. Metaldehyde rapidly breaks down in irrigation and rainwater. Adverse weather conditions such as the rain and wind keep the slugs, especially the juveniles, inactive. They either do not consume enough bait or they are able to recover from the poisoning.