Innovative agricultural producers in the North Central United States are constantly developing techniques and strategies for maintaining a profitable farm operation, while lessening negative environmental impacts. Pest management is an area of agricultural production that depends heavily on environmentally questionable techniques that damage non-targeted habitats and organisms. Ecological principles reveal that, when their basic needs are met, beneficial predatory invertebrates can prevent pest outbreaks through biological control. lnterseeding cover crops into stands of growing cash crops may help support predators in cropland through various means. The aim of this study is to assess how interseeding cover crops into standing corn affects beneficial and pest invertebrate communities, as well as farmer productivity and net profitability. Eight interseeded and non-interseeded plot pairs will be established on volunteer, producer-owned farms in Eastern South Dakota. Cover crop species will be seeded between corn rows without damaging the focal cash crop. Foliar and soil-surface dwelling invertebrate communities in both habitats will be examined three times, prior to corn canopy closure, during corn anthesis, and after corn harvest. In addition to characterization of invertebrate communities, generalist predator activity will be monitored by using sentinel prey items to gain a better understanding of the importance of in-field plant diversity on biological control of pests. Upon completion of field collections and data analyses, extensive outreach efforts including informational videos and research summaries in the popular press will be dispersed by soil health organizations and our local farmer network. As this is a partnership between a non-profit foundation, a land grant university, and agricultural producers, there will be a broad audience interested in the implications of our results for their own operations. Pest management is a major contributor to the cost of corn production in the U.S., and reducing input costs associated with managing pests has the potential to improve the resilience of corn production in the North Central Region. Current focus on valuing interseeding does not consider the pest management benefits of the interseed; our project will fill this critical information gap.
A growing number of agricultural producers recognize the importance of conserving natural resources and biodiversity on their farms to ensure operational sustainability and longevity. This research aims to provide farmers across the Midwest with applicable information regarding interseeding cover crops to be translated into major changes in insect pest management. Gaining an education about conservation biological control of potential pests will likely spark farmer’s interests in harnessing natural forces for the benefit of their operations rather than substituting natural forces with costly and harmful agrichemicals. Conducting this research on volunteer farmers’ land will mitigate the “ordeal of change” that so frequently prevents land owners from trying something new. Changing farmer’s attitudes, and forging relationships through this producer/researcher partnership will open avenues for further communication between stakeholders and scientists seeking to mitigate natural resource perturbation while maintaining profitability.
• Relative effects of adding interseeded species on predation of insect pests.
• Economic benefits of interseeded systems on reducing pest management input costs.
Producers will gain skills to plan and practice pest management using more tools than they previously had, resulting in less dependence on insecticidal inputs. Realization of plant diversity’s role in farmland productivity will propel farmers into seeking out opportunities to incorporate cover crops into their rotation whenever possible; benefiting not only pest management efforts but also erosion reduction, soil carbon sequestration, weed suppression etc. An important long-term outcome of the proposed research is building rural communities through land stewardship and opening previously unrealized economic opportunities.
In 2018, 36.4 million hectares of corn were planted in the United States (USDA-NASS, 2018), 4.7% of the entire landmass of the 48 contiguous states (USDA-ERS, 2019). Nearly all cornfields are planted under monoculture conditions, and these monocultures have replaced what was historically highly diverse perennial grasslands (Rashford et al., 2011; Wimberly et al., 2017). Plant-diverse habitats tend to support greater biodiversity of non-plant species (Schmid, 2014). In addition, diverse habitats perform numerous ecosystem functions which are lost or severely impaired under simplified agricultural production (DeFries et al., 2004; Fiedler et al., 2008). One such ecosystem function provided in plant-diverse environments is the maintenance of herbivore communities at sub-outbreak populations. When plant diversity is robust herbivore population growth is curtailed through multiple means including both top-down and bottom-up antagonisms (Landis et al., 2005; Moreira et al., 2016). Contrarily, in simplified landscapes, such as those hosting corn in monoculture, herbivores face few agents of biological control because the habitat does not provide necessary resources to maintain natural enemies. Farmers often resort to chemical control of herbivores in these enemy-free spaces (Fausti et al., 2018; Fausti et al., 2011). Adding plant diversity to would-be monocultures is gaining popularity amongst agricultural producers due to the positive effect of plant diversity on ecosystem functions including crop pest control (Gurr et al., 2017).
Despite widespread use of corn monocultures throughout industrialized agriculture, corn has historically been grown with other plant species, and this is a practice still commonly used in subsistence agriculture (Landon, 2008). Advancements in agricultural technology and research elucidating underlying plant synergisms has led to a recent increase in farmland being planted to multiple plant species simultaneously (CTIC, 2017). Corn producers are accomplishing this in their fields by planting cover crops between establish rows of corn. Cover crop species can be selectively chosen to add benefits to the ecosystem without interfering with crop growth or harvest.
