I hope you or your clients were not among those that suffered from slugs this spring, but if you were I thought I would take the opportunity while slugs are fresh in our minds to discuss some longer-term slug-management options that might help decrease future slug challenges.
Check out our factsheet that addresses slugs in no-till production provides some good context for this discussion and details on biology and management options. And 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 2017 may become more common, making crop establishment and troubles from slugs even more likely and challenging.
One reliable approach to decreasing pest populations in the future is to diversify rotations as much as possible. This point has been reinforced for slug control by our work in Penn State's Sustainable Dairy Cropping Systems project, which is lead by Heather Karsten in the Department of Plant Science. In this project, a large group of scientists is studying two diverse, six-year rotations that include cover crops and perennially hay (alfalfa or alfalfa/grass mix) and we are comparing these rotations to a two-year, corn-soybean rotation without cover crops. We have found that slug populations are significantly lower in the more diverse rotations than the two-year rotation. There are likely multiple causes for the larger populations in the two-year rotation and I will address two.
First, the rotation itself likely helps because it disrupts life cycles and generally makes 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, four-year is better than three-year (and so on); pest control capacity increases with duration of the rotation. Second, aside from disrupting pest populations, diverse rotations also foster improved populations of beneficial arthropods that can assist with pest control. Simple two-year rotations (including ours) 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 these beneficial arthropod populations by providing more varied habitats, particularly when they include cover crops and/or perennially hay crops. The more you can grow these populations by diversifying and using insecticides only when necessary (even seed treatments), the more help you will get help against your slugs populations. To be clear, I am not advocating for no insecticides, I would like folks to use 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, and then spraying the cover crop with an herbicide (often glyphosate) one to seven days after planting, so the cover crops dies slowly. This planting strategy is not for the faint of heart and often requires some mentorship by an accomplished practitioner, but anecdotally during the spring 2017 season, those farmers that planted green appear to have had less of a challenge from slugs. Our research is continuing to look into how this works, but preliminarily it seems that planting green gives slugs an alternative food source (slugs prefer the dying cover crop, often cereal rye, over the growing cash crop) while fostering improved, natural-enemy populations, particularly ground beetles can help suppress slug populations. These beetles are vital because they eat slugs, but importantly their populations can be suppressed by insecticide use, including seeds treated with neonicotinoid insecticides, so if you are interested in planting green, untreated seeds and IPM are the best companions for this approach. 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 I am becoming more convinced that it is a viable approach to 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. I welcome your feedback.