If you build it, will they come? Insect community responses to habitat establishment at solar energy facilities in Minnesota, USA

Insects serve many roles for ecosystem function, including nutrient cycling, plant pollination and seed dispersal, maintaining soil quality, and occupying important trophic levels as both natural predators and prey (Scudder 2017). Observed declines in insect populations world-wide have understandably raised concerns regarding impacts to these ecosystem functions and implications for human well-being (Sanchez-Bayo and Wyckhuys 2019). Most notably has been the impact of insect declines on agricultural production through loss of pollination services and natural pest management (Wratten et al 2012, Potts et al 2016). Approximately 75% of global crop production is at least partially reliant upon pollination by insects (Klein et al 2007), underscoring the importance of insect pollinator conservation for human food production. In addition, insect biodiversity in agricultural landscapes is important for natural pest control, and loss of beneficial insect predators can result in reduced crop yield and increased use of pesticides (Kovács‐Hostyánszki et al 2017).

Primary factors contributing to the decline in global insect biodiversity include habitat loss, pesticides, and climate change (Sanchez-Bayo and Wyckhuys 2019). Paradoxically, habitat loss due to agricultural intensification has contributed to insect population declines in many rural landscapes (Ekroos et al 2016). As a result, conservationists have suggested that habitat restoration and enhancement in agricultural landscapes could mutually safeguard insect populations and their pollination services for crop production. This has been put into practice across the U.S. through the Conservation Reserve Program (CRP), which was implemented by the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) over 3 decades ago. The program has aided conservation efforts by retiring millions of acres of farmland from cultivation in marginal or ecologically sensitive areas and restoring those areas to native grasses and pollinator-friendly vegetation (Arathi et al 2019, Dolezal et al 2021, United States Department of Agriculture [USDA] 2023a). In addition, evidence suggests that restoring habitat close to pollinator-dependent crops could increase biodiversity and pollination services (Morandin and Kremen 2013, Blaauw and Isaacs 2014, Feltham et al 2015, Kordbacheh et al 2020, Levenson et al 2022, Mota et al 2022). For example, Blaauw and Isaacs (2014) found that wildflower plantings near blueberry (Vaccinium corymbosum L.) fields were temporally associated with greater blueberry pollination visitation by native bees and resulted in significant increases in blueberry yield parameters.

Given the associations between insect biodiversity, habitat, and agricultural production, land use and land cover changes in agricultural landscapes can have profound effects on insect populations and their ecosystem services (Millard et al 2021). In agricultural landscapes across the U.S., renewable energy developments represent a growing form of land use change (Hernandez et al 2015, Walston et al 2021). Utility-scale solar energy developments (ground-mounted photovoltaic (PV) facilities >1 MW) require large amounts of relatively flat, open land, making former agricultural fields ideal locations to site these projects (Adeh et al 2019). A 2021 examination of utility-scale solar energy facilities in the Midwestern U.S. found that 70% of all solar facility footprints in the region were previously used for row crop agriculture, equating to a land cover/land use change of nearly 2400 ha (24 km²) (Walston et al 2021). This relatively rapid rate of land cover/land use change is expected to continue in the coming decades. For example, by 2035, as much as 1000 gigawatts (GW) of utility-scale solar development is needed to keep the country on track for its net-zero carbon emission goals ([DOE] U.S. Department of Energy 2021), representing over a 10X increase in current solar energy deployment ([EIA] Energy Information Administration 2023). The 2035 U.S. solar energy target could require up to 3 million ha (3000 km²) of land, based on an assumed solar energy land use requirement of approximately 3 ha per MW (Ong et al 2013, Walston et al 2021). While future solar developments may be sited in a variety of locations, many agricultural lands suitable for solar energy are likely to be developed.

Siting facilities to avoid environmentally sensitive areas is one of the most important ways to minimize the environmental impacts of solar energy development (Jager et al 2021) and dual use land use strategies can lessen the impact of developments in agricultural landscapes (Hernandez et al 2019). Dual land uses, such as the co-location of solar energy with agricultural activities ('agrivoltaics') and habitat restoration, take advantage of the solar facility's relatively large footprint to improve the site's land use efficiency and ecosystem services output (Barron-Gafford et al 2019, Walston et al 2022). One form of habitat restoration at solar sites, commonly referred to as 'solar-pollinator habitat', focuses on the establishment and maintenance of grasses and forbs among the solar panels which, if managed properly, can provide habitat for insects and other wildlife. Under this broad concept, solar-pollinator habitat has the potential to minimize solar energy impacts by supporting biodiversity, agricultural health, and other ecosystem services such as preserving water quality, runoff and erosion control, and carbon sequestration (Graham et al 2021, Walston et al 2021).