Cover crops growing alongside corn have the potential to alter agroecosystem habitats in ways which make these production areas more suitable for a diversity of arthropod species, not just those whose life histories are supported by corn. Nectar and pollen produced by interseeded cover crops, and alternative prey species consuming cover crop tissues are all resources by which natural enemies can sustain themselves in the absence of crop pests (Lundgren, 2009; Manandhar and Wright, 2016). Adding plant diversity during the corn growing season by incorporating cover crop mixtures may provide a means by which farmers can mimic the diversity and functionality of natural systems row crops have replaced.
Very little work has been done to characterize the effect of added plant diversity through interseeding cover crops on invertebrate community structure and on important functional guilds in corn production systems. The main null hypothesis that we tested was that interseeded cover crops have no effect on insect community structure and the abundances of herbivores, predators, and detritivores. We also tested the null hypothesis that interseeded cover crops in corn do not affect predation rates on the soil surface. Finally, we examined whether these interseeded cover crops affected corn yield and plant density.
2.1 Arthropod communities in cornfields with bare soil or cover crops
2.1.1 Field sites
Research was conducted at three locations over two years. The first location was near Estelline, SD (44.58, -96.79), and two locations were near Gary, SD (44.91, -96.40 [Gary-17]; 44.92, -96.40 [Gary-18]). At study locations eight fields measuring 42 × 42 m each were established in a 2 × 4 orientation separated by 15 m borders.
At Estelline, corn (Elk Mound Seed Company, Elk Mound, WI, 54739; variety: EMS 8100; maturity: 80 d) was no-till planted on May 26, 2017 at 79,000 seeds/ha in 76 cm wide rows. Roundup™ (rate: 2338 mL/ha; a.i.: glyphosate; Monsanto™, St. Louis, MO, 63167) was applied as a preplant herbicide to eliminate emerged weeds. No post-emergent herbicides or fertilizers were used at Estelline (the farmer intended on using the land for fall animal grazing, not for grain harvest).
At Gary-17, corn (Legend Seeds, Inc.™, De Smet, SD, 57321; variety: A10946, A10258; maturity: 97 d) was planted on May 5, 2017 at the same density and row spacing as at Estelline. The field had been cultivated in the spring prior to planting to prepare the seed bed. SureStart II™ (rate: 2923 mL/ha; active ingredients: acetochlor, flumetsulam and clopyralid; Dow AgroSciences ™, Indianapolis, IN, 46268) was applied on May 1 as a pre-plant herbicide. Fertilizer was broadcasted into research fields at a rate of 157 kg nitrogen/ha as urea, 67 kg phosphorous/ha as MicroEssentials®SZ™ (Mosaic™, Plymouth, MN, 55441) and 56 kg potassium/ha as potash. Neither location was treated with insecticide.
At Gary-18, all plots were cultivated to prepare the seed bed. Corn (Blue River Organic Seed™; Ames, IA, 50014; variety: P1000684; maturity: 96 d) was planted on May 26, 2018 in 76 cm wide rows at a population of 79,000 seeds/ha. Research fields were sized and orientated similarly to those in Gary-17. On May 20, pre-emergent herbicide, SureStart II™ (rate: 2923 mL/ha), was applied in research fields with no further herbicide use. Fertilizer was broadcasted into fields at a rate of 157 kg/ha nitrogen as urea, 56 kg/ha potassium as potash and 56 kg/ha phosphorous as diammonium phosphate. No insecticides were used during 2018.
An eight-species cover crop mixture [coated in calcium carbonate (1: 1, seed: CaCO3, by weight)] of hairy vetch (Vicia villosa, 3.5 kg/ha), lentils (Lens culinaris, 3.5 kg/ha), mung beans (Vigna radiata, 5 kg/ha), oats (Avena sativa, 5 kg/ha), flax (Linum usitatissimum, 9 kg/ha), cereal rye (Secale cereale, 14.6 kg/ha) and field peas (Pisum sativum, 14.6 kg/ha) was planted into four of the established plots in an alternating pattern. At Estelline and Gary-17, this seeding was broadcasted immediately following corn emergence (Gary: May 16; Estelline: June 2). At Gary-18, seeds were planted following corn emergence using a seven-row homemade tractor-drawn cover crop interseeder (Figure 1). The device planted a single row of cover crops between each pair of 76 cm-spaced corn rows.
2.1.2 Insect sampling
Soil-dwelling arthropods were sampled four times in each plot during a given site year (corn stages: V2, V4, V8 and anthesis) by taking five soil cores (diameter: 11 cm, depth: 10 cm) within corn rows at random locations within a given plot. This resulted in a total of 160 cores taken from one site during a single field season, or, 480 cores across all three site years. Arthropods within collected soil were extracted for 7 d using Berlese funnels into 70% ethanol. Arthropods were stored in 70% ethanol until identification and curation.
