Slug Control

Rory Mc Donnell and Nicole Anderson
March 2021

Slugs are some of the most common and persistent pests of home gardens and commercial crops in western Oregon and Washington, and if left unmanaged 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. However, they tend to be active primarily at night, but 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 in closed canopy crops such as legume seed, forage crops, or sugar beets. 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 large slug populations. Slugs are relatively inactive when temperatures drop below 38°F or rise above 88°F. They take cover during windy periods and driving rain. Be aware that supplemental irrigation, post-harvest residue buildup on soil surfaces and crop plant structures (e.g., closed canopy) can affect the microclimate of a crop and promote otherwise unexpected slug activity.

Slug damage can be distinguished from that of cutworms and other pests by the presence of slime trails and their small sausage-shaped feces on damaged plants as well as on the soil surface around damaged plants. Underground feeding on roots and tubers is characterized by shallow (0.12 inch) to deep (0.5 inch), smooth-sided pits that are usually less than 0.5 inch in diameter. 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 cereal crops, slugs favor newly planted seeds. Wheat is most susceptible to slug damage from seeding to plant emergence.

Slug damage to vegetable and cereal crops, grasses and legumes can be extensive around field margins. Weedy, grassy or wooded borders serve as excellent habitat for slugs. Grass seed crops, cereals, or vegetable plantings that immediately follow a perennial legume or pasture are likely to sustain slug damage. Large populations of the gray field slug, and smaller numbers of several less common species (e.g. white-soled slug) build up on most perennial legumes in western Oregon and Washington.

In addition to plant damage, verifying that slugs are present and in damaging numbers in a home garden or a crop is usually achieved by putting out a metaldehyde-based slug bait in late afternoon and returning early the next morning to check for dead slugs. Scrape a small area (12 x 12 inches) of the soil surface free of vegetation and debris (making it easier to see small slugs), and scatter four to six pellets of bait inside. You can cover the areas with a scrap of wood or an old carpet tile. 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 inch of rain fall in September or early October when slugs become active on the soil surface after having spent the summer underground. Late September to mid-October are usually good months to control slugs, however depending on the weather, other control windows may occur to apply “follow-up” bait. After October, or when weather becomes too cold (< 34°F) and rainy, baits are less effective, and slugs tend not to be active or feed above ground. In these cases bait use is not advised. As days shorten, eggs are produced and these can hatch in fall or when the temperatures warm in the spring. In late fall and early spring, the new juvenile slugs are difficult to spot in the field but can cause significant damage to your crop. Slug eggs are also laid during the spring.

Our most economically important species is the gray field slug, also known as the gray garden slug (Deroceras reticulatum). The European black or red slug (Arion rufus), the white-soled slug (Arion circumscriptus), the garden slug (Arion hortensis), the hedgehog slug (Arion intermedius), the dusky slug (Arion subfuscus), the black greenhouse slug (Milax gagates), the marsh slug (Deroceras laeve), and the three banded slug (Ambigolimax valentianus) are also important pests. The vast majority of the economically important species in the Pacific Northwest are invasive from Europe. The native slug, Prophysaon andersoni, can also be a minor pest in certain crops.

Slugs are hermaphrodites: every slug has both male and female reproductive parts and theoretically is capable of laying fertile eggs. Mating occurs primarily in the fall and spring. Small, round, pearl-like, white or translucent eggs are laid in clusters in sheltered cavities near the soil surface or under debris on the soil surface. They typically hatch within 2 weeks to a month. Occasionally, these eggs overwinter if they are laid in late October or November. The greatest egg-laying activity in non-irrigated environments usually occurs after fall rains and again in the spring. The life expectancy for the gray field slug is approximately 1 year, but other slug species may live longer.

Chemical control

Slug baits are poisons and therefore can be dangerous to humans, pets, and other wildlife. It is important to use baits properly, follow all label instructions and heed all label warnings. Metaldehyde (e.g., Durham, Deadline M-Ps, Metarex, Slug-Fest), methiocarb (e.g., Mesurol), iron phosphate (e.g., Sluggo, Sluggo Plus, Natria, Slug Magic, Escar-Go and Worry Free), and sodium ferric EDTA (e.g., IronFist, Ferroxx) are four common active ingredients used to control slugs in the Pacific Northwest. Pellet baits have traditionally been the most commonly used product for homeowners. Unfortunately, even when “good” control is achieved, < 60 percent of the slug population will be removed. This usually suffices for economic crop protection if slug pressure is light, but does allow the population to recover over time.

