The role of cover crops as a floral resource for native bee conservation in agroecosystems

Final Report for GNE12-037

Project Type: Graduate Student
Funds awarded in 2012: $14,929.00
Projected End Date: 12/31/2014
Grant Recipient: Penn State University
Region: Northeast
State: Pennsylvania
Graduate Student:
Faculty Advisor:
Mary Barbercheck
PSU Dept. of Entomology
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Project Information


Conservation of natural processes such as pollination, pest control, and nutrient cycling are essential to maintaining a healthy agroecosystem. The incorporation of cover crops into annual crop rotations is one practice that is used in the Northeast U.S. to manage soil fertility, weed suppression, and erosion control. Additionally, cover crops that have a flowering stage have the potential to support beneficial insect communities, such as native bees. Because of the current decline facing managed honeybee colonies, the conservation of native bee communities is critical to maintaining ‘free’ pollination services. However, native bees are negatively affected by agricultural intensification and are in decline across North America. This project assesses the potential of flowering cover crop species to act as a conservation resource for native bee communities, in addition to providing benefits to soil fertility and agricultural production.

Three flowering cover crop species were studied in this project over four fall planting dates. I evaluated each cover crop and planting date treatment for differences in winter survival, spring blooming time, flower density, and the subsequent influence that each of these factors had on native bee visitation. The three cover crop species bloomed at different times throughout the spring and attracted significantly different native bee communities. Planting date did not majorly influence spring flowering time, but did have a significant influence on winter survival. Across all three cover crop species, the planting date treatments with the lowest winter survival had lowest spring flower density which then resulted in the lowest bee visitation frequency. Overall, the main influences on bee community visitation were cover crop species and total flower density. Additionally, as the variations in winter survival and spring flowering time demonstrated, it is highly important to consider the cash crop rotation window limitations when selecting cover crop species so that flowers are produced within the appropriate cover cropping timeframe.


Pollination as an ecosystem service is vital to the reproduction of much of the world’s food crops and other flowering plants. In fact, animal-mediated pollination (primarily by bees) is required for 35% of the world’s total food production and 87.5% of all flowering plants (Klein et al. 2007, Ollerton et al. 2011). While managed honey bee colonies are most often used for agricultural pollination, native bees are also known to play an important role in crop pollination (Kremen et al. 2002, Ricketts 2004, Morandin and Winston 2005, Greenleaf and Kremen 2006a, 2006b). However, despite the fact that pollination services are often essential for agricultural production, the importance of native bee communities extends far beyond this purpose.

Unfortunately, the world’s pollinators are in decline. While the recent decrease in managed honey bee colonies is now well-documented (VanEngelsdorp et al. 2009, Williams et al. 2010), there is also evidence for a global decline in other pollinator groups as well as many pollinator-dependent plants (National Research Council 2007, Potts et al. 2010). The possible causes of this decline include loss of natural habitat, agricultural pesticides, pathogens, disease, and climate change (Potts et al. 2010). However, it is most likely the combined interaction of these factors that has led to much of the widespread global pollinator decline that we see today. Habitat loss and fragmentation, in particular, are often listed as some of the greatest and most common threats to wild pollinators, particularly bees (Kremen et al. 2002, Ricketts 2004, Goulson et al. 2008, Winfree et al. 2009).

Historically, agriculture is often associated with negative influences on biodiversity and increased land simplification (Matson et al. 1997, Tilman et al. 2001). Indeed, bees are the insect group shown to be the most negatively affected by agricultural intensification (Hendrickx et al. 2007). Because total land-use change has been predicted to have the greatest effect on global biodiversity of terrestrial ecosystems over the next 100 years (Sala et al. 2000), determining alternative scenarios that limit the effects of habitat change on native pollinators is a significant consideration for the pollinator research and conservation community.

One strategy for increasing agricultural conservation and ecosystem health is a trend toward organic or diversified farming. Compared to conventional farming, organic agriculture can increase biodiversity and ecosystem services (Hole et al. 2005, Kremen and Miles 2012) and support a greater diversity of native bees (Holzschuh et al. 2006). Additionally, temporal variations in resource availability or location can significantly affect arthropod populations (Vasseur et al. 2013). All in all, by focusing on an array of techniques that take into account a combination of practices including preservation of habitat refuges, wildlife-friendly farming, and the seasonal resource variations across the landscape, an optimal conservation strategy may be found for a wide range of farm types or locations (Hodgson et al. 2010). As a partial solution to this conservation need, we consider the incorporation of winter cover crops into organic farming systems to help enhance floral resources both spatially and temporally across the landscape.