Epigeic invertebrates were sampled three times (corn stages: V4, V8 and anthesis) during a growing season at each location. Manual aspirators were used to suck arthropods off the soil’s surface within confined 0.5 × 0.5 m sheet metal quadrats pressed > 2 cm into the soil to prevent arthropod escape. Four quadrat collections were taken from each plot on every sampling date for a total of 96 taken from one location over the course of a field season, and 288 samples across all three site years. Before leaving a sampled plot, arthropods from each quadrat were placed in 70% ethanol until identification.
Foliar arthropods were surveyed in research plots three times (corn stages: V4, V8 and anthesis) during each season by conducting whole-plant dissections (Lundgren et al., 2015). On a given sampling date 15 corn plants were collected from random locations within a plot (> 5 m in from the plot edge) and dissected on white cotton sheets. Invertebrates were identified upon sight to the lowest possible taxonomic unit and recorded. At each location a total of 360 corn plants were examined during a single season, or 1080 plants across all three site years.
Yield and plant density were measured following corn maturation. Corn ears were collected, and plants were counted within a 3 m row section at four points located 8, 16, 24 and 32 rows from the plot’s edge; row series were sampled so the final collection points represented a diagonal pattern across each field. Corn kernels were removed using a hand-sheller (item number: 530065; Premier1Supplies™, Washington, IA, 52353), and weighed (weights were adjusted to 15% moisture for comparison).
2.2 Predation of sentinel prey in corn monocultures and corn interseeded with cover crops
2.2.1 Field sites
Predation experiments were conducted at four separate locations, two in 2016, and two in 2017. The 2017 observations were made at the Estelline and Gary-17 experimental fields that are described above. In 2016, one site was located near Canby, MN (44.81, -96.36) and one site at the Dakota Lakes research farm near Pierre, SD (44.29, -100.00;). At Canby, alfalfa (Medicago sativa) had been established for 3 y when it was chisel plowed in the fall of 2015. The seed bed was cultivated in spring, which reduced the alfalfa stand density to approximately 10 plants/m2 growing under the corn canopy in the interseeded treatment. In corn monoculture fields at Canby alfalfa was terminated by rototilling prior to corn planting. Corn (Blue River Organic Seed™; Ames, IA, 50014; variety: P1000684; maturity: 96d) was planted in similarly sized and oriented fields as in Gary-17 on May 24, 2016 in 76 cm rows at a population of 76,600 seeds/ha. No fertilizer, herbicides or insecticides were used at Canby.
At Pierre, corn (Pioneer™, Johnston, IA, 50131; variety: P0533AM1; maturity: 105d) was planted on May 3, 2016 at a population of 94,000 seeds/ha. Seeds had been treated with the insecticide, clothianidin (0.25mg/seed). Every two corn rows, spaced by 55 cm, were separated by one row of Roundup Ready® alfalfa in interseeded plots. Monoculture and interseeded corn were planted in adjacent blocks where four research fields were established in each (similar size and separation to the other locations). Glyphosate (rate: 2338 mL/ha) was used for control of weeds at corn planting, and Brox™ 2EC (rate: 1169 mL/ha; active ingredient: bromoxynil; AgriStar ™, St Joseph, MO, 64504) was sprayed on June 8 to suppress alfalfa growth. Fertilizer was side-banded at planting as 50.5 kg N/ha as a blend of urea and ammonium sulfate (9: 1, by weight, respectively), and 88 kg/ha of a monoammonium phosphate and potassium chloride blend (8: 2, by weight, respectively). Additional nitrogen was applied at the R1 plant stage through irrigation water based on soil testing to result in a total of 240 kg N/ha throughout the growing season.
2.2.2 Prey sentinels
To measure general epigeic predatory activity, wax moth (Galleria mellonella) larvae were used as sentinel baits in both treatments. Although wax moth larvae are not crop pests, they have been used successfully in previous field experiments for comparing generalist predatory activity between different agricultural habitats (Lundgren et al., 2007; Meehan et al., 2012). Prepupal larvae were individually pinned (#0 black enameled insect pins; model: 01.10; Entochrysis™, Pardubice, 53002, Czech Republic) to 1 cm tall pyramids made of sculpting clay (Sculpey™ original, Polyform Products Co. Inc.™, Elk Grove Village, IL, 60007) through the larvae’s posterior segments. Larvae were deployed in corn fields during anthesis within 1 h of being pinned to clay and were only used if obviously alive and active. Clay pyramids were buried in the soil so that pinned larvae were presented flush with the soil surface. Once deployed, sentinels remained undisturbed for 1 h, at which time they were recollected to assess the proportion of sentinels which had been predated. Sentinels were considered predated if there were invertebrates actively feeding, or if the wax moth larva had been partially or wholly consumed. Predators present at sentinels were identified upon sight to the lowest taxonomic rank possible and recorded.
