Slugs and snails on Plants

Symptoms

Slugs chew irregular holes in leaves, and they can clip off succulent plant parts entirely. Seedlings are especially likely to be damaged or destroyed. Flowers, fruit, roots, and tender bark can also be chewed.

Look for shiny to whitish slime trails on the plants and on nearby surfaces. A trail of mucus or slime is left behind wherever a slug moves. You may also find dark to pale feces on the plants.

Slugs are active mostly at night and on cool, cloudy, damp, and foggy days. In mild winter areas of Southern California and coastal locations, they are active throughout the year.

Plants listed as hosts include basil, beans, cabbage, dahlia, delphinium, hosta, lettuce, marigold, strawberry, tomato, and citrus. Check them for holes, clipped growth, slime trails, and feces, especially on seedlings. These signs appear on the plants themselves and on the surfaces around them. Damage to flowers and fruit is worth checking for too, along with chewed roots or bark.

Causes

Slug activity increases in cool, cloudy, damp, or foggy weather. Slugs are active mostly at night and on those cool, damp days. Warm, damp weather in spring and fall favors slug development. Temperatures above 40°F (4°C) let slugs move, feed, and lay eggs.

Humid or consistently moist growing conditions are another cause. Overhead irrigation that keeps surfaces wet belongs on the same list. In mild winter climates, slugs stay active year-round. In mild winter areas of Southern California and coastal locations, snails and slugs are active throughout the year.

Shelter such as dense ground covers, boards, debris, stones, and weedy areas also contributes. These spots give slugs shelter. Loose boards, bricks, stones, trash piles, compost piles, and weeds are also named as hiding and breeding sites. A damp spring bed with dense ground cover and overhead irrigation combines several of these causes at once.

Diagnosis

No cited source names a look-alike for slug damage, so this section uses only the symptoms and causes on this page. Slugs leave shiny to whitish slime trails on plants and nearby surfaces, so a trail points to slugs rather than to a care fault. Dark to pale feces on the plants are a second sign.

The damage is irregular holes chewed in leaves, or succulent plant parts clipped off entirely. Seedlings that are damaged or destroyed, and chewed flowers, fruit, roots, or tender bark, fit the same pattern.

Then compare your site with the causes: cool, cloudy, damp weather, moist conditions, overhead irrigation, and shelter such as boards, stones, and weeds. Slugs are active mostly at night, so you may not see them in daylight even when the signs are there.

Boards or wetted burlap bags set out in the evening act as traps, and slugs found underneath the next morning show that slugs are present. Check the plant against the host list too, since citrus, hosta, lettuce, and strawberry are among the plants slugs damage.

Treatments

Remove hiding and breeding sites such as loose boards, bricks, stones, trash piles, compost piles, and weeds. Cultivate the soil to dry the surface and reduce slug activity. Irrigate early in the day and lengthen the interval between irrigations, so surfaces dry before slugs become active in the evening. Grow plants off the ground on metal benches rather than wood, which stays moist longer. Inspect new plants and propagation stock to make sure they are pest-free.

Copper bands or screens on benches or around plants work because slugs and snails will not cross unfouled, unoxidized copper. Handpick slugs regularly and thoroughly. You can also set out boards or wetted burlap bags in the evening as traps, then collect and destroy the slugs found underneath the next morning. A shallow tray sunk level with the soil and filled with beer attracts and drowns slugs. Collected slugs can be dropped into soapy water or a diluted ammonia solution to kill them.

Two molluscicides are described as organically acceptable. Iron phosphate bait can be more effective and persistent than metaldehyde during prolonged wet conditions. Bordeaux mixture or copper sulfate can be brushed or sprayed on trunks or bench legs to repel snails and slugs.

A metaldehyde bait is reapplied at intervals while snails and slugs remain active. Methiocarb bait is a restricted-use rescue treatment, limited to two applications per cropping cycle. A sodium ferric EDTA bait is spot-applied near pots rather than broadcast. The product label is the legal instruction and must be followed for the crop and site being treated, and this page gives no rates, dilutions, or spray intervals.

Prevention

Keep surfaces dry by irrigating early in the day and leaving longer intervals between irrigations. Overhead irrigation that keeps surfaces wet is a listed cause of slug trouble, so water in a way that lets surfaces dry. Cultivating the soil dries the surface and reduces slug activity.

Clear away loose boards, bricks, stones, trash piles, compost piles, and weeds, which act as hiding and breeding sites. Dense ground covers, debris, and weedy areas are also listed as shelter. Grow plants off the ground on metal benches rather than wood, which stays moist longer.

Inspect new plants and propagation stock before they join your collection, to make sure they are pest-free. You cannot change the weather, but you can change the conditions around your plants.

Plants most often affected

The listed hosts include lettuce, cabbage, beans, tomato, hosta, dahlia, delphinium, and marigold. Basil, strawberry, and citrus are also on the list. Citrus are especially susceptible to damage. If you grow any of these, check them first when you see chewed leaves or slime trails.

• American Century Plant
• Autumn Crocus
• Aztec Marigold
• Bear's Breech
• Belladonna
• Cardinalflower
• Chameleon
• Common Asparagus Fern
• Common Comfrey
• Creeping Jenny
• Kitchen Sage
• Little Hogweed
• Nightblooming Cactus
• Pot Marigold
• Purple Queen
• Purpletop Vervain
• Rose Campion
• Silver Cock's Comb
• Spinach
• Sweetwilliam
• Wild Celery
• Witch's Moneybags