Cover crops are plant species grown within a cultivated field during fallow periods in annual cash crop rotation schedules, or intermixed within cash crop plantings. They can be almost any species of plant, but are mostly commonly grasses and legumes, and can be planted almost anytime during the year depending on the crop rotation and local climate. Most farmers plant cover crops for within-field erosion control, soil fertility management, or weed suppression (Lal et al. 1991, Snapp et al. 2005, Clark 2007). However, because the addition of cover crops into an annual crop rotation potentially increases spatial and temporal plant diversity levels, it can also act as an agricultural conservation strategy. By selecting cover crops that also produce insect-visited flowers attractive to native pollinators, this technique can benefit crop productivity as well as supplement resources to native wildlife populations.

The addition of supplemental flowering resources to an agricultural landscape has been shown to be beneficial to native bee communities and is often used as a pollinator conservation strategy (Tuell et al. 2008, Winfree 2010). Our study focuses on whether the addition of a spring-blooming cover crop species could achieve the same purpose. This timeframe is especially important because some native bee species benefit from an increase in springtime floral resources (Elliott 2009, Williams et al. 2012). However, agricultural landscapes are often lacking these early-season flowers compared with other natural or fallow areas (Winfree et al. 2007, Williams et al. 2012, Mandelik et al. 2012). It is for this reason that increasing spring flowering resources within cultivated fields would likely have a large influence on overall resource availability for native bees during this time of the year.

Incorporating flowering cover crops into a grower’s rotation schedule is an agricultural conservation strategy with the opportunity to consider the needs of both the grower and the native pollinator community. However, to make appropriate recommendations to farmers interested in achieving these dual benefits, it is important to select cover crops appropriate for a particular production system. Some of the factors that need to be considered in the selection of cover crops include: when will these species bloom, how will that bloom be influenced by fall planting or spring termination dates (crop rotation windows), and what bee species will visit each of these cover crop flowers? For example, one grower may be interested in a cover crop species that benefits the greatest diversity of pollinators, while another may want to focus on those bees that are needed to pollinate a summer cash crop. This project focuses on pursuing answers to these questions in order to better inform future decisions on the pollinator conservation potential of a variety of cover crop species and crop rotation schedules.

References Cited

Clark, A. (Ed.). 2007. Managing Cover Crops Profitably 3rd ed. Sustainable Agriculture Research and Education (SARE).

Elliott, S. E. 2009. Subalpine Bumble Bee Foraging Distances and Densities in Relation to Flower Availability. Environmental Entomology 38:748–756.

Goulson, D., G. C. Lye, and B. Darvill. 2008. Decline and conservation of bumble bees. Annual review of entomology 53:191–208.

Greenleaf, S. S., and C. Kremen. 2006a. Wild bees enhance honey bees’ pollination of hybrid sunflower. Proceedings of the National Academy of Sciences of the United States of America 103:13890–5.

Greenleaf, S. S., and C. Kremen. 2006b. Wild bee species increase tomato production and respond differently to surrounding land use in Northern California. Biological Conservation 133:81–87.

Hendrickx, F., J.-P. Maelfait, W. Van Wingerden, O. Schweiger, M. Speelmans, S. Aviron, I. Augenstein, R. Billeter, D. Bailey, R. Bukacek, F. Burel, T. Diekötter, J. Dirksen, F. Herzog, J. Liira, M. Roubalova, V. Vandomme, and R. Bugter. 2007. How landscape structure, land-use intensity and habitat diversity affect components of total arthropod diversity in agricultural landscapes. Journal of Applied Ecology 44:340–351.

Hodgson, J. a, W. E. Kunin, C. D. Thomas, T. G. Benton, and D. Gabriel. 2010. Comparing organic farming and land sparing: optimizing yield and butterfly populations at a landscape scale. Ecology letters 13:1358–67.

Hole, D. G., a. J. Perkins, J. D. Wilson, I. H. Alexander, P. V. Grice, and a. D. Evans. 2005. Does organic farming benefit biodiversity? Biological Conservation 122:113–130.

Holzschuh, A., I. Steffan-Dewenter, D. Kleijn, and T. Tscharntke. 2006. Diversity of flower-visiting bees in cereal fields: effects of farming system, landscape composition and regional context. Journal of Applied Ecology 44:41–49.

Klein, A.-M., B. E. Vaissière, J. H. Cane, I. Steffan-Dewenter, S. a Cunningham, C. Kremen, and T. Tscharntke. 2007. Importance of pollinators in changing landscapes for world crops. Proceedings of the Royal Society B: Biological Sciences 274:303–13.