In 2016, 30 sentinel larvae were placed in each research field. Three rows of 10 larvae were placed at the base of corn plants. Within each row, sentinels were spaced 3 m apart, and each row was separated by 4 m. In 2017, 40 larvae were used in each field. The same orientation was used as in 2016, except there was an additional row of 10 sentinels. In total, 1120 wax moth sentinel larvae were deployed to assess predation. Methods for sentinel predation were modeled after those described by Lundgren and Fergen (2011).
2.3 Data analysis
2.3.1 Insect communities
Shannon diversity index was calculated, and species richness reported for the invertebrate communities in each of the three habitats sampled, corn foliage, epigeic, and subterranean for both treatments. Invertebrate species collected from the soil surface and within soil were categorized into three functional groups, predators, herbivores and detritivores. Invertebrates collected on corn foliage were categorized into two groups, predators and herbivores. Mean ± SEM arthropods per plant or per m2 of soil in the various arthropod guilds from corn monocultures and interseeded corn were determined. Individual taxa were not included in functional groupings if their life histories were unknown. Two-way repeated measures ANOVAs (rm-ANOVAs) coupled with Tukey’s HSD all-pairwise comparisons were conducted to examine the effect of treatment (monoculture and interseeded corn), corn stage, or an interaction of both on diversity, species richness, and arthropod abundance in the different cornfield habitats. If an interaction was revealed subsequent one-way ANOVAs were conducted to determine treatment differences for individual corn stages. Individual taxa driving overall trends in abundance for each habitat were determined by performing rm-ANOVAs on taxa representing ≥ 0.5% of the total arthropod community abundance in corn foliage, on the soil surface, or within the soil. To gain a better understanding of treatment effects on the invertebrate community springtails (Collembola) and mites (Acarina) were excluded when determining common epigeic and subterranean taxa due to their disproportionately large abundances. Two-way ANOVAs were conducted on corn yield and density to determine if there was a significant interaction between site year and treatment. If an interaction existed subsequent one-way ANOVAs tested treatment differences at individual sites.
Mean ± SEM percent sentinel predation per plot was determined for each treatment at individual sites and across all site years. A two-way ANOVA was used to determine treatment and site effects on rates of sentinel predation, and if an interaction existed between the two. Subsequent one-way ANOVAs were used to examine the levels of significance between treatments at individual study locations. Linear regressions were conducted to examine if correlations existed between diversity (Shannon H) or epigeic predators/m2 (during anthesis) and predation rate at Gary-17 and Estelline as invertebrate community assessments and sentinel predation experiments were done at these locations. Data analyses were conducted using Statistix® 10 software (Analytical Software, Tallahassee, FL, U.S.A.).
3.1 Invertebrate communities
A grand total of 63,868 invertebrates were collected from epigeic, subterranean and foliar cornfield habitats. The invertebrate community consisted of 516 species from 22 orders, including: Araneae, Acarina, Cephalostigmata, Coleoptera, Collembola, Diplura, Diptera, Ephemeroptera, Hemiptera, Hymenoptera, Lepidoptera, Neuroptera, Odonata, Opiliones, Orthoptera, Protura, Pseudoscorpiones, Psocoptera, Stylommatophora, Thysanura and two unidentified orders, one each from classes Chilopoda and Diplopoda.
3.1.1 Epigeic invertebrates
A total of 8098 invertebrates were collected from the soil’s surface using quadrats. Across all site years and sampling dates, more than twice as many invertebrates were collected from the epigeic environment in interseeded corn (n = 5722) compared to corn monocultures (n = 2376) (F1,71 = 10.36, P < 0.01; Figure 2A).
The five most abundantly collected taxa from the soil surface were Collembola (all springtails, 24.46%), Rophalosiphum padi (Aphididae) (12.07%), Stenolophus comma (Carabidae) (7.26%) Acarina (all mites, 6.74%) and Elaphropus sp. (Carabidae) (3.52%). Based on known life histories, 165 species were designated as predators, 52 species were classified as herbivores, and 16 were detritivores. Throughout the growing season, both predators and herbivores were more abundant in interseeded plots compared to monocultures (predators: F1,71 = 17.90, P < 0.01, herbivores: F1,71 = 9.93, P < 0.01; Figures 2B-C), but detritivore abundance was statistically similar between treatments (F1,71 = 1.87, P = 0.19; Figure 2D). Increased predator abundance on the soil surface in interseeded plots was largely driven by three predatory groups and four commonly collected individual taxa. Ground beetle (Carabidae: F1,71 = 5.42, P = 0.03), rove beetle (Staphylinidae: F1,71 = 4.53, P = 0.04) and spider (Araneae: F1,71 = 6.90, P = 0.02) abundances were higher in cover-cropped corn. Individually, predatory Bembidion sp. (F1,71 = 8.89, P = 0.01), Nabis americoferus (F1,71 = 5.50, P < 0.01), a Tetragnathid spider species (F1,71 = 4.57, P = 0.04) and larval Coccinellidae (F1,71 = 16.69, P < 0.01) were more abundant when cover crops were present. For herbivores, significantly more bird cherry oat aphids (Rhopalosiphum padi: F1,71 = 4.43, P = 0.05) and plant bugs (Miridae: F1,71 = 6.10, P = 0.02) were found in the interseeded fields compared with the monoculture cornfields (Table 1).
