Benefit To Soybean Farmers
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was the most supportive of slugs, but crimson clover was the least supportive of the cover crops tested (Hiltpold, unpublished).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.