Kremen, C., and A. Miles. 2012. Ecosystem Services in Biologically Diversified versus Conventional Farming Systems?: Benefits , Externalities , and Trade-Offs. Ecology and Society 17.

Kremen, C., N. M. Williams, and R. W. Thorp. 2002. Crop pollination from native bees at risk from agricultural intensification. Proceedings of the National Academy of Sciences of the United States of America 99:16812–6.

Lal, R., E. Regnier, D. J. Eckert, W. M. Edwards, and R. Hammond. 1991. Expectations of cover crops for sustainable agriculture. Pages 1–11 in W. L. Hargrove, editor. Cover Crops for Clean Water. Soil and Water Conservation Society.

Mandelik, Y., R. Winfree, T. Neeson, and C. Kremen. 2012. Complementary habitat use by wild bees in agro-natural landscapes. Ecological applications?: a publication of the Ecological Society of America 22:1535–46.

Matson, P. A., W. J. Parton, A. G. Power, and M. J. Swift. 1997. Agricultural Intensification and Ecosystem Properties. Science 277:504–509.

Morandin, L. A., and M. L. Winston. 2005. Wild bee abundance and seed production in conventional, organic, and genetically modified canola. Ecological Applications 15:871–881.

National Research Council. 2007. Status of Pollinators in North America. The National Academies Press, Washington, DC.

Ollerton, J., R. Winfree, and S. Tarrant. 2011. How many flowering plants are pollinated by animals? Oikos 120:321–326.

Potts, S. G., J. C. Biesmeijer, C. Kremen, P. Neumann, O. Schweiger, and W. E. Kunin. 2010. Global pollinator declines: trends, impacts and drivers. Trends in Ecology & Evolution 25:345–53.

Ricketts, T. H. 2004. Tropical Forest Fragments Enhance Pollinator Activity in Nearby Coffee Crops. Conservation Biology 18:1262–1271.

Sala, O. E., F. S. Chapin, J. J. Armesto, E. Berlow, J. Bloomfield, R. Dirzo, E. Huber-Sanwald, L. F. Huenneke, R. B. Jackson, A. Kinzig, R. Leemans, D. M. Lodge, H. A. Mooney, M. Oesterheld, N. L. Poff, M. T. Sykes, B. H. Walker, M. Walker, and D. H. Wall. 2000. Global biodiversity scenarios for the year 2100. Science 287:1770–4.

Snapp, S. S., S. M. Swinton, R. Labarta, D. Mutch, J. R. Black, R. Leep, J. Nyiraneza, and K. O’Neil. 2005. Evaluating Cover Crops for Benefits, Costs and Performance withing Cropping System Niches. Agronomy Journal 97:322–332.

Tilman, D., J. Fargione, B. Wolff, C. D’Antonio, a Dobson, R. Howarth, D. Schindler, W. H. Schlesinger, D. Simberloff, and D. Swackhamer. 2001. Forecasting agriculturally driven global environmental change. Science (New York, N.Y.) 292:281–4.

Tuell, J. K., A. K. Fiedler, D. Landis, and R. Isaacs. 2008. Visitation by Wild and Managed Bees ( Hymenoptera?: Apoidea ) to Eastern U . S . Native Plants for Use in Conservation Programs. Environmental Entomology 37:707–718.

VanEngelsdorp, D., J. D. Evans, C. Saegerman, C. Mullin, E. Haubruge, B. K. Nguyen, M. Frazier, J. Frazier, D. Cox-Foster, Y. Chen, R. Underwood, D. R. Tarpy, and J. S. Pettis. 2009. Colony collapse disorder: a descriptive study. PloS one 4:e6481.

Vasseur, C., A. Joannon, S. Aviron, F. Burel, J.-M. Meynard, and J. Baudry. 2013. The cropping systems mosaic: How does the hidden heterogeneity of agricultural landscapes drive arthropod populations? Agriculture, Ecosystems & Environment 166:3–14.

Williams, G. R., D. R. Tarpy, D. vanEngelsdorp, M.-P. Chauzat, D. L. Cox-Foster, K. S. Delaplane, P. Neumann, J. S. Pettis, R. E. L. Rogers, and D. Shutler. 2010. Colony Collapse Disorder in context. BioEssays?: news and reviews in molecular, cellular and developmental biology 32:845–6.

Williams, N. M., J. Regetz, and C. Kremen. 2012. Landscape-scale resources promote colony growth but not reproductive performance of bumble bees. Ecology 93:1049–58.

Winfree, R. 2010. The conservation and restoration of wild bees. Annals of the New York Academy of Sciences 1195:169–97.

