Slugs and snails belong to a class of animals called the Gastropoda. Snails have well developed shells, which are generally spiral in shape. In contrast, slugs do not have a hardened spiral shell (see Figure 1). As a result, slugs can move through relatively small holes and crevices in soil habitats.
Figure 1. A Slug
The basic morphology of all gastropods include: (1) the head, which includes a mouth, sensory tentacles and simple eyes, (2) the foot, which may be described as a creeping sole, (3) the mantle, which is a fold of skin over the back that forms the respiratory cavity and covers the viscera, and (4) the viscera, which include the internal organs. The skin of slugs regulates body water content and secretes a covering of slime which protects the animal from desiccation. The foot includes a gland that produces mucus on which the animal crawls. As a result, movement of slugs or snails across a plant or substance is marked by a characteristic trail of mucus which is sometimes referred to as a slime trail.
In Ohio, slugs found in the field may be native or of European origin. Two species of slugs of European origin that commonly affect field crops are the gray garden slug (also called the gray field slug or nettled slug), Agriolimax reticulatus (Muller), and the marsh slug (also called the smooth slug), Agriolimax laevis (Muller). The gray garden slug is often pale cream to grey in color with a mottled pattern of spots and streaks. In contrast, the marsh slug ranges from grey to black in color with minimal markings. A mature gray garden slugs is generally an inch or two in length. A mature marsh slug is generally less than an inch in length. Many species of slugs may be found in the field, but field observations described in this circular primarily represent the species described above.
Crop injury due to slugs may include (1) early injury to seeds and seedlings reducing stand establishment, and (2) defoliation of established stands that may delay plant development.
The mouth of a slug includes a tooth covered tongue called a radula, which is used like a rasp to grate its food. As a result, a slugs feeds on the surface of a seed or foliage by rasping the surface. The damage may appear as streaks or holes. When slugs feed on germinating seeds, they will hollow out distinct holes appearing like damage due to wireworms.
Field damage to germinating seeds (corn and soybeans) and early developing seedlings can be devastating. In no-tillage fields, slugs will actively feed on germinating seeds and plant embryos, especially if the seed furrow is covered by a moist surface residue. In years having favorable conditions for slug development, stand loss ranging from 50 to 90% have been observed. When such damage occurs in the seed furrows and soil surface is damp, the slugs often can be readily found near the site of damage.
Figure 2. Proportion of corn fields per tillage practice exhibiting slug injury exceeding levels ranging from 0 to 80%
Severe foliar injury by slugs to corn in the pre-whorl and early whorl stage and soybeans in the early vegetative stages may delay crop development. Since foliar injury is generally observed during the day when slugs are not active, the tell-tale sign that the damage was caused by slugs is the trail of dry mucus that remains from earlier movement by slugs across the foliage.
The extent of the damage caused to a plant stand depends on (1) the abundance of slugs present, (2) the age of the prevailing generation, and (3) the environmental conditions favoring slug activity and development. In general, slugs prefer cool and wet conditions. The ideal temperature and moisture conditions for field slug activity is between 63 and 68 deg. F at 100% relative humidity. Slugs exhibit nocturnal behavior and prefer to feed after dark and before dawn. However, they may become active following a rain.
The occurrence of slug damage to field crops especially on corn and soybeans is directly associated with the reduction of tillage. The accumulation of soil surface residue associated with no-till cultivation provides an optimum environment for development of slug populations, especially when ideal temperature and moisture conditions prevail. In general, most cases of severe economic injury due to slugs occurs in no-till production systems (see Figure 2), especially if a field is maintained under no-till practices for an extended period of time. Slug problems are uncommon in a field under either conventional or reduced tillage.
Most field slugs pass through a single generation per year and generally overwinter in the egg stage. However, if the winter is mild, adults may survive the winter. Since field slugs may live 12 to 15 months and eggs are laid both in the early spring and fall, overlapping generations of adult and juvenile stages may be observed. In the winter, adult slugs may enter a state of hibernation, and in the dry and hot summer conditions they enter a similar inactive state of aestivation. Peak slug activity generally occurs in late spring and early summer when the spring hatch attains adult growth and again in the early fall when cooler temperatures resume.
Slugs are hermaphrodites (individuals are both male and female). As a result, when they copulate, there is a reciprocal exchange of spermatozoa. Self fertilization is possible in some species such as the marsh slug, but in general pairs of slugs are needed to reproduce.
No-tillage corn and soybean fields should be scouted routinely to detect and evaluate the presence of slug problems, especially when cool and wet conditions prevail during the months of May and June. If significant stand loss due to slug activity is detected to warrant replanting, a rescue treatment of applying a slug bait may be justified prior to replanting if continuation of damp weather is forecasted.
If a crop stand has been established and foliar injury due to slugs appears to be at a level that may impact plant development, the extent of defoliation should be evaluated. In general, both corn in the early whorl stage and soybeans in the vegetative stage can tolerate up to 30 to 40% defoliation without a significant impact on yield. However, if growing conditions are unfavorable and stand loss is anticipated, then corrective action may be needed. In general, if the corn or soybean crop has advanced to a point where only the lower foliage is being affected, the crop will overcome the impact of slug defoliation.
Severe slug injury to Ohio crops occurs infrequently and a combination of factors is required to cause slug outbreaks to occur. Factors favoring development of slug outbreaks include the following:
When corrective action is required to reduce slug population, commercial formulated baits can be applied. Currently, the only baits registered for slug control on corn and soybeans are metaldehyde based baits. Such commercial formulations can be expensive to apply, but the cost of such treatments can be maintained at about $10.00 to $15.00 per acre if applied selectively.
When applying baits for slug control, it is very important that the bait be applied when the slugs are at peak activity above the soil surface. Thus, bait application should only be applied during periods of ideal temperatures and wet conditions favorable to above ground slug activity.
In prior years, special exemptions were provided under State 24c and Section 18 labelling to allow growers to formulate specific carbamate insecticides with cracked corn for application of low cost slug bait treatments. The combination of public concern regarding potential bird kill and risk to manufacturers of defending such chemical treatments has eliminated such actions for future implementation.
Cultural options for reduction of slug problems do exist. Occasional use of reduced tillage decreases the development of slug problems in fields that were maintained under long term no-tillage practices. Slug problems in minimum tillage systems such as ridge tillage are rare. Mechanical devices on planters that remove residue over the seed furrow may reduce slug damage to seeds and emerging seedling.
In the case of no-tillage corn and soybean production, growers must learn to tolerate sub-economic levels of slug injury. Reduction of slug populations, once they have become established, is difficult to achieve since the bait treatment only reduces the slug activity buying time to enable the crop to outgrow the problem. However, many Ohio growers have learned over time how to minimize or tolerate the impact of slugs in no-till corn and soybean production.
Copyright 1993, The Ohio State University
OPMS Circular: FC-20 SLUGS
Prepared by: Harold R. Willson, Dept. of Entomology
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-6181