Slugs should have survived our mild winter without a problem, and our early spring temperatures and rainfall pattern are ideal for them. Slugs use their rasping mouthparts to tear tissue from leaves, stems, and flowers. New transplants and small seedlings are especially vulnerable to these creatures. Feeding damage and silvery slime trails already are apparent on bedding plants (Figures 1 & 2).
Soft-bodied slugs require cool, humid conditions, so they usually feed at night and hide in the soil or under shelter during the day. They can do a lot of damage to plants in shade gardens and in heavily mulched areas. It can be difficult to manage them while leaving the conditions that favor them unchanged.
Here are tactics to reduce slug problems. Employing a combination of several options will be more successful than relying on only one.
- Sanitation—Keep the area free of plant debris (leaves, pulled weeds, etc.) and decorative objects on the ground that can provide cool, moist hiding places.
- Manage moisture—Water in the morning when necessary so that the foliage and soil surface are as try as possible in the evening when slugs are active.
- Promote sunlight—Improve sunlight penetration and air movement as much as practical. Prune or remove low foliage, especially leaves touching the ground. Avoid excess mulch to allow drying.
- Alternate plant selection—Use plants that slugs do not prefer (begonias, geraniums, impatiens, etc.) when possible.
- Partially bury small, shallow containers of beer so they are even with the soil surface. Slugs, which are attracted to the beer, will climb into the container and drown. Change the beer as needed.
- Provide limited shelter to trap slugs. Lay a few objects (overturned flower pots, pieces of cardboard, wood, flat rocks, etc.) flat on the ground. Slugs hiding under them during the day can be collected and destroyed (drop in soapy water). Be sure to remove competing shelter sites.
- Diatomaceous earth, wood ash, lime, sawdust, or copper striping can be used to protect areas that are slug-free.
Common active ingredients are metaldehyde and iron phosphate. Some formulations also contain an insecticide.
Metaldehyde kills slugs and snails by absorption or ingestion. The chemical causes them to overproduce mucous, which causes dehydration and an inability to move. Metaldehyde affects the central nervous system and is toxic to many organisms that ingest it. Some products contain Bitrex (recognized by the Guinness Book of Records as the world’s most bitter substance) as a safety measure to prevent accidental ingestion by humans and non-target organisms.
General slug bait use tips (always follow label directions):
- If the ground is dry, water areas to be treated. Scatter product directly from the container or use a hand-held spreader.
- Do not place granules into piles; scatter them in problem areas (in groundcovers, on lawns, and on soil surfaces around plants).
- Do not apply to foliage or edible parts of plants.
- Sprinkle lightly with water after application, but withhold heavy watering for at least 2 days after application.
Example baits include:
- ORTHO Bug-Geta Granular Snail and Slug Killer (3.25% metaldehyde + Bitrex) kills snails and slugs within 1 to 3 days, and remains effective after rain or sprinkling for up to 3 weeks. It is labeled for use around vegetables, fruit trees, citrus, berries, ornamentals, shrubs, flowers, trees, lawns, gardens, and in greenhouses.
- ORTHO Bug-Geta Plus Granular Snail and Slug Killer (2% metaldehyde + 5% carbaryl + Bitrex) kills snails and slugs, plus ants, cutworms, armyworms, earwigs, sowbugs, crickets, pillbugs, grasshoppers, and millipedes. Do not contaminate edible parts of plants.
- Iron phosphate causes slugs and snails to stop feeding and become less mobile after ingesting the bait; they die within 3 to 6 days. This active ingredient is OMRI-listed.
- SLUGGO Slug and Snail Bait (1% iron phosphate) is labeled for use in or around all food commodities and may be used up to, and including, the day of harvest.
- BUG-N-SLUGGO Insect, Slug and Snail Bait (0.97% iron phosphate + 0.07% spinosad) kills ants, earwigs, cutworms, pillbugs, as well as slugs and snails.
Snails and slugs (University of California-Davis)
By Lee Townsend, Extension Entomologist