Winfree, R., R. Aguilar, D. P. Vazquez, G. LeBuhn, and M. A. Aizen. 2009. A meta-analysis of bees’ responses to anthropogenic disturbance. Ecology 90:2068–2076.

Winfree, R., T. Griswold, and C. Kremen. 2007. Effect of human disturbance on bee communities in a forested ecosystem. Conservation biology?: the journal of the Society for Conservation Biology 21:213–23.

Project Objectives:

OBJECTIVE 1: To identify (a) the blooming time frames of three common Northeast U.S. flowering cover crop plant species, and (b) the effect of cover crop planting and termination date on peak bloom.

Objective 1 accomplishments:

The original project proposal was designed to study six cover crop species over two fall planting dates. After review this goal was altered to three cover crop species and four planting dates so as to better represent a fall planting gradient and allow for a more robust model for how these planting dates would fit into a variety of Northeast cash crop rotations. This also allowed for a greater variety of fall climate effects on each planting date observed.

All portions of this objective were met. Data was collected and analyzed regarding overall spring blooming time of each of the three cover crop species as well as how this blooming time differed across the four planting dates. Each crop and planting date was monitored outside of a standard cash crop rotation window and thus the effect of spring termination time is represented by where the date of cover crop termination would fall relative to when each treatment bloomed. All treatments were monitored for starting and ending date of flower production as well as for relative flower density across time.  

OBJECTIVE 2: To identify (a) the native bee species that visit cover crop flowers for pollen and nectar resources, (b) when those bee species are most abundant in the environment and how that relates to cover crop bloom, and (c) what potential pollinator benefits may be lost through early termination of cover crops in standard crop rotations.

Objective 2 accomplishments:

All portions of this objective were met. Bee visitors to the three cover crop species and planting dates were monitored once per week throughout the blooming period of each crop. On each data collection day, pollinators were monitored both visually and with a round of hand net collection. Bees collected were returned to the laboratory for identification to species level.  

Concurrent to the collected plot-level bee data, a series of landscape-level bee traps were set to monitor the complete bee community in the local environment. The goal of these traps was to be able to compare the potential native bee community to the actual sub-set that was utilizing the flowering cover crops; however, the traps did not effectively collect all bee species in the area as several bees were collected on the cover crop flowers but were not found in any of the landscape-level traps.

Again, as this study was performed outside of a standard cash crop rotation window, the potential pollinator benefit lost in different rotation windows was considered and simulated based on the blooming time of each cover crop species in relation to the number of bees served by that crop and a standard cover crop termination date of a specific cash crop rotation schedule. No specific values for pollination loss were calculated, but rather were represented visually through a series of illustrations giving blooming period, number of bee species served, and likely cover crop termination date.

OBJECTIVE 3: (a) To participate in a series of public outreach activities highlighting the benefits of flowering cover crops for conservation of native bee species. (b) To create and distribute an extension publication that illustrates the results of this project and assists growers in determining the optimal species for use in their rotation.

Objective 3 accomplishments:

Both portions of this objective were well met over the time period of this project and beyond. The data and insight gained from this study has been shared at multiple public outreach and seminar events. Such events include: two Penn State Extension field day events (September 2013), three scientific conferences (August and November 2013, March 2014), a public graduate defense seminar (December 2013), and a free public pollinator conservation seminar (August 2014).

Additionally, I created and published online an extension document detailing the benefits of cover crops to both growers and the native bee community. It was published to the Penn State Entomology website as part of their Extension Fact Sheet series in January 2014 and is freely available to the public. A peer reviewed scientific publication including the data collected from this project is also in review and should be formally published in the coming months.


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  • Dr. Mary Barbercheck


Materials and methods:

Site and Plot Establishment.

This experiment was conducted on approximately 0.25 ha of land at the Pennsylvania State University Russell E. Larson Research and Education Center (RELREC) near Rock Springs, Pennsylvania. The dominant soil type at the site is Hagerstown silt loam with soil texture being predominantly clay loam with variability in silt, clay and sand. This land is in transition to organic certification and was managed in accordance with the USDA National Organic Standards (USDA 2013). No pest control materials have been applied at the site since the initiation of the transition.

