By Dusty Sonnenberg, CCA, Ohio Field Leader: a project of the Ohio Soybean Council and soybean checkoff
Farmers plant cover crops for a number of reasons. Improving soil health, increasing water infiltration, reducing soil erosion, enhancing soil life and microbial biodiversity and breaking up soil compaction layers are just a few benefits cover crops provide. It is often said that to ensure a successful cover crop stand, a farmer should to be just as intentional when planting and establishing a cover crop as they are for their cash crops. One factor not often considered when establishing a cover crop is the threat of slugs.
Liz Bosak, an Extension Educator in Perry County with Penn State University, was recently featured on the “Cover Crop Strategies’ podcast discussing when during the growing season to look out for slugs, how slugs damage cash crops and cover crops, the weather conditions slugs prefer, and more.
“Slugs should be scouted for in the spring, once daytime temperatures are in the 50-degree range. Slugs prefer cool rainy and cloudy weather in both the spring and fall,” Bosak said. “These are a good times to be scouting for slugs. Sudden intense rainfall followed by dry warmer weather is not the most favorable for slugs, but continued cool and rainy weather is preferred.”
Cover crops are most at risk in the fall when they are planted, and the weather turns cool and wet as the cover crop seedlings are emerging.
“Slugs go after the seeding stage of plants. They favor seedlings versus older plants. They do not have a preference for either corn or soybeans, one over another. There are plenty of studies evaluating feeding preferences for other plant species,” Bosak said. “Studies show that there are some preferences for different plants. The level of slug feeding damage depends what the plant species is and the age of the plant, and even how old the seedling is.
“Slugs will feed on cash crop seedlings as they are emerging. Depending on how many slugs are present also determines the level of damage. Under high slug populations, a slug can feed on soybean leaves until only the stub of the stem remains above the soil line. If a corn and soybean field are located side by side, the damage can appear grater in soybeans than corn because the growing point of the soybean is at the top where the slugs are feeding, as compared to the corn plant. Under heavy slug populations in a soybean field, the growing point can be consumed, and then the seedling stops growing.”
For corn, the growing point is at the bottom of the plant until about V6, so if the slugs clip the seedling off, the growing point may still be at ground level, and recover.
“Slugs like to hide in an open seed furrow caused by planting too wet, and the seed furrow not closing,” Bosak said. “Heavy crop residue, especially if it is plugged into the open seed furrow is favorable for slugs. In good growing conditions, a crop can outgrow the slug feeding. Under heavy slug feeding conditions, and cool wet weather, the slugs can destroy a stand, and cause the need for the crop to be replanted. When soil conditions are dryer and more favorable for the planter closing the seed furrow, and there is wind, there is less likelihood for the threat of slug damage to the crop.”
When cover crops are emerging in the fall, slug damage to cover crops can appear similar to cash crops.
“The feeding intensity is dependent on what crop is in the field to eat, and at what stage the plant is at,” Bosak said. “If they feed heavily and lay eggs in the soil of a cover crop field in the fall, those eggs can emerge in the spring, and the slugs will move to a newly seeded and emerging cash crop. This is one reason why some farmers equate cover crops with an increased chance of slug issues, but this is not always the case.”
If the cover crop is seeded late in the fall, and does not emerge until the spring, the slugs may feed on the cover crops seedlings in the spring.
Planting green into a cover crop can potentially mitigate slug issues.
“Observations have found that once a cash crop is planted green into a cover crop and then the cover crop is terminated, this can potentially be a positive or a negative,” Bosak said. “The thought is that if slugs are present in the field, then they will feed on the cover crop while the cash crop is emerging, and the cash crops will grow fast enough to avoid serious damage. There is also the thought that the cover crops could shelter and favor predator insects to the slugs.”
Other observations have not found this to hold true in all cases, and slug feeding in the green planted fields did occur.
“Slug problems do not happen every single year. Using a no-till system that has cover crops incorporated is a great way to improve soils. Avoiding cover crops with the thought of avoiding slug feeding to the cash crop is not a certain strategy. There are cover crop fields that have slug issues, and there are cover crop fields that have absolutely no slug issues,” Bosak said. “The focus should be on what reasons a farmer first selects to grow cover crops and desired benefits.”