I received two reports of slug feeding on corn over the last week, with questions about this occasional crop pest. One report came from Ted Bay, UW-Extension Grant County in southwestern WI. Additionally, Paul Sturgis, Croptech Agronomics, reported slug damage in corn fields from his area in north central WI (northern Wood, western Marathon, northern Clark and southern Taylor counties).
Slugs in Corn
Eileen Cullen, Field Crops Extension Entomologist
I received two reports of slug feeding on corn over the last week, with questions about this occasional crop pest. One report came from Ted Bay, UW-Extension Grant County in southwestern WI. Additionally, Paul Sturgis, Croptech Agronomics, reported slug damage in corn fields from his area in north central WI (northern Wood, western Marathon, northern Clark and southern Taylor counties).
Slugs are not insects, they belong to the class Gastropoda. This is important because insecticides are not labeled for slugs, and have no control effect on slugs.
Unlike snails, slugs do not develop a shell. They can move through relatively small holes and crevices in the soil or soil surface residue. Slugs are active at night, it is rare to find them during the day.
Immature slug, Photo: R. Youngman (Virginia Tech University)
There are a few different species of slugs, most have one generation per year and overwinter in the egg stage. If winters are mild, adults can overwinter. Because field slugs can live 12 to 15 months, and eggs are laid in early spring and fall, overlapping generations of adult and juvenile stages can be present in the field. During dry, hot summer conditions, adult slugs enter a period of inactivity. Slug activity is at its peak in late spring and early summer, and again in early fall.
Cold, damp, overcast cloudy weather favors slug activity and delays crop development, extending the period of crop susceptibility to slug injury. Rainfall and saturated soils favor slug activity. When weather conditions and moist soil surface residue persist, slugs can be found in tilled fields with a normal amount of crop residue. However, no-till fields are more prone to slug damage than reduced tillage or conventional tillage fields.
Slugs feed on a wide host range, including soybeans and corn. Weather conditions have been ideal for slugs for much of May and June. With late and re-planted corn and soybeans this spring, plants are smaller than usual for this time of year.
Crop injury may have occurred early to seeds and seedlings causing stand loss. Slug feeding can also cause defoliation in established stands. The calls this week fall into the second category.
The slug mouthpart includes a tongue-like structure used to scrape its food as it eats plant foliage. Damage to corn leaves may appear as streaks or holes, usually both. With sunny, dry weather, as soil residue dries, established soybean and corn stands will outgrow slug defoliation from earlier in the season and treatment is not necessary. However, 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. If you’ve identified a field with slug damage, check to make sure new leaves are not defoliated and the plant is outgrowing damage. For example, you only see slug injury on older, lower leaves.
Slug feeding injury to corn. Photo: Ted Bay, UW Extension Grant County 2008.
You will see foliar damage during the day when slugs are not active. Slime trails (dry mucus from previous movement of slugs across foliage) are a sign of recent slug presence.
Both corn in the early whorl stage and soybeans in the vegetative stage can tolerate up to 40% defoliation without significant yield impact. If crop growing conditions are unfavorable, weather conditions remain conducive for slug activity and crop development is being delayed as a result of feeding injury, treatment may be necessary.
Commercially formulated metaldehyde baits can be applied. These are slug baits, not insecticides. Treatments are expensive, typically in the range of $10 to $15 per acre. One trade name is “Deadline M-P’s”, and most other products have ‘metaldehyde bait’ in the trade name. Product information can be found in Crop Data Management System (CDMS) pesticide label database www.cdms.net/LabelsMsds/LMDefault.aspx?t=
If applying baits, follow label instructions. It is important that application takes place when slugs are still active, typically during periods of cooler temperatures (63 – 68 deg. F) and wet conditions favorable to above ground slug activity. For this reason, slug baits are often applied aerially.
Reduction of slug problems, once they have become established, is difficult because bait treatment only reduces slug activity by “buying time” to enable the crop to outgrow the problem. (remember, slugs will enter a summer period of inactivity). To deal with the problem long-term, occasional use of reduced tillage can decrease development of slug problems in no-till fields. Mechanical devices on planters that remove reside over the seed furrow may reduce slug damage to seeds and emerging seedlings. (Willson, 1992).
References
Baute, T. 2007. Slugs will be Active Soon. CropPest Ontario Newsletter. Ontario Ministry of Agriculture Food & Rural Affairs.