Under favorable conditions, slugs can significantly damage a seedling crop in just 1 or 2 days. As the crop emerges (or in the case of cereals, as the seed swells with moisture soon after planting), slugs begin feeding. Therefore, application timing, the amount of bait used, bait density (number of pellets per square foot), and bait quality are crucial for successful treatment.

In cereal crops, the greatest risk comes during the first week after planting. Gray field slugs enter the seed furrow and begin to hollow out the endosperm shortly after the seed swells with moisture. One slug can destroy 10 to 15 wheat seeds before seedlings emerge. Depending on slug density, baits may be applied prior to planting, at planting (broadcast or in the furrow), and/or shortly afterward. In broadleaf crops and grasses, slugs do not feed on seeds but instead target seedlings by feeding upon and destroying the tender growing points. The most effective timing for application in these crops is at planting (if slugs are active) or just before seedlings emerge, as this is the most vulnerable plant stage. Preventive treatments are advisable on fields with a long history of slug damage, in no-till situations or in situations where surface residue is retained.

The more effective commercially available baits contain cereal bran or flour as an attractant and are formulated into pellets much smaller than the pencil-eraser-size pellets of the past. These so-called mini pellets, or shorts, are smaller and allow for more pellets per unit area than the larger baits. For instance, some slug bait pellets (e.g., Metarex) are a uniform 2.5 mm long. Look for slug bait in which the pellets are uniform in size, have a high bulk density, are food-based (i.e., smell strongly like cereal to attract slugs from a distance), contain Bitrex to prevent unintentional ingestion by mammals, birds and house pets, and are relatively dust free. The result upon broadcasting these pellets is a very dense and uniform pellet distribution per unit area treated. This is important because slugs tend to encounter these pellets at a greater frequency than the larger, older type. Generally, it is recommended to reapply bait after 10 to 14 days if slug pressure persists, plant damage continues, all bait has been consumed, or the bait has broken down (due to weather). Be sure that the label on the bait product applied will allow for reapplication if needed within this time frame.

The kill rate of a pellet depends on the attractiveness and quality of the carrier, palatability, weather conditions at the time of application, and the toxicant concentration. If the carrier material is not attractive and palatable to the slugs, they may avoid the bait or consume a sublethal dose of toxicant, from which they can recover.

Methaldehyde, iron phosphate, sodium ferric EDTA, and methiocarb

Several chemicals are formulated into slug and snail baits for use on food crops, seed crops, and ornamentals. Metaldehyde has been used since the early 1940s, iron phosphate since 1998, and sodium ferric EDTA since the early 2000s. The most recent generation of products has been developed from metal chelates (e.g. sodium ferric EDTA) incorporated into an ingestible bait. These baits (IronFist and Ferroxx) have been trialed in Oregon and showed positive results in terms of reducing feeding and slug control in grass seed, clover seed, and cereal production.

Baits that contain methiocarb can be effective but they currently have limited labels and are used primarily in nonfood or ornamental crops. For example, Mesurol 75W is used as a spray in nonfood crops and also has activity on certain insect pests as listed on the Gowan label.

Iron phosphate formulations (e.g. Sluggo and Sluggo Plus) are approved for organic production. They are formulated as a uniform and dust-free cereal-based mini pellet. Time to mortality is somewhat slower (5 to 7 days) compared to metaldehyde. The slugs, however, cease feeding after having eaten the iron phosphate bait. Trials with this active ingredient have shown it to be as effective in controlling gray field slugs as metaldehyde, although slightly greater rates of the iron phosphate formulations per unit area are usually needed. Slugs that ingest iron phosphate or iron chelate baits usually die underground or under a source of cover, and not above ground as happens when metaldehyde is consumed.

Metaldehyde is available in various formulations for slug and snail control. These include liquids, sand granules as well as traditional cereal-based baits. Meal formulations (for home use, usually a 2 percent metaldehyde pellet with an insecticide to control other pests) are also available. Liquid metaldehyde and meal formulations may give fast plant protection due to the good coverage, but they do not last more than 2 or 3 days, at best, because UV light and moisture cause metaldehyde to degrade into non-mollusk-killing compounds. Slug-Fest is one such liquid sprayable product and is labeled for use on many food as well as nonfood and ornamental crops. It is often used to control immature slugs prior to canopy closure in establishing a stand.

