Slug Damage and Management in Field and Forage Crops
Slugs eat a wide array of broadleaf plants and grasses, including most crops and many weeds. They harm crops both by killing seedlings outright, causing poor stands, and by damaging leaves on young plants. They feed by scraping the surface of their food, which can include seeds, roots, stems, and leaves. The appearance of their damage varies by crop. In wheat, slugs feed on recently-planted seeds, hollowing them out and killing them. In corn and many small grains, slugs scrape strips in the leaves, leading first to window-pane damage, and then to leaf shredding. In soybeans, slugs create craters in the cotyledons, and then ragged holes in the leaves. Similar ragged holes are seen on slug-damaged canola, alfalfa, and other broadleaf crops. Slime trails are often seen in close association with their damage. Seedlings are especially at risk when the seed slot is left open, creating a dark, cool slug “highway” leading right to the next seedling.
Slugs have also been documented to eat fungi, plant residue, organic matter, and occasionally one another or other invertebrates. The importance of these alternative foods in their diet in crop fields is unknown.
Economic thresholds are not available to guide slug control decisions, but scouting for slugs can help predict when a problem is imminent. In spring prior to seeding, slug eggs and overwintered slugs can be found by looking under crop residue, especially on mild days soon after rain. Another approach to find slugs is to place artificial shelters in the field, such as roofing shingles, old boards, wet cardboard, or anything that will create a dark, cool, moist environment. Several days after putting them out, slugs can be found under the shelters in the morning. Once crops have emerged, slugs can be found by inspecting crops in the evening with a flashlight. With all of these methods, be sure to look closely as juvenile slugs can be very small.
Unfortunately, management options for slugs are limited. Moreover, recognized tactics are occasionally ineffective; therefore, an integrated management approach that relies on several control tactics is preferred. Most growers who experience slug problems are committed to no-till or reduced-till practices, so while tillage will certainly help control slugs, it may not be an option. Nevertheless, it is clear that tactics that reduce the amount of surface residue will decrease slug populations. For example, shallow disking (three inches deep) or “turbo tilling” can significantly decrease slug populations.
Because older crop plants are not as susceptible to slug feeding as young plants, several management tactics aim to foster early plant growth to get crops growing as quickly as possible to try to “outrun” the slug threat. For example, early planting may give crops a jump on slugs if crops emerge and have significant growth before eggs hatch in large numbers. Early planting in spring can reduce slug damage to new forage stands. In contrast, some growers have tried planting later, after soils are dried and warmed. This approach is meant to encourage quicker germination and growth by the crop during the time when slugs are already active. The choice of early or late planting likely will vary by region, depending on the timing of slug egg hatch relative to crop planting dates. Using row cleaners on the front of planters to move crop residue away from the row can also be beneficial because it allows sunlight more access to the soil, increasing temperatures and improving emergence. Growers can further contribute to better early growth by selecting crop varieties that are rated “excellent” for emergence and seedling vigor. Good agronomic practices such as ensuring seed slots are closed can mitigate some slug damage as can choice of crop rotation and cover crop, but these factors have been little explored.
Slugs do have predators including ground beetles, rove beetles, centipedes, harvestmen (aka daddy longlegs), firefly larvae (aka glow worms), soldier beetle larvae, frogs, and birds, particularly ducks and chickens (mob-stocking ducks can be quite effective!). Invertebrate predators of slugs can certainly contribute to slug control and can be conserved by increasing crop diversity and using insecticides sparingly (i.e., avoid them as much as possible or reducing the amount used by banding insecticides directly over the row rather than broadcasting it over the entire field) and in accordance with IPM principles. Some research at Penn State has found that ground beetle populations can be low in areas where slug populations are high and populations of these same beetles can be high where slug populations are low, suggesting that ground beetles can influence slug populations. Ongoing work at Penn State seeks to provide insight on the value of ground beetles as slug predators. Regardless of their exact contribution to slug control, it seems clear that predators need to be part of an integrated slug management plan.
Few chemical controls are available for slugs. Metaldehyde-based baits (e.g., Deadline products) can be used in many crops, but are often not economical and significant rains can wash away much of the product. Some growers have considered applying metaldehyde baits at planting, but because planting date can coincide fairly closely with slug egg hatch in spring, it is important to ensure that juveniles are present before baits are applied or the baits will lose their effectiveness before the eggs hatch. Efficacy trials at the University of Maryland suggest that baits can be banded over the row to reduce their cost. Pellets based on iron phosphate (e.g., Iron Fist, Ferrox) are also available and some are OMRI-listed (e.g., Sluggo) for organic systems. These are also expensive on a large scale. Because slug control can be frustrating, some growers have experimented with home remedies. Chief among these is spraying crops at night with nitrogen solutions, which act as a contact poison and burn slugs. A common approach is to use a 30% urea-based nitrogen solution, mix it with an equal amount of water, and apply 20 gallons per acre. This tactic should be repeated a few nights in a row to maximize its effectiveness because nitrogen solutions provide no residual control and all slugs in a field will not receive a killing dose in a single application. This approach will burn plant leaves, but growers that rely on this approach feel they gain more from controlling slugs than the damage they do by burning their plants a bit. Some farmers are experimenting with different strategies involving cover crops or companion plants to reduce the amount of feeding on the focal crop, often corn. Much research remains to be completed to develop better slug management options.