The establishment and management of solar-pollinator habitat is a relatively novel concept and most of what is reported about the ecological and ecosystem services benefits of this practice is theorized from models and results from other habitat restoration studies (e.g. Walston et al 2021). There is little empirical evidence on the establishment of vegetation at solar facilities and resulting biodiversity responses (but see Graham et al 2021). Fundamental research is therefore needed to systematically describe the biodiversity outcomes of this novel land use across different regions and vegetation management practices. To address these basic research needs, we conducted a longitudinal field study across 5 years (2018–2022) to understand how insect communities respond to newly established habitat on solar energy sites in agricultural landscapes. We were also interested in understanding whether the establishment of solar-pollinator habitat resulted in any 'spillover' effects in pollinator and beneficial insect visitation to nearby crop fields such as soybeans (Glycine max). Soybean is one of the most extensively grown crops worldwide and while it is capable of self-pollinating, studies have shown yield increases as a result of bee visitation (Garibaldi et al 2021, Levenson et al 2022). Our objectives were to address the following three research questions:

• Does flowering plant abundance and diversity on solar sites increase over time?
• Does insect abundance and diversity within the solar sites increase over time?
• Does proximity to solar-pollinator habitat influence bee visitation to croplands near the solar facilities?

2.1. Study sites

Our study took place between 2018 and 2022 at two utility-scale solar energy facilities in southern Minnesota, USA: the Atwater Solar Site (45.1410° N, −94.7702° N) and Eastwood Solar Site (44.1545° N, −93.9164° N). Both solar energy sites are operated by Enel Green Power North America and are located in rural landscapes between 100 and 200 km from Minneapolis, MN. The solar sites are separated by 160 km and 1° of latitude (figure 1(A)). Construction for both sites was completed in 2017. The Atwater Solar Site is 14 ha in size with a nameplate electrical capacity of 4.0 megawatts (MW) (alternating current [AC]). The Eastwood Solar Site is 17 ha in size with a nameplate electrical capacity of 5.5 MW AC. Both solar facilities are constructed as arrays of monocrystalline PV solar panels mounted on 2 m high racking with single axis tracking systems. Light sensors in the trackers allowed the panels to rotate and follow the sun throughout the day. At both sites, arrays were positioned north-to-south, allowing panels to track the east-west daily orientation of the sun (figures 1(B) and (C)). At lowest vertical orientation each day, the lower edge of panels is approximately 0.9 m above ground.

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Prior to solar energy development, both sites were previously used for decades for row crop agricultural production. The solar facilities were constructed with plans to minimize impacts to soils. For example, in an effort to preserve soil quality and productivity, the solar sites were not graded (except for the construction of access roads). Both sites remained adjacent to row crop agriculture on at least 2 sides throughout our study (figures 1(B) and (C)).

2.2. Habitat restoration & management

2.3. Field work & experimental design

2.4. Data & analyses

We made a total of 358 transect observations in the onsite solar-pollinator habitat areas between August 2018 and August 2022. Aggregating repeated within-year transect observations resulted in a final dataset of 54 transect observations in which we calculated average beneficial insect abundance, maximum floral abundance rank, and total flowering plant species richness. Of these aggregated transect observations, 20 occurred on the Atwater Solar Site and 34 occurred on the Eastwood Solar Site. We detected a total of 10 943 beneficial insects across both solar sites, spanning 4 orders (Hymenoptera, Diptera, Lepidoptera, Coleoptera) which we summarized to insect groups. The most numerous insect groups we observed were beetles (primarily goldenrod soldier beetles [Chauliognathus pennsylvanicus]; 35.1% of total observations), Syrphid flies (primarily Toxomerus sp.; 19.5% of total observations), and moths (17.2% of total observations) (figure 2). We observed distinct temporal shifts in the insect community. Syrphid flies were the dominant pollinator group observed from years 0–2. In Years 3 and 4, our observations were dominated by soldier beetles (C. pennsylvanicus).

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3.1. Habitat metrics over time

Throughout all 5 years of this study we observed 37 different flowering plant species within the onsite habitat transects (table S2). Floral abundance and flowering plant species richness increased over time following seeding in year 0 (figures 3(A) and (B)) and there were no differences in these responses between the two solar sites (table 1). Using the best-ranking GAMs, year explained 61.4% and 84.8% of the variability in floral abundance and flowering species richness, respectively (table 1).

