Slugs in Soybeans

Figure 1. Soybean cotyledons damaged by slugs. Image by farmer, Danny Liggett.

Chad Lee, Director, Grain and Forage Center of Excellence, University of Kentucky

Slugs are lurking in numerous fields in Kentucky. The weather, crop growth stage and soil conditions all have combined to make a perfect all-you-can-eat buffet for the slugs and a nightmare for little soybeans. Here are some very quick and general comments about slugs in soybeans.

Slimy Little Slugs

1. Slugs like cool, cloudy, moist weather. They are more active in the evening and early morning most days. On cool, cloudy days, they are active all day.
2. Slugs do not like warm, sunny weather. Slugs dry out too quickly under the sun. On such days, slugs seek the shade and hide under residue.
3. Cool, cloudy days make for slow crop growth. Warm sunny days with adequate soil moisture make for more rapid crop growth.

The Poor Innocent Young Soybean Plant

Soybeans are most sensitive to yield losses from slugs when the plants begin emergence. As the unifoliates unfold (VC growth stage), and then the first trifoliolate leaf (V1 growth stage) unfolds, the plants become less and less susceptible to stand losses and yield losses from slugs.

If you have emerging soybeans, cool, cloudy weather and heavy residue, this is a slug feeding fest. If the soybean is chewed below the cotyledons, then the plant will die. If the cotyledons are severely eaten, the young plant could die. If the cotyledons only have a couple holes from chewing, the plants are more likely to survive. Any feeding on cotyledons after the soybeans are at VC or V1 or later, then the feeding is harmless to the soybean plant. 

Getting Rid of the Slugs

Best options for control:

Warm weather and sunshine. Rapid growth and bright sunshine are the best defense against slugs. Until warm weather and sunshine occurs for a couple days, a farmer may need to try something else.

Slugs do not like any salt. The salt will desiccate the tiny critters in a hurry. Most of our fertilizers are salts. Apply the fertilizer when slugs are active and more likely to come into direct contact with the salt. The slugs must come into contact with the salt for it to work. If a fertilizer is applied and a half-inch of rain occurs, the salts will be diluted out and will not have any effect on the slugs. So, to make this work, timing is most critical. Options of fertilizers include:

• Dry potash (0-0-60). Apply about 30 to 40 pounds of product per acre (or more). While any salt fertilizer will work, potassium is preferred since any potassium not needed by the crop will remain in the soil for the next crop.
• Liquid UAN (28% or 32% N). Some farmers report using UAN as a carrier in their burndown and do not have slug problems. They are using about 10 to 15 gallons per acre. That rate would burn the leaves on the soybeans. It probably won’t kill the soybean plants, but while fighting slugs, we are trying to preserve as much leaf areas as possible. Burning the leaves may not be the best option now. Also, most herbicides that allow UAN to be the carrier, only allow UAN for preplant or preemergence applications. So, if you used UAN, you could not mix most herbicides in the tank.
• Other fertilizers could work if they are applied at similar rates to those fertilizers mentioned above. Most foliar fertilizers contain too little salt to burn plant leaves and too little salt to kill slugs.

Slug baits and slug-icides. There are some products registered for this. I’m not sure they have much better success than the salts.

Beer in tin pans. Apparently, the beer will attract the slugs into the tin pans and the alcohol combined with potassium salt in the beer will kill the slugs. This seems logical. Reports on the success of this are sketchy and lead one to wonder if the beer ever made it to the tin pans… or if this was simply a way to write-off beer as a “business expense”.  

To Replant or Not to Replant?

Some soybean fields will have reduced stands. Generally, soybean stands will need to be below about 50,000 to 75,000 plants per acre to justify a replanting. Generally, most farmers cannot tolerate soybean stands that low.

Since most farmers are used to planting double crop soybeans and/or river bottom fields into mid-June, they will likely want to replant even if we think a stand will produce a reasonable yield.

Three scenarios where a replant is almost always justified.

1. If the field is on a major road.
2. If the field is next to your farm shop or your house.
3. If the field is close to a landlord’s house. 

Take Home

Slugs are a nuisance and take a make a beautiful field of soybeans ugly.

Warm weather and sunshine are the best control method. Fertilizers salts are an option, but require excellent timing.

Once soybeans get to about VC or V1, the risk damage from slugs is minimal.

Figure 2. Soybean damaged beneath the cotyledon by slugs. This plant will not survive. Image by farmer, Danny Liggett.

Figure 3. Slug feeding on soybean cotyledons. This plant likely will not survive. Image by UK assistant professor, Erin Haramoto.

Figure 4. Soybean cotyledons damaged by slugs. The plant second from left most likely will not survive (if feeding stopped and had the plant remained in the field). The other three plants likely would have survived. Image by farmer, Steve Carver.

Figure 5. Soybean yield response to final plant populations. Maximum yields in high-yield environments can be achieved at populations less than 100,000 plants per acre.