As farmers strive to implement best management practices to improve nutrient management, there are sometimes unforeseen consequences that challenge those practices.
A good example is no-till or reduced-till fields and the common pests, slugs. To many crop producers this is a “four letter word.”
Slugs are particularly prevalent in fields with heavy residue and little soil disturbance. They can eat virtually all crops and inflict most of their damage during crop establishment and early growth in the spring and fall.
This damage tends to be most severe under cool, wet conditions, which slow crop growth and favor slug activity. Slugs typically feed at night and hide in residue or soil during the day, so it can be challenging to assess their activity.
The increased adoption of no-till methods in recent years, as well as the limited control options available to no-till farmers, has elevated the importance of slugs as pests of field crops in Pennsylvania.
John F. Tooker and his graduate student Maggie Douglas from the Department of Entomology have been participating in a Sustainable Dairy Cropping Systems project funded by Northeast SARE/USDA.
They have been assessing under very diverse cropping strategies the impact slugs have on field crops. The biggest challenge is the limited management options available. It is for this reason that the Penn State researchers are examining integrated approaches to control slugs.
Although tillage will control slugs, many producers who use no-till are committed to the practice because of the economic and conservation benefits it provides.
That said, practices that reduce the amount of surface residue will decrease slug populations. For example, shallow disking (3 inches deep) or “turbo tilling” can significantly decrease slug populations. High-residue cultivation also shows some promise in depressing slug populations.
Other approaches to slug management involve cultural practices. Because older crop plants are not as susceptible to slug feeding as young plants, several management tactics aim to foster early plant growth to get crops growing as quickly as possible to try to “outrun” the slug threat.
For example, early planting may give crops a jump on slugs if crops emerge before eggs hatch in large numbers, particularly for crops such as alfalfa that can be planted in very early spring.
Farmers can monitor slugs using simple shelter traps to get an idea of when slug activity picks up in their regions relative to crop planting dates.
Also, using row cleaners on the front of planters to move crop residue away from the row allows sunlight more access to the soil, increasing temperatures and improving emergence.
Growers can further contribute to better early growth by selecting crop varieties that are rated “excellent” for emergence and seedling vigor.
Good agronomic practices such as ensuring seed slots are closed can mitigate some slug damage, since slugs use open seed slots as “highways” to feed on germinating seedlings.
Choice of crop rotation and cover crop can also influence slug pressure, but more research is needed before specific recommendations can be made on these factors.
Slugs do have predators, including ground beetles, rove beetles, centipedes, harvestmen (aka daddy longlegs), firefly larvae (aka glow worms), soldier beetle larvae, birds and frogs.
Invertebrate predators of slugs can be conserved by increasing crop diversity, using cover crops, and using insecticides sparingly (e.g., banding insecticides directly over the row rather than broadcasting it over the entire field) and in accordance with integrated pest management principles.
Few chemical controls are available for slugs. Metaldehyde-based baits can be used in many crops, but are sometimes not economical, and significant rains can wash away much of the product.
Because slug control can be frustrating, some growers have experimented with home remedies. Chief among these is spraying crops at night with nitrogen solutions, which act as a contact poison and burn slugs.
A common approach is to use a 30 percent urea-based nitrogen solution, mix it with an equal amount of water, and apply 20 gallons per acre. This tactic should be repeated a few nights in a row to maximize its effectiveness, because nitrogen solutions provide no residual control and all slugs in a field will not receive a killing dose in a single application.
Some farmers are experimenting with different strategies involving cover crops or companion plants to reduce the amount of feeding on the focal crop, often corn. Hopefully, developing research will provide no-till growers with a few more tools against these slimy pests.
More information on slugs can be found at http://ento.psu.edu/extension/factsheets/slugs-as-pests-of-field-crops or by contacting Tooker at tooker<\@>psu.edu or Douglas at mrd276<\@>psu.edu
Virginia “Ginny” Ishler is a nutrient management specialist and manager of Penn State University’s dairy complex.