Twenty-seven species individually represented ≥ 0.5% of all surface-dwelling arthropods (excluding the abundances of mites and Collembola). Of these 27, the abundance of six species, two herbivorous and four predatory, were significantly increased in interseeded corn (Table 1). There were no species whose abundance was lower in interseeded corn compared to corn in monoculture.
An interaction existed between treatment and corn stage for surface-dwelling invertebrate abundance, and for the abundance of predatory and herbivorous guilds (Figures 2A-C). Total arthropod abundances did not differ during V4 and V8 plant stages, but during anthesis more invertebrates were collected in interseeded plots (F1,23 = 19.23, P < 0.01). Predators were more abundant in interseeded plots during all of the sampled corn stages (V4: F1,23 = 14.06, P < 0.01, V8: F1,23 = 6.75, P = 0.02, anthesis: F1,23 = 4.15, P = 0.05), whereas herbivores were more numerous in interseeded plots during V4 and V8 (F1,23 = 7.89, P = 0.01 and F1,23 = 4.42, P = 0.05, respectively), but not anthesis.
There were 298 epigeic invertebrate species collected over the season (in both treatments), with greater species richness in interseeded corn fields than in monocultures (F1,71 = 13.07, P < 0.01; monoculture: 39.50 ± 3.04, interseeded 61.25 ± 4.03 species). Across treatments, invertebrate richness increased incrementally among sampled corn stages (F2,71 = 30.70, P < 0.01). Interseeding cover crops did not affect the diversity (Shannon H) of surface-dwelling arthropods (F1,71 = 0.00, P = 0.99), but invertebrate diversity did vary among corn stages (F2,71 = 14.16, P < 0.01), with V4 corn being less diverse than corn at the V8 and anthesis stages.
3.1.2 Subterranean invertebrates
A total of 45,797 invertebrates were extracted from soil cores throughout the duration of the experiment, with 19,254 from corn monocultures and 26,543 from corn interseeded with cover corps. Despite collecting 7289 more invertebrates from interseeded plots total arthropods per square meter was not significantly different between treatments (F1,95 = 0.72, P = 0.41, Figure 3A). When the community was separated into functional guilds (predators: n = 167 species; herbivores: n = 38 species; detritivores: n = 41 species) an interseeded cover crop did not affect invertebrate abundance for predators (F1,95 = 1.03, P = 0.32), herbivores (F1,95 = 1.20, P = 0.29) or detritivores (F1,95 = 0.73, P = 0.40) (Figures 3B-D).
Mites and collembola dominated the subterranean community, representing 56.83% and 33.01% of all individuals collected, respectively. Diplurans were next most abundant, representing 0.68%. Abundances of taxa fell sharply following diplurans, without distinctly abundant individuals. 27 species each represented ≥ 0.5% of all collected subterranean arthropods, and although an overall treatment effect did not exist for the abundances of subterranean invertebrates, the abundances of four individual taxa differed between interseeded and monoculture corn. One Lycosidae spider species, a detritivorous Cryptophagidae beetle and all Thripidae (herbivorous) were increased when cover crops were present, whereas Beetle Larvae 15 (see Table 1 footnote for specimen description) was found less frequently in cover-cropped fields (Table 1).
Interseeded corn possessed a greater diversity (Shannon H) of subterranean invertebrates than corn monocultures (F1,95 = 4.17, P = 0.05). There was a total of 361 species collected throughout the growing seasons in both treatments with no difference in terms of species richness between monoculture and interseeded fields (F1,95 = 2.00, P = 0.17; monoculture: 55.25 ± 5.32, interseeded 64.50 ± 6.39 species). Across treatments, both species richness and diversity varied between corn stages (F3,95 = 12.08, P < 0.01 and F3,95 = 4.17, P < 0.01, respectively), with corn having the greatest number of species and diversity during anthesis compared to all earlier sampled stages.
3.1.3 Foliar invertebrates
A total of 9973 invertebrates were collected from plant samples, with 4681 from interseeded corn and 5292 from corn monocultures. The abundance of total invertebrates per dissected corn plant did not vary between treatments (F1,71 = 0.53, P = 0.47, Figure 4A). Likewise, interseeding corn with cover crops did not affect the number of predators (n = 29 species) or herbivores (n = 18 species) per corn plant (F1,71 < 0.01, P = 0.99 and F1,71 = 0.89, P = 0.36, respectively, Figures 4B-C). Across treatments, abundance varied among corn stages for total arthropods, predators and herbivores, with corn plants during anthesis possessing more invertebrates than V4 or V8 plants (Figure 4).
