Slug Damage to Soybeans—Do Cover Crops Help or Hurt?
Figure 1. A meadow slug feeding on an emerging soybean.
Updated: November 16, 2021
, Sydney Garbitzer
, and Ray Weil
Slug Damage to Soybeans—Do Cover Crops Help or Hurt?
Reports of slug damage to emerging soybean seedlings (Glycine max) have increased in recent years, though their activity is highly variable from field to field, and within a field from year to year. Spring, 2021 weather was both cool and wet, conditions that many slug species find ideal. Slug eggs overwinter in the soil, hatch in the spring, and the juvenile slugs can begin causing damage to crops in just a week’s time after hatching (Hammond et al. 2009). Because slugs lay eggs at various times of the year, it can be difficult to predict when slugs will become a problem, but in general, they have higher activity when temperatures are relatively mild during the spring months of April-June and then in the fall months of September - October (Godan 1983). Managing slugs in conservation tillage and no-till systems can be especially challenging because slugs do well in the absence of tillage that disrupts their lifecycle and crop residues on the soil surface provide them with excellent habitat and hiding places (Horowitz et al. 2010). Corn residues often provide better slug habitat than soybean residues.
The type of slugs that are commonly found in field crops in the Mid-Atlantic, (Douglas & Tooker, 2021) include: meadow slug (Deroceras leavae) see Figure 1, gray garden slug (Deroceras reticulatum), dusky slug (Arion subfuscus), and the banded slug (Arion fasciatus).
Soybean is more vulnerable to slug damage than corn and is especially vulnerable to slugs in the early seedling stages. Slugs feeding during this time can “decapitate” a soybean plant by eating the apical meristem or “growing point”, killing the whole plant. Thus, slugs can rapidly reduce the number of soybean plants surviving in a stand. Corn seedlings with the growing point below the soil surface are more likely to outgrow slug damage. Slugs can feed on soybean seeds before they even germinate, but also when the soybeans have spread their first cotyledons and leaves. If the soybean plant survives long enough to put out true trifoliate leaves and the apical meristem stays intact, the plant may continue growing even as slugs cause significant defoliation (Hammond, 2000).
With the increasing adoption of cover crops and their importance for soil health, many farmers are wondering if cover crops may make slug problems worse. We have been conducting a set of cover crop experiments on a pair of fields at the Central Maryland Research and Education Center Beltsville facility for several years. While these experiments were designed to study cover crop management effects on nitrogen cycling and losses, we noticed a substantial slug population present in one of the fields in early April 2021. We used this opportunity to study the effects of cover crop management on slug activity and damage to soybean seedlings.
This study focused on the impact of cover crops on the slug damage sustained by soybean seedlings during the critical early seedling growth stage. Data was collected in a field with 2 to 5% slopes and dominated by Christiana-Russett soils which are slowly drained with a silt loam topsoil over a silty clay loam subsoil. The field had large plots, from which corn or soybean had been harvested in fall 2020.
These plots were split into three subplots (9.1 m x 54.9 m) with either No cover, 3-way (a radish, rye, crimson clover mixed-species cover crop), or Rye (cereal rye cover crop). In the plots that had grown corn in 2020, these cover crop treatment plots were further subdivided into sub-subplots (9.1m x 18.2 ft) in which the cover crop (or weed growth in the No cover) was terminated with glyphosate herbicide either Early (April 7th), Mid (April 26th) or Late (May 13th). The whole experiment was replicated four times. On May 6th soybean was no-till planted in plots that had corn stubble from the previous year and corn was planted on plots that had grown soybean the previous year. Ten days before the soybean and corn were planted, 40 asphalt shingles (28 cm x 30 cm) were pinned to the ground, one per plot, in all three termination-date sub-sub-plots in the rye and 3-way cover crop treatments and in the mid termination date in the no-cover plots (28 sub-sub-plots) as well as in all three cover crop treatments in the corn plots (an additional 12 sub-plots). The shingles served as devices for counting slugs: all slugs present under the shingle were counted in mid-morning every few days from 10 days before to 21 days after soybean and corn planting. Emerging soybean and corn seedlings in two 1-m sections of the row adjacent to each shingle were scored for slug damage on 5 dates and soybean stands were counted on June 3rd. In addition, each time slug data were collected, the soil temperature at the surface adjacent to the shingle was recorded using an infrared non-contact thermometer and the volumetric water content of the upper 5 cm of soil was measured using a capacitance probe (Meter Group ECHO5).
