This is an updated proposal for the 2020 proposed project “Cover Crop Selection and Termination Implications for Slugs”. Funding was approved for year 1; this updated proposal requests funding for year 2 of the project. The amount requested for the proposed work has been adjusted based upon anticipated expenses from the current project. Four species of cover crop (rye, barley, crimson clover, tillage radish) were broadcast at three locations in Sussex County in October 2021. Plots measured 60’ x 100’. Shingle traps have been deployed and slug populations are being monitored. Monitoring will continue in 2021 with weekly sampling from March to the end of May. In September 2022, cover crops will be sown in the same plots as 2021 and also in at least 1 additional plot in New Castle County with larger plots than the 2021 plots.

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

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.

Updated February 2, 2023:
Data for the cover crop experiments from 2021-2022 were entered and analyzed showing no significant effect of cover crop species on slug numbers under shingles. The 2022 experiment was modified to increase the number of shingles to 4 per plot based upon Spring 2022 observations. The three cover crop sites used in 2021-2022 were reseeded again in 2022 with the additional substitution of canola for tillage radish due to the tillage radish winter-killing in 2021-2022. Two additional sites were installed near Middletown DE for the first time, both following corn. Middletown 1 was seeded September 21, Middletown 2 was seeded September 28. Both were broadcast and had excellent stand establishment. Harbeson was seeded on September 21 and incorporated September 22. Georgetown was seeded October 17 and Lewes was seeded October 21 just before double crop soybean harvest. Both the Georgetown and Lewes sites had poor canola establishment. Shingles were deployed at the Middletown and Harbeson sites October 25 and October 26; Georgetown on November 8 and Lewes on November 22. At least 1 slug sampling event has been conducted to date. Preliminary observations suggest that slugs have been feeding more heavily on canola than the other cover crops, and late season imported cabbageworm and diamondback moth activity continued into December.

Updated February 9, 2023:
Five sites have been established. Two sites - Georgetown and Lewes, had poor canola establishment. All other sites have good cover crop establishments. All sites have been visited at least twice. The Harbeson site has considerably more slug activity thus far than it did in 2021-2022. Significantly more slugs were found in the rye plots over all others on January 2. Over the course of the sampling dates thus far, the Georgetown site also has greater slug abundance in rye plots. However, at the Lewes site although there have been no significant treatment differences, the crimson clover sites have numerically more slugs than other plots. Slug scouting will resume between the end of February and the beginning of March.

Updated February 2, 2023:

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In year 1 of the project, different cover crop species were sown in large plots on cooperator farm fields. No significant slug populations were detected in the various species plots. Plots were re-seeded in Fall 2022 to see if populations will increase gradually overtime.

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.

There has been a heavy emphasis over the last several decades to improve soil health by reducing soil disturbance and implementing cover crops. Slug populations are favored by no-till environments and emerged in the 1980’s as major pests of both field corn and soybean (Hammond 1985). However, reduced tillage and cover crop presence also favors slug natural enemies.

Furthermore, not all cover crops are of equal benefit to slugs. Laboratory feeding studies have demonstrated differences in cover crop palatability and slug survivorship. Brooks et al (2003) demonstrated that among the legumes, red clover was the most attractive to gray garden slugs and concluded that it could be useful as a diversionary plant in wheat fields. In their study, vetches were among the least attractive plant species. In another laboratory evaluation, brassicas resulted in the greatest slug survivorship while legumes were lowest. Among the legumes, hairy vetch was the most supportive of slugs, but crimson clover was the least supportive of the cover crops tested (Hiltpold, unpublished). In choice tests, slugs fed upon soybeans the least when given a choice with hairy vetch and with daikon radish, while soybean feeding was greatest in the presence of rye and crimson clover (Brichler 2020).

Field experiments also show inconsistencies with slugs and cover crops. Rivers et al. 2020 observed greater slug damage to field crops when planted into a hairy vetch/triticale cover crop than a cereal rye cover crop. In contrast, Mangan et al. (1995) observed more slugs in rye plots than in vetch containing plots when cover crops were tested for brassica production, and Busch et al. (2019) observed no difference between high diversity and low diversity crop rotations,.

Once it is time for spring planting, the farmer needs to put a cover crop management and termination plan into place. Traditionally, it has been recommended to terminate a cover crop at least 3 weeks prior to planting for both agronomic and insect pest management concerns. Small insect larvae developing in the cover crop may starve to death prior to the cash crop emerging from the soil. Conversely, recent research in Pennsylvania has suggested that several years of delayed termination, along with reducing insecticide usage can favor natural enemy build up (Douglas et al. 2015). Slugs will also feed on the green cover crop while the soybeans are emerging, potentially reducing some of the feeding intensity on the cash crop. The rolled cover crop does provide a protective layer of mulch providing shelter to slugs. In 2020, some producers trying delayed termination tactics either baited fields or had to replant, and year to year variation can sometimes result in more activity late than early (Rivers et al. 2020). Delayed termination effects on slugs can be variable (Reed et al. 2019). One grower in our spring slug survey had extremely high marsh slug counts up until he terminated his cover crop using vertical tillage. After the vertical tillage, slug counts plummeted and there was virtually no damage on the emerging corn. Whalen and Cissel (2011) reported large numeric differences in most sample dates from 4 locations testing vertical tillage. This proposed research seeks to build on both cover crop feeding research and termination studies by planting cover crops on producer fields and terminating them early or with vertical tillage.