To determine the effect of fall planting date on timing of spring cover crop flowering, flower density, and native bee visitation, three species of cover crop, canola (Brassica napus L.‘Wichita’), medium red clover (Trifolium pratense L.), and Austrian winter pea (Pisum sativum subsp. arvense L.) were planted in monoculture on four dates during the fall of 2012, each three weeks apart. The first planting date (PD 1) occurred on 1 August, planting date two (PD 2) on 24 August, planting date three (PD 3) on 13 September, and planting date four (PD 4) on 5 October 2012. These variations in fall planting date are representative of a range of possible cover crop planting dates for use in common agronomic cash crop rotations of the Mid-Atlantic region. The experiment utilized a split-block design with crop and planting date as the main effects. Each main plot was approximately 9 m by 11 m, with crop type as a main plot, and planting date subplots of approximately 2 m by 11 m stripped within the main plot. Each treatment was replicated four times with a total of 12 main plots (cover crop type) and 48 subplots (planting date by cover crop type).

Seeds were weighed and measured to provide a seeding rate that was representative of common farmer practices for cover crop monocultures of each species (Clark 2007). Planting was completed using a no-till cone plot drill, which planted nine rows of seed, each 19 cm apart. Planting depth was varied by crop. Canola (12.7 kg/ha) and red clover (13.4 kg/ha) were planted at 1 cm depth and Austrian winter pea (87.3 kg/ha) was planted at 2 cm depth. Plots were managed without irrigation and with manual weed suppression as needed in the fall and early spring.

Floral Density and Phenology.

To assess bloom phenology and density across cover crop species and planting date treatments, one randomly located, 0.25m2 quadrat was flagged in each subplot prior to the onset of flowering. All treatments were monitored for open cover crop flowers, or flower heads for red clover, at least once per week and the total number of open blooms was recorded for each quadrat from the onset of flowering in the treatment until all blooms were gone or the termination of the experiment in early July. The total number of cover crop plants and number of plants in bloom were also counted in all canola treatment plots.

Additionally, three randomly placed quadrats per subplot were monitored within 24 h of the pollinator observations. For these observations, the total number of open blooms per 0.25m2 was recorded to serve as a measure of average bloom density for the subplot across time.

Pollinator Observations and Specimen Collection.

As a method of quantifying abundance of visitation to the blooming cover crop, visual pollinator observations were conducted after cover crop bloom initiation. Each subplot was visually monitored for bee floral visitation for 2 min, twice per day, once in the morning (0900 – 1200 hours) and once in the early afternoon (1230 – 1530 hours). The observer walked at a slow and steady rate along the perimeter of the plot recording all bees that visited the open cover crop blooms during the 2 min period. Each bee that was observed landing on the reproductive parts of the flower was recorded to the lowest taxonomic level possible from visual estimations (modified from Westphal et al. 2008). Groups that were easy to determine on-the-wing were identified to genus (e.g., Bombus, Apis, Xylocopa), whereas those that were smaller or more difficult to identify in motion were grouped into morphospecies categories (e.g., large dark bee, green bee, small dark bee).

After completing visual observations for all plots, each treatment was revisited for an additional 60 s and all bees observed landing on the reproductive parts of the cover crop flowers were collected with an aerial insect net. Netted specimens were killed using a glass kill jar with an ethyl acetate-soaked plaster bottom and returned to the lab. All bees were identified to species. These specimens served as a reference for the morphospecies categories of the preceding observation period as well as an overall indicator of the bee species richness associated with each treatment. As species richness, and not bee abundance, was the goal of collecting netted specimens, bees that were obviously of the same species (e.g., Xylocopa viriginica (L.)) and that had been collected already once during the netting period on that treatment plot were not collected in duplicate, even if observed on the flowers of interest. Apis mellifera L. specimens were not collected often during netting periods as species identification was confident during the visual observations.  

Weather data including air temperature, 30 s average wind speed, and sky condition were collected twice for each session, before and after each morning and afternoon observation and netting period using a thermo-anemometer (Kestrel 2000, Nielsen-Kellerman, Boothwyn, PA). This was repeated for both the morning and afternoon observation sessions. All data for this experiment was collected from late-April to early-July, 2013.

Landscape-level Passive Bee Collection.

To compare the bee community collected from the flowering cover crops to the bee community in the landscape for both experiments, we placed two types of passive traps, pan and plastic vane traps, across the site on a weekly basis from 22 April 2013 until the completion of both experiments in this study. Traps were in place for 48 h with collected specimens removed from the traps every 24 h. Traps were placed in linear transect groups comprised of three pan traps (one each white, yellow, blue) and two vane traps (one each blue and yellow). In total, eight groups of pan and vane traps were deployed across the full study location, an area of approximately 11 ha. All traps were located along grass access roads surrounding the study plots, and were as evenly distributed across the study area as was possible given road spacing constraints and other field operation concerns.