Once seedlings had emerged, slug damage was scored for all visible and counted seedlings on a scale of 1 to 5 (see Figure 2). The scoring was defined as: 1 (none), 2 (1 or 2 small bites, still healthy) 3 (cotyledons or hypocotyl damaged), 4 (severe damage, almost killed), 5 (stem or cotyledon eaten or broken off, the plant will die).
Once the soybeans had put out two true trifoliate leaves and were likely to outgrow further slug damage, stand counts were done by counting soybean plants in two 3-meter sections of the row near each shingle.
Slug Presence Before Crops Emerged Prior to crop emergence, slug counts were higher in soybean residue than in corn residue even though during this period the soil under corn residue was wetter than soil under soybean residue (Figure 3.). Prior to soy or corn crop emergence, the presence of the cover crops did not affect slug numbers (Figure 4).
Cover Crop Termination Date’s Impact on Slug Damage to Emerging Crop
Soybean damage scores averaged across rye and 3-species mix cover crops were lower in the latest killed cover crop (planted green) plots than in the early and mid killed plots, see Figure 6. Damage was similar in the mid killed and early killed plots. By the time trifoliate leaves developed, soybean stand counts were somewhat higher in late-kill “planted green'' plots (Figure 7). The cool spring conditions delayed soybean emergence until after the latekill cover had mostly desiccated.
The benefit of planting green may be greater under conditions better for rapid soybean germination and seedling growth.
Cover Crop Species’ Impact on Slug Damage to Emerging Crop
Studying the cover crop species impact on slugs showed interesting results. When comparing slug presence in no cover, rye, or the 3-way mix plots, the number of slugs were highest in the 3-way and lower in the no cover and rye plots. However, this pattern was not reflected in the damage inflicted by the slugs on the young soybeans. The most slug damage was sustained in the rye cover crop plots and lower in the no cover and 3-way (Figure 9). This suggests that the 3-way mix cover crop supported more slugs but tended to keep them off the soybeans while the rye was not able to do this.
Once the soybeans outgrew the slug susceptible seedling stage the stands varied by treatment from 30,000 to 50,000 plants/acre. This was low enough that we had to replant soybean to get a decent crop. Therefore, we are not able to report any effect of slug damage and cover crops on the soybean yield for 2021. We replanted soybeans as close to the original rows as possible when the first planting was in the 2nd trifoliate stage. This actually worked quite well (see Figure 8) and both the original and second planting soybeans grew well where weed pressure or waterlogging was not an issue.
By harvest time, soybean yields were much lower than normal (2500 kg/ha or 37 bu/acre) in the experimental field (7e, right side of Figure 10.), whether no cover crop, rye or the 3-way mix had preceded the crop (no cover crop effect). We ascribe the low yields to the extremely wet spring and summer conditions resulting in poor seed furrow closure, standing water for extensive periods, and high amounts of grassy weeds. A nearby sandy field (39A, left side of Figure 10) planted at the same time with the same cover crop and soybean seeds yielded more than normal (3900 kg/ha or 58 bu/acre). On the normally droughty sandy field, the unusually wet conditions were favorable to soybean growth and neither weeds nor slugs were a problem.
When terminated two weeks before soybean planting, the rye cover crop, but not the 3-way mix, seemed to make the slug damage worse than with no cover crop.
When averaged across both cover crop types, planting green with termination a week after planting resulted in a significant, though small, reduction in slug damage, but not numbers.
The number of slugs counted was not closely related to the amount of damage they caused to soybean seedlings.
The extremely cool and wet conditions after soybean planting were great for slugs and very poor for rapid soybean emergence and early growth.
Within the context of no-till soybeans, our data from a single site and year suggest that cover crops probably do not make slug damage worse, and termination after planting may be worth considering, especially if planting can be timed for weather warm enough to stimulate soybean emergence.
Douglas, M. R., & Tooker, J. F. (2012). Slug (Mollusca: Agriolimacidae, Arionidae) ecology and management in no-till field crops, with an emphasis on the mid-Atlantic region. Journal of Integrated Pest Management, 3(1), C1-C9. https://doi.org/10.1603/IPM11023
Hammond, R. B., Michel, A., & Eisley, J. B. (2009). Slugs on field crops. Fact Sheet FC-ENT-0020-09. The Ohio State Univ., Agricultural and Natural Resources Extension, Columbus. https://ohioline.osu.edu/factsheet/ENT-20
Horowitz J Ebel R Ueda K. (2010). “No-Till” farming is a growing practice. Economic Information Bulletin No. 70. USDA-ERS, Washington, DC
This article appears on November 2021, Volume 12, Issue 8 of the Agronomy news.
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