Only six individual or grouped taxa represented ≥ 0.5% of total invertebrate abundance in corn foliage. Herbivorous thrips (Thripidae), corn leaf aphids (Aphididae: Rhopalosiphum maidis) and mites (Acarina), represented 34.44%, 32.61 and 1.81% of the total foliar community abundance, respectively. Predatory minute pirate bugs (Orius insidiosus), spiders and ladybeetle larvae (Coccinellidae), represented 5.17%, 3.15% and 0.53% of the total foliar community, respectively. None of these taxa significantly differed in population size between interseeded and monoculture corn plots.
Corn earworms (Helicoverpa zea), European corn borers (Ostrinia nubilalis) and northern corn rootworms (Diabrotica barberi) are particular pests of concern for corn producers (SDSU, 2019), but in this study only 10 individuals of each species were collected from a total 1080 plants. Seven H. zea were collected from interseeded plots and three from monoculture, two O. nubilalis from interseeded and seven from monoculture, and eight D. barberi from interseeded and two from monoculture plots.
In both interseeded and monoculture fields there was a total of 66 species collected throughout the growing season. Species richness did not differ between treatments (F1,71 = 0.13, P = 0.73; monoculture: 17.00 ± 1.24, interseeded 18.50 ± 1.40 species), but differed among corn stages (F2,71 = 36.58, P < 0.01), with greater richness measured at V8 and anthesis than at V4. Similarly, foliar arthropod diversity did not differ between corn monocultures and interseeded corn (F1,71 = 0.07, P = 0.79), but diversity differed among plant stages (F2,71 = 35.55, P < 0.01) with arthropods being more diverse during V8 and anthesis compared to V4 corn.
3.2 Predation experiments
Across all site years, 45.5 ± 7.7% of sentinels were consumed in interseeded corn compared to only 22.6 ± 3.2% in corn monocultures (F1,31 = 27.6, P < 0.01, Figure 5). Variability in treatment effects at different study locations resulted in a significant interaction effect between treatment and site (F3,31 = 18.2, P < 0.01). At Canby and Gary-17 sentinels were consumed at a significantly higher rate in interseeded corn than corn monocultures (Canby: F1,7 = 201.9, P < 0.01; Gary-17: F1,7 = 7.01, P = 0.04), whereas at Pierre and Estelline sentinel predation was not different between treatments (Pierre: F1,7 = 1.65, P = 0.25; Estelline: F1,7 = 0.37, P = 0.57) (Table 3).
Of the 369 total predation events, we observed active feeding by 547 invertebrates on 211 sentinels at the conclusion of the one-hour deployment period. Crickets (Orthoptera: Gryllidae) were most commonly observed consuming sentinels (n = 330, 60.3%), followed by ants (Hymenoptera: Formicidae; n = 119, 21.8%), harvestmen (Opiliones: Phalangiidae; n = 37, 6.8%) and ground beetles (Carabidae; n = 31, 5.7%) (remaining grouped taxa presented in Table 4). When comparing sentinel predation rates among both treatments at Estelline and Gary-17 to epigeic predator abundance and species diversity (Shannon H) during anthesis no significant correlations were revealed (predator abundance: F1,16 = 1.81, P = 0.20, community diversity: F1,16 = 0.50, P = 0.49).