Methodology used for pan trapping was adapted from Westphal et al. (2008) and from The Bee Inventory Plot report (LeBuhn et al. 2002). The pan traps, also referred to as bee bowls, were constructed of 96 ml plastic soufflé cups spray-painted in white (Krylon® Fusion for plastic, Cleveland, OH), florescent yellow (Krylon®, Cleveland, OH), or florescent blue (ACE® Glo Spray, Oak Brook, IL). All yellow and blue bowls were also painted with a primer of the white plastic-bonding paint. Bowls were mounted above the ground on 1.2 m tall, 2 cm diameter wooden dowels. Atop each dowel one painted bowl was attached using a single thumb tack. The final setup consisted of another bowl of the same color placed within the supporting thumb tacked bowl. The sample bowl was filled three-fourths full with soapy water created using 2 liters of water and approximately 1 ml of non-scented dish soap.

The plastic vane traps (SpringStar Inc., Woodinville, WA) are constructed of yellow and florescent blue perpendicular vanes and a collecting tub attached below the vanes. All vane traps were used in their unaltered form. Each trap was suspended from a 1.2 m galvanized steel shepherd’s hook purchased from a local garden supply store. Approximately 200 ml of soapy water mixture were added to the collection tub of each vane trap to act as an insect killing agent.

All non-bee insects collected in the traps were considered bycatch and discarded.


Multiple types of outreach materials were used for the variety of events attended. Such materials included; display collections of pinned, local native bee specimens to illustrate their diversity in body forms, samples of the landscape-level traps, laminated photographs of a variety of flowering cover crops, and power point presentations or posters depending on what was most appropriate for the venue. The box of bees was of particular interest in all instances as it most effectively demonstrated the diversity of native bees in the environment and kept the audience’s attention for the following discussion on their conservation need.

References Cited 

Clark, A. (Ed.). (2007). Managing Cover Crops Profitably (3rd ed.). Sustainable Agriculture Research and Education (SARE).

LeBuhn, G., Droege, S., Williams, N., Minckley, B., Griswold, T., Kremen, C., … Buchmann, S. (2002). The Bee Inventory Plot. Retrieved from

Westphal, C., Bommarco, R., Carre, G., Lamborn, E., Morison, N., Petanidou, T., … Steffan-Dewenter, I. (2008). Measuring Bee Diversity in Different European Habitats and Biogeogrphical Regions. Ecological Monographs, 78(4), 653–671.

Research results and discussion:

In general, because individual bee species respond differently to variations in flower physiology and morphology (O’Toole and Raw 1991, Potts et al. 2003), we expected the pollinator communities to vary across the three cover crop species, and indeed we found this to be the case. Overall, a combined total of 61 bee species were collected from landscape-level passive traps and crop-level netting throughout the course of this project. A total of 36 bee species were collected in canola, followed by Austrian winter pea with 11 species, and red clover with only 6 species. In contrast, 49 bee species were collected in the landscape-level traps, 12 of which were unique to the traps and not collected on any of the three cover crops. However, no single crop or trapping method collected all species observed during the span of this experiment.

Additionally, and contrary to our predictions that planting date would also influence the observed bee communities, we found no difference in the number of bee species across planting date treatments within each cover crop group. Planting date was also predicted to have a significant influence on the timing and duration of the spring cover crop bloom. While we did observe differences in fall plant growth across planting date treatments, with the exception of canola planting date four, planting date did not have a significant effect on spring flowering time for the cover crop treatments. Instead, we saw the highest influence of planting date on winter survival, rather than the initiation or duration of bloom (Table I). As a result, observations of plant growth throughout the spring saw a convergence of crop growth and flowering densities across planting dates as the warm season progressed. Differences in winter survival, however, were cover crop species-specific. Canola and red clover displayed low survival in the latest planted treatments, and Austrian winter pea showed limited survival with the earliest planting date. This, in combination with the differences in spring blooming time and duration across species, demonstrates that the influence of both fall and spring management timing is cover crop dependent.

Besides being the only cover crop to bloom within the field grain rotation window common to the central Pennsylvania study region, this project found canola to be attractive to the greatest diversity of bee species of the cover crops studied. In fact, canola flowers have been shown to be attractive to a wide diversity of pollinators including managed honey bees, native bees, and flies of the Syrphidae family (Free and Nuttall 1968, Jauker and Wolters 2008, Mänd et al. 2010, Viik et al. 2012, Woodcock et al. 2013). Bee community composition was not, however, the only difference observed between cover crop species. We also observed significant differences among the cover crops in bee visitation abundance with significantly more bees visiting canola than either Austrian winter pea or red clover (Figure 1). Given that Austrian winter pea and red clover did not flower until late-May to mid-June, these cover crops would require either rotation windows with summer cash crop planting times or that portion of the cover crop be left in the field to achieve any pollinator floral resource benefit. We conclude that, of the three cover crops studied, canola would have the greatest potential for providing early-season resources for beneficial insects in spring rotation windows for the Mid-Atlantic or in regions with a similar climate regime.

