Management Considerations for Slugs: Do Insecticides Work?

With recent weather patterns, specifically high rainfall leading to wet soil conditions, some farmers have experienced damaging populations of slugs in their no-till fields. No-till fields are particularly affected since increased residue provides a stable, cool, and wet environment for these animals that are prone to desiccation (drying out). Oftentimes, farmers wonder if insecticides or seed treatments are effective at managing these non-insect pests. We will discuss management options for slugs in this article.

Slugs (Gastropoda) are members of the phylum Mollusca, which also contains clams, scallops, snails, squids, and octopi, among others. Molluscs have soft bodies with two regions: a head and a foot. These animals often have a hard exoskeleton, such as a shell or plate. However, slugs do not have a shell. They are essentially snails without a shell. Slugs have four front tentacles: two for the eyes and two that act as antennae. They are covered in slimy mucus and secrete additional mucus to aid in locomotion. Oftentimes, the slime trail is an indicator of slugs in the field.

If you scout for slugs, you will likely find all life stages throughout the growing season. This is because slugs are hermaphrodites, and their mating habits are complex and not well synchronized. Their eggs, which are small gelatinous spheres, can be found under residue or in the soil. In general, slugs are most active and damaging April to June and September to October. They are most common in heavy, wet soils that are not tilled often and prefer slightly alkaline to neutral soils.

Slugs are nocturnal herbivores, so they generally feed from dusk to dawn, but they may feed during the day if it is raining. Activity increases with temperatures below 70°F and with higher humidity (they prefer 100% humidity). Slugs can feed when temperatures are as low as 34°F, although their activity is decreased. Activity is also reduced at temperatures greater than 80°F, because slugs are 80% water and are prone to desiccation; therefore, warm, dry conditions slow slug activity while mild, wet conditions promote activity. Significant slug populations are most likely during a wet spring that follows a mild winter, or any spring following a wet fall, because wet soils promote egg-laying.

Plant injury

Slugs are considered serious pests of wheat, barley, oats, rye, corn, soybeans, tobacco, canola, alfalfa, and other cereals or legume forage species when grown under no-till or reduced tillage. They can feed directly on the seeds or seedlings, leading to plant death and poor stands. Additionally, slugs can damage seeds, roots, stems, leaves, and flowers by scraping the surface with their radula. Feeding tends to be greater on leaves closer to the soil, and young plants are preferred. Slugs may also eat fungi, plant residue, soil organic matter, small invertebrates, and occasionally each other.

Corn and small grains often exhibit window-pane damage and then shredding, as slugs scrape strips in the leaves. In soybean, slugs can create craters in the cotyledons or leave ragged holes in the leaves. In addition, slugs can destroy the apical meristem (growing point), leading to plant death, or they can completely defoliate plants under high populations. Seeds and seedlings are at greater risk of damage if the furrow or slot is left open during planting as this creates a dark, cool “tunnel” for slugs to effortlessly travel from seed to seed.

To estimate the absolute density of slugs in the field, a soil sampling method exists; however, it is labor intensive and impractical for most farmers and crop consultants. Less intensive methods include looking for adults in the fall, looking for eggs and overwintering individuals before seeding in the spring, inspecting emerged crops with a flashlight at night, or placing artificial shelters (shingles, boards) in the field and checking underneath them occasionally. Fields with a history of slug issues, fields with a lot of surface residue, low-lying fields, fields with manure applications, and fields with heavy soils should be prioritized when scouting.

Because they’re soft-bodied with no shell, slugs have many predators, such as frogs, toads, snakes, birds, ground beetles, rove beetles, firefly larvae, marsh flies, harvestmen, wolf spiders, centipedes, and parasitic nematodes. Any insecticide used for management would have an impact on the effectiveness of natural enemies.

Management

Reduce soil residue:

• Shallow disking can reduce populations.
• Using row cleaners to remove residue over the row may help reduce damage when slug populations are low.

Avoid slugs early in the season:

• Planting earlier may limit damage if plants have enough growth before eggs hatch.
• Ensuring seed slots are closed at planting may help minimize damage.
• Having other plant species present in the field may reduce slug damage to the crop: weeds, companion plants, or recently terminated cover crops have been preferentially fed upon in field studies. However, extra residue provided by cover crops may increase slug issues as well.

Promote crop vigor:

• Planting later, when soils are drier and warmer, can promote quick crop growth even though slugs would be active.

• Choosing crop varieties that have higher emergence and vigor ratings can help promote early growth.
• Pop-up fertilizers at planting could encourage quick growth to “outrun” slug damage.

Foliar insecticides have inconsistent results, but baits may be used to manage slugs:

• Insecticides, particularly chlorinated hydrocarbons and organophosphates, do not appear toxic to slugs, are inconsistent, or require very large doses.
• Neonicotinoid seed treatments are not lethal and may actually increase feeding.
• Carbamates can have activity as baits, but not foliar sprays.
• Metaldehyde baits are effective molluscicides; however, in the U.S. they are only labeled for use on ornamental plants, some small fruits and berries, citrus, some vegetables, and grass grown for seed. They are NOT registered for ANY USE in Iowa.
• Other baits are available that include iron phosphate, iron chelate, or sulfur, which must be ingested to be effective and may not be cost-effective for farmers.
• Any bait should be applied when juveniles or adults are present in the field, as their efficacy is easily diminished by rainfall.

Poisoning slugs with nitrogen or repelling them with salts may minimize damage, but these approaches have not been tested in field studies:

• Applying a nitrogen-based spray at night may poison slugs, however, they may also burn the crop.
• Applying dry ammonium sulfate or salt-based herbicides over crop rows may repel slugs.
• Consider the costs and benefits of these approaches. It is possible these tactics are not economically justifiable if severe damage has not occurred.
• Additionally, slugs seek shelter under windy conditions and are nocturnal feeders, so any application should be made under calm, mild conditions at night.

No economic thresholds exist for slugs in crops, but typically crops have most of the growing season to recover from damage that was not lethal. Corn can typically withstand at least 40% defoliation during early stages without yield reduction. Soybean only suffers minor yield loss if the unifoliate leaves sustain 50% defoliation, but no yield loss occurs when trifoliates sustain the same amount of defoliation. However, stand reductions are more concerning, and farmers should decide if treatments or replanting are necessary given the conditions.

Management summary