As we near the end of our slug monitoring project in many counties (monitoring for 6 weeks after plant), we continue to observe warm temperatures, and as a result slug numbers remain low under the shingle traps. Low numbers don’t necessarily mean slugs are no longer around, they mean the slugs are not as active and plants are likely outgrowing feeding damage. If you are observing defoliation on your soybean plants, use the guide provided in Figure 1 to determine if treatments may be necessary. Soybean defoliation thresholds are dependent on the plants growth stage (Figure 1). For instance, plants from V1 (first trifoliate) – R2 (full bloom) have a 30% defoliation threshold before yield loss becomes an issue. In Ohio, many soybean fields are past VE (cotyledon emergence) – VC (unifoliate leaves unrolled), which is most susceptible to slug damage, because the growing point of the plant is within the emerging cotyledons. If your plants succumbed to slug damage at the VE – VC stage, it may be necessary to replant. If you decide to replant with no other treatments against slugs, keep a close eye on the seedlings because slugs remain in the field year-round and may continue feeding on the newly planted seeds.
For advice on soybean replant decisions, view our previous newsletter article:
Bait Treatment Options
As mentioned in previous newsletters, there are not many options available for slug management in Ohio. One option is baits containing metaldehyde or iron phosphate. Before applying baits, it is important to confirm slugs are causing the defoliation in your field. Slugs are nocturnal, making them difficult to spot during the day (especially with the increasing heat). To get a feel for your slug population look for the slugs by carefully examining the soil surface, brushing aside residue and debris. You can put down square foot sections of plywood at a few locations in the field (marked with flags) and check under them periodically in the morning. Unfortunately, we do not have a threshold recommendation for how many slugs is too many, but monitoring the population will give you a feel for whether the problem is increasing or decreasing. If you decide an application is necessary, apply the baits in dry weather, when rain is not in the forecast to avoid the pellets breaking down.
Metaldehyde (3.24 and 4%) remains the most effective bait treatment to date. Concentrations are most effective when applying 10 pounds per acre for soybeans and 25 pounds per acre for corn (spread pellets at a rate of 5 – 12 pieces per square foot). Metaldehyde is labeled in Ohio for both crops, but not labeled for both crops in all states, so be sure to check the label before application. An alternative to metaldehyde is iron phosphate, but it is not as effective. The advantage of iron phosphate is it is approved for use in organic crops.
For more information on slug management, visit our field crop slug factsheet at https://aginsects.osu.edu/sites/aginsects/files/imce/Slugs%20on%20Field%20Crops%20_%20Ohioline.pdf
Ohio Monitoring Update
The number of counties monitoring slugs in Ohio for the multi-state project funded by the United Soybean Board is starting to decrease, as many counties have already reached the monitoring expectation of monitoring for 6 weeks after plant.
The map below shows the average number of slugs found under the shingle trap in each county from June 24th – June 30th (Figure 2). With the high temperatures, the average number of slugs in the majority of counties decreased. Seneca county had the highest average of 10.2 slugs/shingle over the past week, followed by Allen county with 0.8 slugs/shingle. The majority of counties in Ohio continue to report low numbers of slugs under the shingle traps; however, slug activity varies greatly from field to field, and all no-till fields planted recently should be scouted for slugs – especially fields that are newly planted or in the VE – VC growth stage.
Slug Monitoring in Ohio
June 24th – June 30th