While cover crop species influenced bee community use in this project, bee visitation differences were also observed across planting date treatments within each cover crop (Figure 2). In this case, bee visitation frequency was reduced when the flower density was significantly lower. This was particularly pronounced in the cover crop planting dates that had the lowest winter survival rate and therefore the fewest number of blooming plants and flowers per area (Figure 3). We conclude that floral resource density was the primary factor in the differential response of the bee community across treatments. This conclusion is supported by studies of pollination services in canola fields (Viik et al. 2012) as well as other plant diversity studies (Potts et al. 2003, 2009, Holzschuh et al. 2006, Tuell et al. 2008).

Additionally, it is important to consider variations in weather patterns from year to year and their consequences on winter survival, plant growth, flowering, and insect use. For example, in preliminary data collected on cover crop bloom and bee visitation in the spring of 2012, canola bloomed as early as the first week of April in central Pennsylvania. In contrast, the first blooms of 2013 did not appear until the fourth week of April. This difference would have been specifically evident in field grain rotation windows such as between winter wheat harvest and spring corn planting. Assuming that corn would be planted on the same date in both years, the difference in weather conditions as demonstrated between 2012 and 2013 would have greatly influenced the quantity of canola bloom in the environment in that rotation window. Indeed, other studies have shown great differences between canola oilseed crop production between study years, which were attributed mostly to variations in weather across multiple growing seasons (Lutman et al. 2000).

Weather variations do not only affect plant growth but also influence the timing of insect emergence and foraging activity. In general, the suitable weather conditions for pollinator activity is considered to be low wind, no rain, dry vegetation, and temperatures above 15C (Westphal et al. 2008), with larger bodied bees (e.g., bumble bees) more adapted to foraging in colder temperatures than those with smaller body sizes (Heinrich 1979, Vicens and Bosch 2000). However, as springtime weather conditions in temperate systems are often unpredictable, differences in seasonal temperature or rainfall have the potential to reduce total bee use of a cover crop resource if foraging is limited by ambient weather conditions. Because early-season cover crop bloom will always be subject to fluctuations in spring weather, it is possible that cooler or wetter spring seasons may not provide the same potential resource use as other warmer, drier years.

References Cited

Free, J. B., & Nuttall, P. M. (1968). The pollination of oilseed rape (Brassica napus) and the behaviour of bees on the crop. The Journal of Agricultural Science, 71(1), 91–94.

Heinrich, B. (1979). Bumblebee Economics. Cambridge, MA: Harvard University Press.

Holzschuh, A., Steffan-Dewenter, I., Kleijn, D., & Tscharntke, T. (2006). Diversity of flower-visiting bees in cereal fields: effects of farming system, landscape composition and regional context. Journal of Applied Ecology, 44(1), 41–49. doi:10.1111/j.1365-2664.2006.01259.x

Jauker, F., & Wolters, V. (2008). Hover flies are efficient pollinators of oilseed rape. Oecologia, 156(4), 819–23. doi:10.1007/s00442-008-1034-x

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Research conclusions:

Flowering cover crops have great potential as an agricultural conservation tool that can benefit both production and conservation goals. While the idea of using flowering cover crops for pollinator benefit is not necessary new, the information available to the public at this time is mostly anecdotal. Because of this, the information is often deficient in detail on how various agronomic management properties such as fall cover crop planting date and spring termination date affect the blooming resource of various cover crop species. However, as evidenced by this experiment, increasing our knowledge about the applied use of cover crops for this purpose requires a more complete understanding of the factors that can influence the timing, availability, or quantity of the floral resource. This project does not provide definitive answers about selecting the ‘proper’ flowering cover crop for a particular rotation. Rather, it highlights the importance and influence of factors such as cover crop fall planting date, spring termination time, and cover crop species selection on potential floral resources. While future study is necessary before we have a complete understanding the utility of cover crops as a tool for pollinator conservation in annual, temperate cropping systems, this study can serve as a baseline for supplementary studies and as an indicator of relevant factors that conservationists, land managers, extension agents, and university researchers must consider to successfully expand and refine the use of flowering cover crops in conservation agriculture.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