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Table 1. Generalized additive model (GAM) results for the effect of (A) year and (B) site on measured habitat and biodiversity metrics. Results are for the best-ranked GAM for each response variable (see supplement).

(A) Year effects (fixed effect)
Response variable	Test statistic a	p	R2
Floral abundance	14.63	<0.001	0.616
Flowering species richness	17.74	<0.001	0.896
Insect group diversity	1.961	0.045	0.400
Total insect abundance	79.57	<0.001	0.636
Native bee abundance	13.27	0.002	0.667
(B) Site effects (random effect)
Response variable			p
Floral abundance			0.487
Flowering species richness			0.148
Insect group diversity			0.686
Total insect abundance			0.132
Native bee abundance			0.491
(C) Floral rank effects (random effect)
Response variable			p
Floral abundance			—
Flowering species richness			0.576
Insect group diversity			0.528
Total insect abundance			0.001
Native bee abundance			0.003
(D) Flowering species richness effects (random effect)
Response variable			p
Floral abundance			0.244
Flowering species richness			—
Insect group diversity			0.181
Total insect abundance			0.675
Native bee abundance			0.023

^a Test statistic is Z value for linear models without smoothing parameter (Floral abundance, Insect group diversity) and Chi-square value for GAM models with smoothing parameter.

3.2. Biodiversity metrics over time

3.3. Offsite agricultural visitation

We made a total of 52 transect observations in the soybean fields between 2019 and 2022 during the soybean bloom period, which generally occurred during the month of July each year. Within these transects, we observed a total of 509 pollinators and beneficial insects, of which 23 were bees (14 native bees, 9 honeybees). 70% of the insects observed in the soybean fields were syrphid flies (family Syrphidae). All native bees observed visiting soybean flowers were bumblebees (Bombus sp.). Visitation of pollinators and beneficial insects to soybean flowers differed by field location (table 2). Bee visitation to soybean flowers adjacent to solar-pollinator habitat was comparable to bee visitation adjacent to CRP grasslands, and approximately 2 times and 2.5 times greater than bee visitation at roadside and soybean field interior transects, respectively. For all pollinators and beneficial insects, there were no differences in visitation between solar-adjacent soybean transects and CRP-adjacent or roadside soybean transects. However, these locations did have greater visitation than soybean interior transects (figure 4).

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Table 2. Effect of field location group on bee visitation to soybean flowers in agricultural fields near the solar facilities. Results are for the best-ranked GLMM for each response variable (see table S8 in the supplemental materials).

Response variable	df	χ2	p
Total pollinator and beneficial insect visitation	4	38.111	<0.001
Bee visitation	4	116.11	<0.001

Our results provide the first empirical field evidence on the interannual, temporal changes in insect communities following the planting of pollinator habitat at solar energy facilities. Consistent with other studies, our observations highlight the relatively rapid (<4 year) insect community responses to grassland restoration activities (Griffin et al 2017, Onuferko et al 2018, Lanterman Novotny and Goodell 2020, Purvis et al 2020). As predicted, all habitat and biodiversity metrics increased each year following conversion from cropland to solar-pollinator habitat. By the end of the 5 year study period, we observed a 7-fold increase in flowering plant species richness, on average, within the onsite habitat transects. In that same time, abundance of insect pollinators and beneficial insects tripled, and insect group diversity increased by an average of 13% per year. Remarkably, we observed an exponential increase in the abundance of native bees, which increased over 20-fold during this study, with most observations occurring after Year 2.

Our findings on the solar sites were restricted to the experimental test plots which were planted with additional plant species and managed differently than the rest of the solar sites. A total of 66 species of native grasses and forbs were planted throughout the two solar sites. An additional 61 species were planted in the experimental test plots (supplemental table S1). Areas outside the experimental test plots were mowed more frequently and were also strategically grazed with sheep during the last two years of the project. Despite these differences, the dominant flowering native species observed in the experimental test plots were consistent with the observations of groundcover establishment within the larger PV array area (as documented in McCall et al ). The dominant flowering native species observed at both solar sites included Black-eyed Susan (Rudbeckia hirta), Spotted Bee Balm (Monarda punctata), and Goldenrod (Solidago sp.); see supplemental table S2 for a complete list of flowering plant species observed within the experimental test plots. