Grey garden slug adult found under a refuge trap.

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The Pennsylvania Slug Project is back for another year as part of the Soybean On-Farm Network, funded by the Pennsylvania Soybean Board. Penn State Extension educators throughout the state are beginning to monitor for slugs and will be regularly sharing in this newsletter their observations and the risk posed by slugs to no-till crop fields. This year, we are trying to collect data early and focusing our efforts on the gray garden slug. Our goal is to develop a model to predict when gray garden slug egg hatch occurs in the spring and provide information on slug outbreaks and risk across the state.

As planting gets started and progresses, be prepared for slugs if there are wet and cool weather conditions. Four slug species can be encountered in Pennsylvania field crops and are monitored in our project. See our Slugs as Pests of Field Crops fact sheet for more details. If you want to deploy slug traps on your farm, check out our Scouting for Slugs in Field Crops video.

Slug Management

Our research indicates that managing slugs requires an integrated pest management (IPM) approach that should be planned well before the spring planting season. The most diverse rotations tend to experience the fewest slugs, and adding cover crops can increase rotational diversity. Our research also shows that higher ground beetle populations can help suppress slug populations. Populations of these beetle predators are reduced by insecticide use, including broadcast applications and insecticidal seed coatings.

For tips on how to manage slugs in the spring, see our article on Best Management Practices at Planting to Reduce Slug Damage. If you are monitoring slugs with refuge traps and average 1–2 slugs per trap and, there is severe slug feeding damage or dying seedlings, a slug bait is justified. Slug baits are most economical as a rescue treatment in portions of fields that are experiencing severe slug infestations. These baits can be ineffective, especially when it rains.

Week of June 10, 2025

This will be the last report for the season. Grey garden slugs have continued to decline and were in low numbers across the state, with minimal damage reported. Marsh slug numbers also remained low and were found in Berks, Centre, and Chester Counties. If soybeans have not emerged yet in some fields, continue to scout over the next few weeks. Slugs tend to disappear in the heat of the summer and return in the fall.

Figure 1. Average grey garden slugs captured from June 4-10. The number on the left indicates the average grey garden slug count per trap; the number on the right indicates the total traps set up in that county.

Week of June 3, 2025

Grey garden slugs have started to decline in numbers across the state. Soybean defoliation remains low at most locations. Franklin County continued to see grey garden slug numbers decrease compared to past weeks, with damage not quite reaching thresholds. Marsh slug numbers have also declined and were found in low numbers in Huntingdon County. With planting delayed in many areas due to wet weather, continue to scout newly emerged fields for slug damage.

Figure 2. Average grey garden slugs captured from May 28-June 2. The number on the left indicates the average grey garden slug count per trap; the number on the right indicates the total traps set up in that county.

Week of May 27, 2025

Grey garden slugs continue to be active. Soybean defoliation remained high in York County. Indiana County continues to have high numbers of grey garden slugs with little damage. Franklin County saw grey garden slug numbers decrease compared to past weeks, with damage not quite reaching thresholds. Marsh slugs were found in low numbers in Bradford, Centre, and Chester Counties. Continue to assess your situation to determine whether slug baits are justified. Remember to watch the weather forecast if you plan to use baits. Rain will wash the baits away, rendering them ineffective.

PA map showing average grey garden slugs captured from May 21-27 by county — Figure 3. Average grey garden slugs captured from May 21-27. The number on the left indicates the average grey garden slug count per trap; the number on the right indicates the total traps set up in that county.

Week of May 20, 2025

Grey garden slugs continue to be active and have been observed causing damage to soybeans. Low numbers of grey garden slugs were found in York County this week, however, one field had severe damage estimated at 50% defoliation across the field. Slug populations increased above threshold numbers in Indiana County, but little damage has been observed so far. If you are seeing 1-2 slugs per refuge trap and have severe slug defoliation (the early-season defoliation threshold for soybeans is 25%) in your field, a slug bait would be justified. Remember to watch the weather forecast if you plan to use baits. Rain will wash the baits away, rendering them ineffective.

PA map showing average grey garden slugs captured from May 14-20 by county — Figure 4. Average grey garden slugs captured from May 14-20. The number on the left indicates the average grey garden slug count per trap; the number on the right indicates the total traps set up in that county.

Week of May 13, 2025

Juvenile grey garden slugs continue to be active as soybeans emerge. Although numbers have reached threshold in Franklin County and are getting close to threshold in Dauphin County, there has been little feeding damage observed. The majority of grey garden slugs found this week were juveniles. Marsh slugs were found in low numbers in Bradford, Chester, Centre, Dauphin, Huntingdon, Indiana, and Venango Counties. Slug eggs were found in York County. With rain forecasted through the week, it would be best to hold off on applying slug bait at this time.

PA map showing average grey garden slugs captured from May 7-13 by county — Figure 5. Average grey garden slugs captured from May 7-13. The number on the left indicates the average grey garden slug count per trap; the number on the right indicates the total traps set up in that county.

Week of May 6, 2025

Juvenile grey garden slugs increased in numbers in Franklin and York Counties. Although numbers increased and are starting to reach threshold in Franklin and York Counties, minimal feeding damage has been observed. Grey garden slugs are continuing to hatch out in southern to central Pennsylvania. Adult grey garden slugs were observed laying eggs in Berks County. Slug eggs were also found in Centre County. Other slug species were noted in small numbers at field sites, with the exception of marsh slugs, averaging 1 slug per trap in Centre County.

PA map showing average grey garden slugs captured from May 1-6 by county — Figure 6. Average grey garden slugs captured from May 1-6. The number on the left indicates the average grey garden slug count per trap; the number on the right indicates the total traps set up in that county.

Grey garden slug laying eggs — Grey garden slug actively laying eggs. Image Credit: Sarah Frame, Penn State Extension.

Week of April 29, 2025

Juvenile grey garden slugs were found in Franklin, Berks, and Huntingdon Counties. Grey garden slugs are continuing to hatch out in southern to central Pennsylvania. Other slug species were noted in small numbers at field sites, with the exception of marsh slugs, averaging 2.8 slugs per trap in Centre County.

Figure 7. Average grey garden slugs captured from April 25-29. The number on the left indicates the average grey garden slug count per trap; the number on the right indicates the total traps set up in that county.

Week of April 22, 2025

Most grey garden slugs found this week were large adults; however, juvenile grey garden slugs were found in Franklin and York Counties. This indicates that grey garden slugs are hatching in the southern part of the state. Other slug species were noted in small numbers at field sites, with the exception of marsh slugs, averaging 2.7 slugs per trap in Centre County.

Map of Pennsylvania showing average grey garden slug captured from April 21-22 — Figure 8. Average grey garden slugs captured from April 21-22. The number on the left indicates the average grey garden slug count per trap; the number on the right indicates the total traps set up in that county.

Week of April 15, 2025

Few slugs have been reported so far this year. All sites were reporting less than one slug per trap. All of the grey garden slugs were large adults, indicating that they overwintered.