3.3 Corn yield and density
Significant interactions between treatment and site year existed for both corn yield (F2,23 = 44.26, P < 0.01) and corn density (F2,23 = 14.47, P < 0.01), owing to the uniquely large treatment effect observed in both measurement types at Estelline. Neither yield nor density differed between interseeded and monoculture corn plots at Gary-18. At Gary-17 corn density was also unchanged between treatments, but yield was marginally-significantly greater in cover-cropped corn (F1,7 = 4.74, P = 0.07). At Estelline corn plant density was greater in interseeded plots (F1,7 = 58.93, P < 0.01), but corn yield was reduced in the presence of cover crops (F1,7 = 117.97, P < 0.01). (Table 2)
Over 63,000 specimens collected across several plant stages and from multiple cornfield strata resulted in a robust bioinventory of cornfield-dwelling invertebrates, making this the most comprehensive bioinventory of cornfields yet compiled. Demographics of this community were relatively consistent with other cornfield bioinventories described previously (LaCanne, 2017; Lundgren et al., 2015; Stevenson et al., 2002). Cornfields hosted thousands of invertebrates per m2, representing hundreds of species from a total of 22 orders. By far, most invertebrates (approximately 70% of specimens and 70% of species) were collected in the top 10 cm of the soil column. These numbers are particularly stark when one considers the relative spatial area sampled over the study; soil communities were assessed for approximately 15 m2, whereas the epigeal and foliar communities were collected from 72 and 137 m2, respectively. Within this community in the soil column, Collembola and mites dominated the community, and diversity of the mites would have added even more species to our tally. Biomass of invertebrates in the soil represents a significant source of nutrients and ecosystem function within the soil, and one that can be readily managed by farmers (Altieri, 1999; Landis et al., 2000; Pearsons and Tooker, 2017). The diverse community revealed in this study provides many services to farmers, but the diversity found in cornfields still pales in comparison to the invertebrate communities found in ancestral habitats that cornfields have replaced (Nemec et al., 2014; Schmid et al., 2015; Standen, 2000; Wimberly et al., 2017). Species of this community that receive the most attention from land managers are those considered pests. Despite the lack of insecticides used in fields assessed for community characteristics, corn pests were never found at actionable levels, and foliar (corn earworms and European corn borers) and root pests (corn rootworms) of special concern were particularly scarce (fewer than one per 100 dissected corn plants for foliar pests, and corn rootworm larvae were not found). These results call into question the need for prophylactic insecticidal products meant to control these arthropods, which has been pointed out by previous researchers (Bredeson and Lundgren, 2015; Hutchison et al., 2010). Clearly, a broader view of biodiversity that transcends managing the handful of problematic species to managing the function of insect communities seems justified by the current study (Coll and Wajnberg, 2017).
Interseeding covers into standing corn increased the community complexity of invertebrates over corn planted in bare soil. Foliar communities were largely unaffected by the cover crop. The reason for this may have been that the low-growing cover crops attracted specialists to this habitat stratum. This pattern has been seen in orchard systems as well, where enhanced ground level community structure did not move into the orchard canopy (Altieri and Schmidt, 1986; Horton et al., 2009). Within the soil column, invertebrate abundances were consistent between the two systems. We imagine that the rate of dispersal of invertebrates throughout this habitat is curtailed relative to the soil surface and foliar habitats (Ojala and Huhta, 2001). It is feasible that communities within annual, ephemeral cropping systems simply don’t have the time to respond to the added plant diversity. The observation that species diversity (Shannon H) increased may be the initial rebalancing of the successional community in the soil in response to the cover-crop mediated habitat change (Longcore, 2003). We hypothesize that most niches in the soil column in this disturbed habitat were occupied by early successional species that are adapted for cropland, and as plant diversity increased, it began to change the relative abundances of the species that were there. If this is true, then we might expect richness and diversity to increase in the interseeded cropland over monocultures over time (Siemann et al., 1999; Suding and Gross, 2006).
The epigeic community was most strongly influenced by the addition of interseeded cover crops. More than twice as many specimens were collected on the soil surface of interseeded corn fields, and all functional guilds (except for detritivores) were increased significantly relative to corn planted in bare soil. Conditions on the soil surface within plant-diverse fields also resulted in a greater number of species inhabiting this environment. We expected that this community would be the most affected, as this community is most proximate to the resources made available by the cover crops [similarly shown by (Horton et al., 2009)]. These resources might be most simply categorized as habitat and trophic in nature. Diversifying plant assemblages directly affect invertebrate communities by ameliorating harsh abiotic conditions (Orr et al., 1997), increasing habitat structural complexity and niche partitioning (Langellotto and Denno, 2004; Letourneau et al., 2011) and providing a variety of nutritional sources (Lundgren, 2009; Venzon et al., 2006), to name a few. Exactly how diversification of plant communities alters habitat suitability for invertebrates is often categorized into these simplified cause-and-effect relationships. In truth, multi-trophic ecological interactions within even the most simplified agroecosystem become nearly infinitely complex (Lundgren and Fausti, 2015), and in some ways, it is impossible to predict why invertebrate communities respond the way that they do to habitat manipulations except for in broad patterns.
Interseeding cover crops increased predation on the soil surface in corn. Adding plant diversity into simplified agricultural production systems often has the resulting effect of elevating biological control (Altieri and Schmidt, 1986; Bickerton and Hamilton, 2012; Lundgren and Fergen, 2010). One explanation is that community structure and function are linked; that changes in predator community structure in response to plant diversification is responsible for increasing predation rates. This was not what we observed. Predator abundance and diversity were not correlated with consumption rates per field. Factors known to influence predation include relative prey and non-prey food sources in a habitat (Nomikou et al., 2010), intraguild interactions (Frank et al., 2010), structural complexity of the habitat that may impede or enhance foraging by predators (Finke and Denno, 2002), etc. Within interseeded fields it is likely that a combination of altered behavioral factors, not predator community structure, interacted to increase predation by the natural enemy community.
