Slugs – Scouting and Control in Field Corn

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Biology of Slugs
Most slugs pass through a single generation per year. Although they generally overwinter in the egg stage, we can often find juveniles and adults all winter, especially if conditions are warm. Since 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. A combination of one or more of the following factors favors slug outbreaks: no-tillage field crop production practices; development of dense weed cover or addition of organic matter such as manure; mild winters which increase the number of overwintering stages, especially adult slugs; prolonged periods of favorable temperatures (63 to 68°F) combined with evenly distributed rainfall that maintains soil moisture at 75% saturation; high pH (6.3 – 6.7); over fertilization with nitrogen and cool growing conditions which delay crop development and extend the period of susceptibility of the crop to slug injury.

Scouting for Slugs
You can still identify fields with the potential for problems before planting by using a shingle or covered pit to provide a humid, sheltered hiding place for slugs. Slugs tend to congregate in large numbers in these shelters. As a rule of thumb, you can expect problems in a field if you find one to five slugs per trap. Once a field is planted, you should examine fields with a potential for damage on weekly basis. Slug damage will appear as a shredding of the leaves since they feed by grating away the surface of the plant tissue. The presence of “slime trails” can also be used to distinguish slug injury. Look for slugs under dirt clods and surface trash around 5 plants in 10 locations in a field. Since slugs are nocturnal, sampling should be done in the evening or when weather is cloudy. As a general guideline, a treatment may be needed if conditions are favorable for slug development and you find 5 or more slugs around each plant from the spike to 3-leaf stage.

Controls
Management options are still limited to the use of baits and cultural practices. If a number of factors are present which favor slug development, then a combination of cultural practices and baits may be needed. Cultural practices, including the use of “pop-up” fertilizer and trash whippers to remove residue over the seed furrow, can help corn grow ahead of the damage. When populations were extremely heavy in the spring of 2003, good results were obtained with Deadline MPs (metaldehyde bait). The label states 10 – 40 lbs per acre (http://www.cdms.net/LDat/ld7CL000.pdf ). We saw good results with 10 lbs. per acre broadcast applied with a cyclone spreader if the spreader was calibrated so you are getting at least 5 pellets per square foot. Also, the best results have been observed when applications were made and there was at least one day of sunny weather after an application. In general slugs stop feeding in 2-3 hours even though they may take 2-3 days to die. If conditions remain extremely wet, slugs sometimes can absorb enough moisture to compensate for the water lost in mucus production so a second application may be needed. Most baits, as well as cultural practices, only reduce the slug activity – buying time to enable the crop to outgrow the problem.

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