Slugs are among 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 whenever the relative humidity in their immediate environment approaches 100 percent, the temperature is above 38°F, and the wind speed is negligible (<5 MPH). By day, slugs usually rest in crevices and cracks in the soil, or under surface debris where it is moist. They tend to be active primarily at night, but also feed and reproduce by day during light rain events, foggy periods, or after irrigating. Even in the summer, when air temperatures peak in the Pacific Northwest and soils are dry on the surface, slugs can be active at night in closed canopy crops such as grass seed, legume seed, pasture/hay crops, or certain vegetable crops. This is because as night temperatures decrease, 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 no-till, minimum tillage, supplemental irrigation, post-harvest residue buildup, and crop plant structures (e.g., closed canopy) can affect the microclimate and promote otherwise unexpected slug activity. Also, some broad-spectrum insecticides can kill slug predators, such as ground beetles, and this reduction in natural enemy pressure can cause a significant increase in slug numbers even in fields which traditionally have not had slug issues.
Slug damage can be distinguished from that of cutworms, armyworms, and other chewing pests by the presence of slime trails and their small sausage-shaped feces, which are found on or around damaged plants. Underground feeding on roots and tubers is characterized by shallow (0.1 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. In cereal crops, slugs favor feeding on newly planted seeds, which they can hollow out. Wheat is most susceptible to slug damage from the time of seeding to plant emergence.
Slug damage to vegetable, cereal, grass seed, and forage crops can be extensive near field margins. Weedy, grassy or wooded borders serve as excellent habitat for slugs. Grass seed, cereal, and vegetable crop plantings that immediately follow a perennial legume or pasture are likely to sustain slug damage. Large populations of the gray field slug and fewer numbers of several less common species (e.g., white-soled slug) build up on most perennial legumes in western Oregon and Washington. Our most economically important species in the Pacific Northwest 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 region are invasive from Europe. A native slug, reticulate taildropper Prophysaon andersoni, can also be a minor pest in certain crops such as ornamentals including Christmas trees.
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 piece of wood or an old carpet tile. This prevents other creatures from coming into contact with the bait, and the cover helps to keep slugs poisoned by the bait from moving away. Place bait stations after the first inch or more of rainfall in September or early October when slugs become active on the soil surface after having spent the summer underground. September and October are usually good months to control slugs, however depending on the weather, other control windows may occur when “follow-up” bait applications can be effective. After October, or when weather becomes too cold (< 38°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 day length shortens, eggs are produced and can hatch in fall or spring, when temperatures warm. In late fall and early spring, new juvenile slugs can be difficult to spot in the field but they can cause significant damage to gardens and crops. Slug eggs are also laid during the spring.
Slugs are hermaphrodites, which means that every individual has both male and female reproductive parts and 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.
Slug baits (molluscicides) are poisons and therefore can be dangerous to humans, pets, and other wildlife. It is important to use baits properly by following all label instructions and heeding 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 Snail & Slug Killer, 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 and farmers. Unfortunately, even when “good” control is achieved, typically < 60 percent of the slug population will be removed. This usually suffices for economic crop protection if slug pressure is light, but populations tend 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), bait distribution and bait quality are crucial for successful treatment.
In cereal crops, the greatest risk comes during the first week after planting. Gray field slugs, for example, 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 band in the furrow), and/or shortly afterward. In broadleaf crops and grasses, slugs do not feed on seeds but instead target small 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 history of slug damage, in no-till or minimal till planting systems, or in situations where post-harvest residue is retained from the previous crop.
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 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 molluscicide products has been developed from metal chelates (e.g., sodium ferric EDTA) incorporated into an ingestible bait. These baits (e.g., IronFist and Ferroxx) have been trialed in western Oregon and showed encouraging results in terms of reducing slug populations in grass seed, clover seed, and cereal crops.
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 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 thought to be somewhat slower (5 to 7 days) compared to metaldehyde. The slugs, however, typically 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% 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, 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 and it is best applied at times when slugs are active aboveground.
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 mini pellets. Cereal-based mini pellets and very small pellets, (e.g., Metarex) have the best performance record in our rainy climate and can last 1 to 2 weeks on wet soil.
Research has shown that metaldehyde has a different mode of action than previously suggested. The toxicant does not dehydrate but rather damages 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 Pacific Northwest.
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, but monitor all fields as slugs can suddenly become a problem even in fields which traditionally have not had slug issues. 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.
Control is seldom, if ever, complete. Around the home garden, removing debris, leaf litter, weeds, and other excess vegetation helps to reduce slug habitat and lower 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 barriers have been used with varying degrees of success. However, underground slug movement or environmental degradation of the repellent (e.g., copper oxidizes, salt washes away) negatively impacts efficacy over time.
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. In contrast, open, damp and cloddy seedbeds provide ample shelter for slugs and typically promote higher populations. Take steps to ensure that a crop has the best chance to emerge from the ground quickly.
Some birds, such as starlings, blackbirds, and killdeer, feed on slugs throughout the fall and winter months. Grazing sheep are also known to inadvertently feed on these pests in grass and clover fields. Many insects including 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 the United States because of biosecurity reasons. Research focusing on discovering and testing pathogenic nematodes in the Pacific Northwest will likely prove to be valuable for developing biological control agents for pest slugs and snails.
In areas where the application of certain baits (e.g., metaldehyde) is not recommended (e.g., around dogs and cats), non-toxic attractants can be used to help manage pest slugs. One cost effective option is bread dough (500 g of all-purpose flour, 500 mL of water, and 0.5 oz of active dry yeast). In home gardens for example, the dough can be deployed in 2-inch diameter balls after sunset, and slugs drawn to the attractant can then be removed. The dough balls will need to be replaced nightly because they become less attractive as they dry out. Alternatively, a more watery dough (e.g., use double the amount of water in the dough recipe) can be placed inside a plastic cup (8 oz) that is buried in the soil with the cup lip level with the soil surface. Slugs entering the cup to feed on the dough typically drown. Fresh cucumber slices are also attractive to many slug species that are pests in Oregon and Washington.