Slugs are some of the most common and persistent pests of our home gardens here in Northwest Washington, and if left unchecked they can cause significant damage. Slugs are closely related to snails but have no external shell. They are active above ground by day or night, whenever the relative humidity in their immediate environment approaches 100 percent, the temperature rises above 38°F, and the wind speed is negligible.
By day, slugs are usually found in the soil, in crevices and cracks, or under soil surface debris where it is moist. Thus, slugs tend to be active primarily at night, but they also feed and reproduce by day during rainy spells, foggy periods, or after irrigation. Even in the summer, when air temperatures peak in the Pacific Northwest and soils seem dry on the surface, slugs can be active at night. This is because as night temperatures drop, the humidity of the air between the canopy and the soil often increases, if only for a few hours, even in non-irrigated settings. This “extra time” for feeding and reproduction can eventually lead to very large slug populations.
Slugs are relatively inactive when immediate temperatures drop below 38°F or rise above 88°F. They take cover during windy periods and driving rain.
Slug damage can be distinguished from that of cutworms and other pests by the presence of slime trails. Leaf damage is typified by removal of plant tissue between veins. Seedling grasses and legumes may disappear when slugs feed in the furrow and destroy the growing points of seedlings. In your vegetable garden, slugs favor newly planted seedlings.
In addition to plant damage, verifying that slugs are present and in damaging numbers in a home garden is usually done by putting out slug bait in late afternoon and returning early the next morning to check for slugs or slime. Put out half a dozen bait stations in your yard. Scrape a small area (12 x 6 inches) of the soil surface free of vegetation and debris (making it easier to see small slugs), and scatter bait inside.
You can cover the areas with a scrap of wood or even an old newspaper or magazine. This prevents other creatures from disturbing the bait, and the cover helps to keep slugs sickened by the bait from moving away. Place bait stations after the first couple of inches of rainfall when slugs become active on the soil surface.
Slug baits are poisons and therefore can be dangerous to children, pets, wildlife and edible crops. It is important to use baits properly, follow all label instructions and heed all label warnings. Contact our local WSU Whatcom County Extension office for a list of slug bait specifically for our region.
Pellet baits have been the most commonly used product for homeowners.
Various materials, such as salt-impregnated plastic strips and copper strips, provide a small-scale barrier that can work for a few days to a few weeks in keeping slugs away from plants. These have been used with varying degrees of success. For example, underground slug movement or environmental degradation of the repellent (e.g., copper oxidizes, salt washes away) negatively impacts efficacy.
Slug populations can be reduced by tillage. Typically, when the amount of minimal/zero tillage is increased, so are slug levels. Rototiller tines crush and bury slugs, disrupt their pathways, dry soil, and remove volunteer-plant food for slugs. Control is proportional to tillage frequency, depth, and efficiency.
Many birds, such as starlings, blackbirds, and killdeer, feed on slugs throughout the fall and winter months. Predatory ground beetles feed on slugs.