During this the summer of 2013 I was fortunate to have been able to share the results of my research project with farmers and other sustainable agricultural professionals at two different farmer extension field days. The first event was held on September 7, 2013 in Milton, PA at the home of an organic forage grain farmer and Penn State research collaborator. The topic of this event was ‘Cover Crop Research and Organic Weed Control’ and included side discussions and presentations on other related research projects our group was conducting with cover crops at the University. This opportunity provided me with a platform to give a 20-minute presentation on the importance of native pollinators, why we should conserve them in agricultural landscapes, and how we can consider the use of flowering cover crops as a dual-purpose crop for field-level agronomic benefits as well as ecosystem service conservation benefits. Several of the famers that I spoke with at this event seemed genuinely interested in helping our native pollinators and my talk seemed to peaked their interest into considering what other benefits can be gained from smart cover crop choices. The second extension field day event took place on September 11, 2013 at the Penn State research farm and was a collaborative event between our Penn State cover crop research team and the Pennsylvania Association for Sustainable Agriculture (PASA). The topic of this event was ‘Cocktails & Crimpers: Cover Crop Innovations for Low-Input Soy, Corn & Wheat Production’, and again I was able to speak to this audience for 20 minutes about pollinators and using cover crops for their conservation benefit. Both field day events went very well and were met with interest and enthusiasm from many of the participants. Also included with both field days were questionnaires giving evaluations of the event. For this I was able to ask for information regarding the participant’s previous knowledge of native pollinators, knowledge gained via my presentation, and future interest in planting flowering cover crops for the use of native pollinator communities. We had a total of 41 participants between both field days. Combining the evaluations from both events, 61% of the participants reported leaving with increased knowledge of native pollinator diversity, 61% said that they would likely consider planting cover crops for the purpose of pollinator conservation, and 59% said they were more likely to allow cover crops to produce flowers for native pollinator resource use.  

Other presentations that included results from this project include; a poster presentation at the International Conference on Pollinator Biology, Health and Policy (University Park, PA, August 2013), an oral presentation at the annual conference for the Entomological Society of America (Austin, TX, November 2013), a poster presentation at the 3rd annual Penn State TRIAD Symposium for sustainable cropping systems (University Park, PA, March, 2014), and a free, hour long public talk on native pollinator conservation given at the Aldo Leopold Foundation (Baraboo, WI, August 2014). All events were well attended and received although no specific numbers or evaluations were collected.

In addition to the numerous presentations, two publications were produced regarding the results and purpose of this study. The first is a Penn State Extension fact sheet on cover crops and native pollinators entitled Bees and Cover Crops: Using flowering cover crops for native pollinator conservation and is available free online via the Penn State Department of Entomology extension fact sheet page. The second is a peer reviewed scientific publication entitled Management of Overwintering Cover Crops Influences Floral Resources and Visitation by Native Bees.

Project Outcomes

Project outcomes:

Because this study focused on how flowering cover crops could provide conservation benefit to native bee communities in addition to any other agronomic and economic benefits that cover crop may already provide growers, no specific economic analysis of the pollination or conservation potential is possible via this project format.

Farmer Adoption

As this project was designed primarily to provide baseline research for the future increased application of flowering cover crops for native pollinators, direct farmer contact or information on increased adoption was limited to outreach events and contact with extension agents and other agriculture professionals. I did have, however, many positive comments and conversations before and after each of my outreach and scientific presentations across the country. Additionally, I received an email in early May, 2014 from a Southern Pennsylvania farmer seeking specific cover crop advice for their farm after they had read my online extension fact sheet. Assuming that most farmers who would have encountered my publication would not have taken the time for independent contact, I believe this letter, received only a few months after publication, demonstrates that farmers are reading and positively receiving the ideas that my project supports. I also know that data and photographs from my project have been incorporated into public presentations from Pennsylvanian agricultural professionals, including others from my university as well as regional NRCS offices, as recently as September 2014. The NRCS agent in particular, learned of my project through a presentation that I gave at a scientific conference the previous fall.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

Because, prior to this study, limited scientific data existed on the use of cover crops for pollinator conservation, my project, as a baseline study, has several areas of possible expansion. Such areas include; similar studies as this but considering other species of flowering cover crop (e.g. hairy vetch, crimson clover, buckwheat), comparison studies investigating bee communities in areas with flowering cover crops and areas without to assess actual conservation potential, and monitoring pollen and nectar levels across cover crop species and planting dates to look for differences in actual resource availability. Each of these additional areas could be designed as independent and complete research studies whose results would build upon and expand the knowledge base created from this initial experiment.

Information Products

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.