Slugs are some of the most common and persistent pests of home gardens and commercial crops in western Oregon and Washington, and if left unchecked 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 reaches 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 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 becomes saturated, 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 stay underground 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 on the 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 or grassy borders serve as excellent habitat for slugs. Grass seed crops, cereals, or vegetable plantings that immediately follow a perennial legume or pasture are quite likely to sustain slug damage. Large populations of the gray garden slug and smaller numbers of several less common species build up on all 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 field 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 the yard or field. 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 four to six pellets of bait inside. You can cover the areas with a scrap of wood. 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 rain fall in September or early October when slugs become active on the soil surface after having passed the summer underground. Late September to mid-October are usually good months to control slugs, however depending on the weather, other windows of control 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 are not moving or feeding 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.
Our most economically important species is the “gray field slug” or gray garden slug, Deroceras reticulatum. The European black or red slug, Arion rufus, and in recent years the brown-banded Arion, Arion circumscriptus, the hedgehog slug, Arion intermedius, the dusky slug, Arion subfuscus or A. fuscus, the black greenhouse slug, Milax gagates, the large spotted garden slug, Limax maximus, the marsh slug, Deroceras laeve, and the reticulated slug, Prophysaon andersoni, can also be important pests.
Slugs are hermaphrodites: every slug is born with both male and female reproductive parts and theoretically capable of laying eggs. Mating occurs primarily in the fall and spring. Small, round, pearl-like, white or translucent eggs are laid in clusters of a dozen or more (over 500 eggs in a lifetime) in sheltered cavities near the soil surface or under debris on the soil surface if the soil is moist. 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 from 6 to 18 months, but other slug species may live longer.
Slug baits are poisons and therefore can be dangerous to children, pets, wildlife and edible crops. When using baits use them 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 iron chelate (e.g., IronFist®, Ferrox®, sodium ferric EDTA®) are four common and effective chemicals used to control slugs in the Pacific Northwest. Pellet baits have been the most commonly used product for homeowners. Unfortunately, even when “good” control is achieved, only about 60 to 70 percent of the slug population may 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 significant 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 garden slugs are attracted to the seed furrow and begin to hollow out the endosperm within hours after the seed swells with moisture. One medium-size 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 shortly afterward. In broadleaf crops and grasses, slugs do not feed on seeds but instead make short order of seedlings by feeding upon and destroying the tender growing points. The most effective timing for bait 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 or in no-till situations.
The more effective commercially available baits contain cereal bran or flour 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 provide 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 significant because slugs tend to encounter these pellets at a greater frequency than the larger, older style type. Research in the PNW indicates that a pellet density approaching 5 to 6 per sq ft is an optimum density. This density can be achieved by applying a per-acre rate of just 5 lb of a 2.5 mm bait, or about 8 to 10 lb of the mini-pellet bait. Doubling or tripling the bait density does not necessarily increase control proportionally. Generally, it is recommended to reapply bait in 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, weather conditions at the time of application, and the toxicant level of the bait. If the carrier material is not attractive and palatable to the slugs, they may refuse the bait or consume a sublethal dose of toxicant, from which they can recover.
Methaldehyde, iron-phosphate, iron-chelates, and methiocarb
Several chemicals are formulated into slug and snail baits for use on food and seed crops. Metaldehyde has been used since the early 1940s, iron-phosphate since 1998, and iron-chelates since the early 2000s. Major efforts have been applied to finding new chemical baits for managing slugs in agriculture. A new generation of products has been developed from metal chelates incorporated into an ingestible bait to make an effective molluscicide. These iron chelate baits (IronFist® and Ferrox®) have been extensively trialed in Oregon and showed positive results in reducing the feeding and controlling of slugs in grass seed, clover seed, and cereal production.
Baits that contain methiocarb may still be available. They are effective but have had limited labels for use 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.
Currently a couple of commercial iron-phosphate formulations, Sluggo® and Sluggo Plus®, are approved for organic production. They are formulated as a uniform and dust-free cereal-based mini pellets. Mode of action is uncertain, but the iron probably binds with and precipitates protein within the slug. Mortality is somewhat slower (5 to 7 days) compared to that induced by metaldehyde. The slugs, however, cease feeding after having eaten the iron phosphate bait. Agricultural use of this product has shown it to be as effective in controlling gray garden slugs as metaldehyde baits, although slightly greater rates of the former per unit area are usually needed. Slugs that ingest iron phosphate or iron chelate baits usually die underground 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 wind, 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 particularly used for immature slugs prior to canopy closure in establishing a stand.
Big 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 2 to 3 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 exposture 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 sickened slugs. Furthermore, under wet conditions, poor control may follow from low-quality baits and low active ingredient levels 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 a year-round necessity in many crops and sites with no-till or conservation tillage practices. Presume damage from slugs in certain crops and sites 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 more unlikely 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 a large population of the field 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 the following winter or early spring. These small slugs often do not accept bait as readily as larger slugs.
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.
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. Plows, discs, and rototillers 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. 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 get up quickly.
Many birds, such as starlings, blackbirds, and killdeer, feed on slugs throughout the fall and winter months. Predatory ground beetles feed on slugs. Naturally occurring fungal and bacterial pathogens, parasitic nematodes, and marsh fly larvae are potential biological control agents of slugs but are not commercially available for use in the United States at this time.
Slug-pathogenic (malacopathogenic) nematodes, a group of soil-dwelling roundworms, are a novel method and approach that has been used in Europe to kill slugs with some success. One of the most widely established, commercially-available slug biocontrol agents in Europe is the nematode Phasmarhabditis hermaphrodita (Schneider). This nematode has been shown to be associated symbiotically with a bacterium that uses an endotoxin that kills slugs. The nematode locates slugs in the soil and enters the slug’s mantle cavity. After the slug dies, the nematodes multiply over the decaying slug body and then migrate back into the soil where infect more slugs if conditions are favorable. Parasitic nematodes were recently found in CA, but currently restricted in this country due to biosecurity reasons, so are unavailable for purchase. Research focusing on discovering and testing pathogenic nematodes in PNW fields will likely prove to be valuable for use as a biological control agent for control of slugs.