The response of crop yield to undersown cover crops is highly variable depending on a host of factors such as nutrient availability, weed pressure, soil moisture, cover crop species and sowing times (Abdin et al., 1998; Curran et al., 2018). It is possible that reduced yield in cover-cropped fields at Estelline was a result of competition between corn and cover crops for nutrients in fields not receiving fertilizer. Interseeded fields at this location also possessed a challenging weed population, further depleting available soil fertility. Alternatively, marginally increased corn yield in cover-cropped fields at Gary-17 reflects the results of studies identifying synergisms in mixed-cropping systems. For example, Rerkasem and Rerkasem (1988) recorded an increase in corn yield and leaf tissue nitrogen when ricebean (Vigna umbellata) was grown in close proximity. Meng et al. (2015) help explain this synergistic phenomenon by observing the transfer of legume-fixed nitrogen to corn via mycorrhizal fungi. Within published literature the synergistic effects of intercropping typically leads to a yield advantage in comparison to monocultures (Yu et al., 2015). Crop establishment and density also benefit from companion plants by protecting young seedlings from adverse abiotic conditions such as damaging wind and erosion (Rinehart, 2006), or biotic factors such as stand-reducing herbivores (Frank et al., 2010). These protective effects may have contributed to improved corn stand density in interseeded fields at Estelline.
Educational & Outreach Activities
Ecdysis Foundation, Ducks Unlimited and the South Dakota Grassland Coalition organized a well-attended cover crop interseeding field day hosted at Blue Dasher farm with tour to K Creek Ranch where there were interseeding plots established. Approximately 100 attendees learned about interseeding basics and advancements from a suite of speakers including Loran Steinlage and Drs. Mike Bredeson and Jonathan Lundgren.
The interseeder that was developed during this project has been used for two seasons with great success by farmers Lyle Kruse, Roger Svec and Aaron Swanson near Estelline, SD. Interseeding field day attendees were able to experience first-hand a corn field that had a healthy seven-way mixture of cover crops planted into it.
I had the great opportunity to speak about my experiences interseeding in Salina, Kansas at the Soil Health University event to approximately 300 producers. I’ll give a similar talk at the 2020 no-till on the plains meeting in St. Louis, MO to an unknown number of producers.
One peer reviewed journal article has been published based on the work in this project, and an additional paper is in preparation.
Prior to this project being completed there was a dearth of information relating to the effects of interseeding cover crops into corn on any agroecosystem parameters. The data collected in this SARE-funded study will give producers insight into how the invertebrate community reacts to diversification of a would-be monoculture. The collected information can reassure that the biological control of herbivores is improved once you establish a suitable habitat for beneficial invertebrates to persist. Eliminating pesticide use following establishment of a healthy invertebrate community has economic implications as there is less money being spent on pesticides, and has the positive environmental benefit of keeping valuable natural resources such as soil and water free of pesticidal toxins.
Social stigmas and pressures very often keep good ideas off of farmers land. As a result of this project being conducted, producers have been able to witness first-hand a number of successful occurrences where interseeding has been accomplished, and even occurrences where interseeded corn has out-performed corn monocultures. Seeing success rather than taking someone’s word for it can speak volumes. This project did much to shape the local social acceptability of attempting polycultures.
The biggest take-home message that I gained from this research project is that the farmers who have a desire to implement regenerative agricultural practices do an incredible amount of information gathering on their own. The early adopters of regenerative agriculture are well-versed in the ecological principles of what will make their farm’s soils more functional and healthy. What limits these producers from conducting certain regenerative practices is the accessibility of proper tools. Since creating a cover crop interseeder the device has been shared among a number of local farmers, with more always showing interest in using the equipment. One interseeder isn’t enough for all the people who would like to use it.
As a scientist who wishes he was a farmer I have had the privilege of visiting many incredible regenerative farms and have witnessed what is working on the farmscape. What is maddening is that one can see these successful farms being economically viable whilst restoring natural resources but yet these farms are few and far between. Clearly there are deep social and infrastructural issues we must resolve in order to see change on a broader scale and at a faster pace.
A grain and forage farmer in eastern South Dakota has been able to access the data, experiences and equipment derived from this project to now feel confident hosting cover crops on every single acre of corn planted on his farm. The weed suppression, insect recruitment and soil building capacity of having greater diversity in his corn is seen as a major benefit and he says he’ll never plant a corn monoculture again. Results form his fields are impressive. The added diversity allows him to extend the growing season beyond that of just corn and perpetuates the “green bridge”, important to his soil health.
Regenerative agriculture is at the stage where farmer’s have innovative practices they’d like to adopt (or at lease try), but the barrier is often equipment-based. It isn’t feasible for farmers to incur an expense such as for an interseeder or cover crop roller crimper if they haven’t had an opportunity to try it first. SARE could be helpful to farmers by somehow facilitating the trial of necessary regenerative agricultural equipment.