Large pellets containing metaldehyde need higher application rates for good coverage. They usually provide good control in the first few days, but often degrade quickly and do not persist as long as minipellets. Cereal-based minipellets and very small pellets, (e.g., Metarex) have the best performance record in our rainy climate and can last 1-2 weeks on wet soil.

Research has recently shown that metaldehyde has a different mode of action than previously suggested: it does not dehydrate but rather destroys the mucus-producing system unique to slugs (and snails), which severely reduces their mobility and consequently promotes their dehydration through exposure to the sun. Wet conditions, therefore, do not reverse the toxic effect of metaldehyde, as was once thought. However, if slugs do not consume a lethal dose of metaldehyde, they may recover, particularly during wet weather which reduces the likelihood of dehydrating poisoned slugs. Furthermore, under wet conditions, poor control may follow from low-quality baits and low concentrations of active ingredient in the bait. This is usually because of rapid (2 to 3 days) physical degradation or fungal growth on pellets that reduces slug feeding.

Due to metaldehyde’s specific mode of action, beneficial organisms (earthworms or predatory insects) are not directly affected by baiting with metaldehyde even when these organisms feed on the bait. However, when applying an insecticide, such as carbaryl, to control certain insect pests like cutworms, armyworms, or wireworms, many beetle predators that feed on slugs, along with earthworms and harvestmen (daddy long-legs), may be killed as well. Be aware, too, that metaldehyde baits are a leading cause of accidental poisoning and deaths of dogs in the PNW.

In western Washington and Oregon, slug control is often a year-round necessity in many crops and sites with no-till or conservation tillage practices. Presume damage from slugs in certain crops and fields with a history of problems. Bait early if slug activity is apparent. In some cases, it may be best to bait for slugs before you work the soil (particularly if tillage is shallow and light). Irrigate before baiting in home gardens in order to bring more slugs to the surface during the night. In vegetables, such as brassicas, baiting must be done before the buttons form or canopy closes, because once the slugs have a chance to enter the head, they are less likely to be attracted to the bait.

Fall baiting usually is recommended for non-irrigated crops. Apply bait after the first rain showers of the season, when slugs become surface active after a summer of hiding deep in the soil to avoid high temperatures and dry conditions. Bait applied immediately after the first fall rains can kill large numbers of slugs before they lay eggs. However, spring applications are also necessary in most fields with minimum or no-tillage practices. 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. These newly hatched slugs often do not accept bait as readily as larger slugs. Slugs also lay eggs during the spring in the PNW.

Control is seldom, if ever, complete. Around the home garden, removing debris, leaf litter, and other excess vegetation helps to remove slug habitat and reduce slug numbers.

Alternative control

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 barriers 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, slug numbers increase when the amount of minimal/zero tillage is increased. Plows, discs, and rototillers crush and bury slugs, disrupt their pathways, expose their eggs to desiccating conditions, dry soil, and remove volunteer-plant food for slugs. Control is more or less proportional to tillage frequency, depth, and efficiency. Plowing followed by disking can be sufficiently effective, so that no further control is needed. A fine seedbed will protect seeds and help prevent slugs accessing seedlings before emergence. Take steps to ensure that a crop has the best chance to emerge from the ground quickly.

Biological control
Many birds, such as starlings, blackbirds, and killdeer, feed on slugs throughout the fall and winter months. Some predatory ground beetles and rove beetles feed on slugs. Naturally occurring pathogens, and parasitic nematodes are potential biological control agents of slugs but are not commercially available for use in the United States at this time.

Some nematodes are lethal to slugs and snails, and one species, Phasmarhabditis hermaphrodita, is being used successfully in Europe as a commercially available biological control agent (Nemaslug). This nematode is associated symbiotically with a bacterium that uses an endotoxin to kill a wide range of pest slugs and snails, including many of the species that are economically important in Oregon and Washington. After the slug dies, the nematodes multiply on the decaying slug body and then migrate back into the soil to infect more slugs if conditions are favorable. Phasmarhabditis hermaphrodita was recently found in Oregon and California, but Nemaslug is not available in this country because of biosecurity reasons. Research focusing on discovering and testing pathogenic nematodes in the PNW will likely prove to be valuable for developing biological control agents for slugs and snails.