Over twenty species of slugs are found in Pennsylvania. The gray garden slug (Deroceras reticulatum) is typically the pest species found in field and forage crop fields. In September, the two most common life stages present in fields are eggs and adults (Figure 1). Typically, eggs are the overwintering stage but in warm winters it is thought that the adults will also overwinter and can begin laying eggs in early spring. The weather this past winter, spring and summer has been ideal for optimum slug growth and reproduction. Warm winters, and cool wet spring and summer days drastically favor slug development.
Figure 1. Gray garden slug life cycle. Graphic: Liz Bosak
What are slug management options? There are a lot of unknowns when it comes to slug management. However, an integration of management tactics is probably the best approach. Residue management, correct planting procedures, natural predator populations, molluscicide application, and warm, dry weather can all help to limit slug populations.
Tillage in some form can reduce slug populations but for many Pennsylvania farmers dedicated to maintaining a 100% no-till system this is not an option. Researchers from Ohio found that slug populations were lowest in conventionally tilled and reduced tilled fields compared to no-till fields. The reduced tillage fields had a single disking pass in the spring. Based on the life cycle, a single disk or vertical till pass may be more effective in the fall compared to the spring. In annual ryegrass seed production, fall tillage passes have reduced slug populations but there was no comparison with spring tillage. Perhaps of more interest, in their system, alternate year tillage or tillage every three years significantly reduced slugs.
Good agronomic practices can help to limit the worst slug damage. The most important tactic is to make sure that the seed furrow is completely closed to avoid the slug super highway (Figure 2). An open seed furrow is the perfect environment for slugs to happily avoid predators and cool off while feeding a row on newly germinated corn and soybean seedlings. In fields with high residue, more than 75%, clumps of stalks can be pushed into the seed furrow making a perfect nest for juvenile slugs. Trash wheels push the residue away from the row and help with furrow closure. Selecting a variety with fast emergence in cooler planting situations can help to get the crop growing and outpace the slugs.
Figure 2. Open seed furrow. Only a few soybean plants remained in the row. Photo: Liz Bosak
Slugs may have seemed invincible this spring but there are many natural predators including birds, frogs, toads, snakes, spiders, centipedes, and many insect predators such as firefly larvae, ground beetles, and rove beetles. Using insecticides in crop fields only when an insect pest reaches an economically damaging level will help to conserve natural insect predators.
Rescue treatments are often a last resort and vary in their effectiveness. Baited pellets (e.g. Deadline M-Ps) containing metaldehyde, a molluscicide, can effectively reduce slug populations. However, this can be an expensive option because re-application is necessary after rainfall and typically multiple applications are needed during cool, wet weather. At-planting application is only effective if eggs have already hatched and juveniles are present in the field. Researchers in Ohio evaluated bait applications at 1, 2, 3, 4, and 5, weeks after planting and the most effective applications coincided with presence of juveniles in the field. A home remedy of urea-based nitrogen applied at night has been used with variable success.
What can you do this fall? Scouting newly seeded cover crops for slug damage or for slug eggs beneath residue or in combine piles can help to predict problem fields for next spring. This requires close monitoring because slugs prefer to feed on seedlings and it is easy to miss the infestation if more than a week passes between field checks. This will help to start building a slug field history for your farm, in particular, whether infested fields in the fall are an issue for seeding corn and soybean in the next spring.
Resources and References:
Douglas, M.R. and Tooker, J.F. “ Slugs as Pests of Field Crops ” fact sheet.
Slug Activity Predictor on the Slug Portal citation.
Dreves, A.J. and Mc Donnell, R. 2017. Slug-Portal ; Oregon State University, College of Agricultural Science, Dept. of Crop and Soil Science, Corvallis, OR.
Douglas, M. R. and Tooker, J. F. 2012. Slug (Mollusca: Agriolimacidae, Arionidae) Ecology and Management in No-Till Field Crops, With an Emphasis on the mid-Atlantic Region. Journal of Integrated Pest Management. 3(1): 1-9.
Hammond, R.B.; Smith, J.A.; Beck, T. 1996. Timing of Molluscicide Applications for Reliable Control in No-Tillage Field Crops. Journal of Economic Entomology. 89(4):1028-1032.
Hammond, R.B. and Stinner, B.R. 1987 Seedcorn Maggots (Diptera: Anthomyiidae) and Slugs in Conservation Tillage Systems in Ohio. Journal of Economic Entomology. 80: 680-684.
Sullivan, C.S. and Salisbury, S.E. 2015. Evaluating the response of slug populations and activity to tillage practices in annual ryegrass grown for seed. 2014 Seed Production Research, Oregon State University 151:1-3.