Scouting For Slugs This Fall Will Help Predict Spring Problems

Scouting for slugs in the fall will help you to determine if they’ll be a problem in your fields next spring. 

Slug problems mainly occur in no-till fields where heavy crop residue on the soil surface gives slugs a moist sheltered area to live and dodge the drying sun. Cool, wet conditions in the spring and fall also help them to venture out more, without having to worry about dehydrating from drier, sunnier conditions. We tend to see more plant injury in cool wet conditions when the slugs are more active than the crop that is trying to grow. Most of the serious slug injury is experienced in canola, soybeans, wheat underseeded with red clover, and newly seeded alfalfa. 

In the fall, adult slugs have one of two goals. For some of them, it is to lay eggs that will overwinter and hatch to become juveniles in the spring. For others, their priority is to get ready to overwinter so that they can come out next spring to lay their eggs. Regardless, they will all be there in the spring ready to feed on your emerging crop. So knowing what the population level is in your fields now will give a good indication of what will be there next spring. 

Scouting Methods

Fall scouting for slugs is not that labour intensive. There are two ways of trapping slugs, though some of you may feel that one method is a waste of good beer. The first option is to take small pieces of plywood (approximately 2-3 feet long) or roofing shingles and position them on the soil surface in fields that have been harvested. 10 to 15 boards randomly scattered across the field will provide a good indication of population levels. These boards will act as shelters for the slugs. Visit the boards every 5-7 days for the next month and count how many slugs you see under the boards. Morning is the best time to look since slugs will still be hanging out in their shelters. Putting flags near the boards will also help you to find the traps again. 

The second method is to dig holes the size of a plastic coffee cup randomly in your field. Place a plastic cup in this hole with a few inches of beer in the bottom of the cup. Position the cup so that the lip of the cup is level with or slightly below the soil surface. The beer will attract the slugs and they will drown in it (though they die happy!). Again, place flags next to these traps so that you can find them again. If it rains (which is highly likely this fall) empty these cups and refill them again with beer. Return to these traps every 5-7 days for the next month to determine the number of slugs that have drown themselves in beer. 

Action Thresholds

There are no thresholds, but if you are frequently finding slugs at these trap sites, you should consider that field at high risk for slug injury in the spring. Those fields with only a few slugs found are at less risk of having issues in the spring. These fields should again be scouted in the spring just to make sure there is little slug activity. 

Managing Potential Problems

Fields indicating a potential problem should be planted early next spring to help get the crop up and growing before slugs become active in late-May. Also, some form of tillage should be considered. Tillage will help the crop emerge more quickly and will also disturb the shelter sites that help these slugs survive. If these methods are not an option, then rotating into another crop in that field may be a wise decision. Although slugs will also feed on corn, it is a good option since the growing point is not affected and therefore it can grow out of the injury. One slug bait product is showing some promise, though it is not registered for field crops and is still very expensive when applied at the rates required. 

Given the wet fall that we have had, slugs are living the good life right now, ready and waiting for a crop to feed on. Determining which fields are at high risk of slug problems next spring will help you to stay one step (or should I say slime trail) ahead of them.

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