Cool, wet spring weather in 2020 contributed to widespread slug injury to corn, soybean, and even sorghum. The adoption of cover crops and no-till or conservation-tillage, together with high soil surface residue, particularly after a corn crop, favors slug populations. There has been much recent interest in how cover crops and cover crop management might influence slug damage to the cash crop. In field and laboratory experiments, slugs demonstrate feeding preferences among plant species. Delayed cover crop termination has been suggested as a means of retaining beneficial predators in a field and giving slugs alternative food sources to concentrate on rather than the cash crop. It is possible though that some cover crop species may be more nutritious and therefore be more stimulating to slugs than others. Delayed cover crop termination also results in greater biomass which can provide favorable slug microclimates. Some of this residue could be incorporated through vertical tillage. Previous work in Delaware showed a reduction in slug activity following vertical tillage in fields without late cover crops. Vertical tillage also had limited impact on beneficial insects. The objectives of this proposal will build on that research by examining vertical tillage in delayed-termination cover crop plots, as well as examining cover crop species for their impact on slug population.

Large field plots will be planted on two cooperator farms in the same location of the field in 2021 and 2022. At each location, 4 cover crops (rye, barley, crimson clover, tillage radish and winter rape, will be planted in October. Plots will be at least 50’ x 100’. In the spring, plots will be subdivided with half being terminated early and half terminated late along with vertical tillage. Four plots will be left bare ground as a no-cover control. Plots will be sampled in the fall for slug activity and then in the spring starting in early March. Sampling will consist of weekly inspection of 2 shingles and a square meter soil residue sample in the center of each plot until the cash crop has outgrown the potential for slug damage (V5 for corn, V1 for soybean). The cash crop will be rated for slug injury and stand loss.

1) Examine potential influences between cover crop species and slug populations.
2) Investigate vertical tillage impacts on slugs.

Results will be interpreted as risk or non-risk of various cover crop species and delivered to farmers.

Updated August 1, 2021:
Cover crop seed is being ordered. Cover crops will be planted at participating farms in October.

Updated December 31, 2021:
Three sites in Sussex County were selected and sown with tillage radish, crimson clover, barley, and rye. Plot sizes were 60-ft by 100-ft. At each site and in each plot, two shingle traps have been placed to monitor slugs for the fall. Currently, sites have been visited three times to record slug numbers under shingles, with a fourth visit planned for the week of January 3. Site 1 cover crops were sown on a farm with a history of marsh slugs, although the exact field used has not been surveyed previously. Cover crops following lima bean were sown on 29 Sept. with an Earthway broadcast hand seeder. The cooperator farmer incorporated cover crop seed on 30 Sept. Previous herbicide use has impacted cover crop growth. Site 2 cover crops were sown in a field with a history of marsh slugs and treatments to mitigate slug damage to soybean, including tillage and bait. Cover crops were sown on 1 October and incorporated by the cooperator farmer on 2 October. Site 3 cover crops were sown in a field with a history of both marsh and gray garden slugs and has been used in previous extension projects. Cover crops were sown on 8 October and were not incorporated.

Funds for the project have so far been used for wages for a technician assisting with plot layout and sowing and for cover crop seed.

Updated October 18, 2022:

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Slugs are a perennial threat to between 10 and 20% of Delaware soybean (Musser et al. 2018, 2019, 2020), depending on the year and weather conditions. Slugs are most problematic in no-till fields with high residue, especially in years with mild winters and cool, wet springs (Douglas and Tooker 2012). Slug feeding can be severe enough to require replanting. Remedial chemical management of slugs is difficult due to application cost, equipment requirements, and uncertainties regarding timing and necessity. By the time serious slug damage to a soybean stand becomes noticeable, it is unlikely that a bait application will be able to rescue enough of the stand to prevent a replant. Thus, preventative or cultural management strategies need to be developed and refined for slug management.