Building a Longer-term Plan Against Slugs

By John Tooker

I hope you are not among those that suffering from slugs this spring (Figure 1), but if you are I thought I would take the opportunity while slugs are on our minds to discuss some longer-term slug-management options that might help decrease future slug challenges. Longer-term options often need to be considered because rescue treatments (e.g., baits or nitrogen sprays) against slugs can be unreliable and frustrating. Our factsheet  addressing slugs in no-till production provides some context for this discussion and details on biology and management options. Prior to getting into the main discussion, it is wise to acknowledge that most climate forecasts appear to be predicting that Pennsylvania will be getting wetter, not drier, over the coming decades. This means to me that springs like 2021 may become more common, making crop establishment and troubles from slugs even more likely and challenging.

Among the factors contributing to slug challenges seem to be earlier planting dates and heavy residue in no-till fields. Particularly for soybeans, there is a trend to plant earlier in the year. Many farmers can avoid slug problems if they can wait a bit longer to plant. The worst slug situations occur when the weather turns wet and cold after planting and slugs are able to happily feed while the crop struggles to grow. This situation could be avoided by planting when soil temperatures are higher and crops can get out of the ground and grow quickly, hopefully leaving slugs behind. Heavy residue can also be a contributing problem because it gives slugs an easy place to hide, often in proximity to crop plants so they do not have far to travel to get a meal. Taking steps to reduce residue (e.g., shredding, baling, moving to silage for a year, or even a light vertical tillage that does not violate no-till standards) should help. Alternatively, taking a field out of grain production and letting that residue decay should be helpful.

Beyond planting dates and residue management, managing crop fields for diversity can help. Generally speaking, longer rotations have fewer pest problems (and shorter rotations have more pests), but the challenge is finding a balance between a rotation that can help restrict pests and one that is profitable for your operation. Longer rotations disrupt life cycles and generally make pest populations (insect, slugs, pathogens, even weeds) less successful because each crop has its own timing and management practices that contribute to keeping the population in check.

In continuous corn or a corn-soybean rotation, management is often more or less similar each year, allowing pests the opportunity to adapt and thrive despite management. The bottom line is that for pest management, a three-year rotation is better than a two-year, a four-year is better than a three-year (and so on); pest control capacity increases with the duration of the rotation. Aside from disrupting pest populations, diverse rotations can also foster improved populations of beneficial arthropods that can assist with pest control. Simple two-year rotations often include preventative insect management with insecticidal seed coatings and broadcast sprays of insecticides, either by themselves or tank-mixed with herbicides and/or fungicides. These insecticides and the simple rotation (by not providing sufficient habitat) together limit populations of beneficial insects, spiders, and centipedes that can help with insect pest and slug control.

Conversely, diverse rotations benefit beneficial arthropod populations by providing more varied habitats, particularly when they include cover crops and/or perennial hay crops. The more you can grow these populations by diversifying and using insecticides only when necessary (even insecticidal seed coatings), the more help you will get help against your slug populations. To be clear, I am not advocating for no insecticides, I would like folks to use them if they need them within the framework of Integrated Pest Management, which with its economic thresholds can inform when insecticides will be useful. Believe it or not, using insecticides blindly can exacerbate pest problems, including slugs.

Many farmers believe that cover crops tend to be part of the problem when it comes to slugs, but our research indicates that cover crops can be helpful in the fight against slugs. As mentioned above, cover crops can help diversify rotations and will promote better populations of beneficial arthropods, which in turn can help control slugs if their populations are strong and not disrupted by insecticides. Some farmers have even gone as far as planting into standing green cover crops (i.e., "planting green") to help with their slug challenges. This approach involves establishing corn or soybean into standing cereal rye (or other cover crop species), and then spraying the cover crop with an herbicide (often glyphosate) one to seven days after planting, so the cover crop dies slowly. This planting strategy is not for the faint of heart and often requires some mentorship by an accomplished practitioner, but farmers planting green, particularly with soybeans, can have fewer challenges from slugs. I would not advise growers to dive into planting green without discussing the practice with some farmers that have worked with the system for a while, but when paired with IPM that can protect predator populations (avoiding insecticide-coated seeds and unnecessary broadcast sprays of insecticides), it seems to be an option for slug management, in addition to the other benefits it provides (e.g., erosion control, organic matter input, nutrient cycling, etc.).

So, there are a few ideas and principles to help you begin to develop a cropping system that stands up better against slugs. These approaches to farming are more management intensive, but in the long run, appear to be more resilient in the face of pests and most any other challenges that may come along.