Evidence of slugs include slime trails and thin, long droppings. Leaf damage is typified by removal of plant tissue between veins. On roots and tubers, feeding slugs leave shallow, smooth-sided pits that are usually less than 0.5” in diameter. Their damage is normally shaped like irregular holes with smooth edges.
Slugs are molluscs, with only vestigial, internal shells. They are not well-adapted to life on land, surviving only where water is fairly abundant. Although slugs can dig through the soil, usually just seek existing spaces. By day slugs shelter in the soil, in crevices and cracks, or under soil surface debris where it is moist.
Slugs tend to be active mostly at night when it is cooler, but can be active by day when it is wet and cool. They shelter during heavy wind and rain, and are inactive when temperatures drop below 38°F or rise above 88°F. During the summer months most slug populations appear to be low, but adults emerge following heavy rains, suggesting that they disperse deeper into the soil in response to dry conditions.
Slugs are hermaphrodites (each has both male and female sexual organs) but self-fertilization is rare, and they normally pair for fertilization. Slugs reach maturity in 3-6 months, but will continue to grow in size. Most species live one season or less, and adults may deposit eggs throughout the season.
They lay clear to creamy, oval to round eggs in clusters of 3 to 40. Eggs measure 2.0–3.0 mm and are deposited in small cavities in the soil. Eggs hatch in about 2 weeks, but overwinter if they are laid in late fall. The greatest egg-laying activity usually occurs after fall rains and again in the spring.
Although it seems like they will eat anything, they do favor the plants in vegetable gardens and fruit, like strawberries, that are close to the soil. Slugs eat plant tissue, decaying organic matter and sometimes even animal tissue.
Late September to mid-October are usually good months to control slugs. In late fall and early spring, the new juvenile slugs are difficult to spot but can cause significant damage.
Control
Natural Enemies include Tetanocera spp. (marsh flies) , Carabidae spp. (ground beetles), Staphylinidae spp. (rove beetles), Lampyridae spp. (larval fireflies), millipedes, salamanders, newts, robins, sparrows, toads and garter snakes.
The best time for long-term, effective control is to treat the whole garden in the autumn, by removing both slugs and their eggs.
While mulch is beneficial to plants and the soil, it is also a good hiding place. Common advice is to remove debris, leaf litter, and other excess vegetation, thereby reducing slug habitat. However, this practice undermines the overall health of garden ecosystems, by reducing biodiversity. Many species of pollinators and beneficial insects, amphibians and reptiles also depend on leaf litter, etc. for food shelter.
Slugs come out to feast at dusk; watering gardens in the evening makes them even more inviting to slugs. Consider watering in the morning instead, when they need to go back into hiding.
Hand picking is tedious, but effective. Dusk or early morning are the best times. Placing boards or grapefruit skins (as little domes with entrances) on the soil surface, to draw them overnight, may help you catch more with less effort. Check traps and destroy pests every morning until numbers drop, then check weekly. Drop the slugs in a container of soapy water. Wear gloves or use tweezers to avoid slime. Drop the slugs and snails in a bucket of soapy water. Vinegar and warm water washes slime residue off more easily.
Barriers
Barriers slow the slugs down, but do not kill them. With barriers, you would need to create a ridge 1” high and 3” wide to effectively keep slugs out.
Cedar granules are by-products from the manufacture of home aromatic cedar products. They are made from a different type of cedar and are more effective than the cedar mulch sold at most garden centers. Aromatic cedar granules remain effective much longer than other barrier materials like lime, ashes or diatomaceous earth, which wash out with rain and watering.
Salt-impregnated plastic strips and copper strips provide a small-scale barrier that can work for a few days to a few weeks. Strips need to be wide enough to prevent slugs from raising their bodies over it. Copper stripping sold for this purpose is often not wide enough to create an effective barrier. Underground slug movement and environmental degradation (copper oxidizes, salt washes away) can also reduce efficacy.
Baits and Poisons/Molluscicides:
Bait applied in the fall will kill adult slugs before they lay eggs. Apply bait after the first rain showers of the season. This is the time when slugs come to the surface after hiding deep in the soil to avoid high temperatures and dry conditions. Of the eggs that are laid in the fall, some will hatch in 2 to 4 weeks; the rest will hatch during winter or early spring. A second bait application in late winter or early spring will keep the adult population down. A third application a month later ensures minimal slug damage during the upcoming growing season.
Most gardeners have heard of beer traps. Pour about an inch of fresh or leftover beer in a container that is a few inches deep. Bury it in the soil, with the rim level with the soil surface. Slugs will go in for the beer, not manage to crawl back up, and drown. Check traps daily, in case you need to rescue predators that have gone in for the slugs.
Non-drinkers might try preparing a honey and yeast mixture by mixing water with some honey and yeast; proportions are not critical.
Slug baits (possibly even safe/organic baits) can be harmful to humans, pets, and other wildlife. It is important to follow all label instructions. Metaldehyde, methiocarb, carbaryl, iron phosphate and sodium ferric (iron) EDTA are common active ingredients used to control slugs. Metaldehyde is generally more toxic but does not harm earthworms. Metaldehyde is degraded by moisture and UV light, so does not last more than 2 or 3 days after application. Iron phosphate remains active for up to 2 weeks, even with repeat wettings.
Place bait stations during evenings when the garden is cool and wet. They will not risk coming out of hiding in warm, dry conditions. Effectiveness of baits may be increased by placing bait pellets under wooden boards. This also enables you to easily find and remove the killed slugs, in order to remove them from the wildlife food chain.
To save seedlings, apply barriers or baits just before seedlings are expected to emerge. Seedlings are generally kept moist, and planting happens most often in spring when temperatures are still cool.
Common Species
Deroceras reticulatum (gray garden slug), is the most common native slug and widely found in the United States. It is dull gray with black spots, and 1-2” long. In addition to plant material, Deroceras reticulatum also feed on mites and earthworms.
Deroceras leave (Marsh/Meadow/Brown slug) is native to North America and is often found in open areas like fields, back yards or developed lands. They are approximately 1” long at maturity, with color ranging from dark brown to black. In spring, Deroceras laeve become more active a few weeks before other species. They reproduce year-round, by self-fertilization.
Limax maximus (Spotted Garden slug), occurs in the northern states, south to Virginia and California. They are grayish with black spots or faint black longitudinal bands on its back and reach almost 4” in length. The lifespan for Limax maximus can reach 3 years, whereas most slugs live for a year or less.
Deroceras invadens from southern Europe has spread widely and rapidly. However, it has been overlooked due to its similarity to D. leave. It is highly likely that some of the damage to gardens attributed to D. laeve or even D. reticulatum is due to D. invadens.
Tandonia budapestensis, a subterranean slug, is a recent introduction to North America from Europe. It reproduces slowly, requiring 2 years for a single generation.
Please keep in mind that slugs are part of the natural biodiversity in your garden ecosystem. Their absence may negatively impact the wellbeing of other wildlife. Perhaps take action only when you notice an imbalance/infestation.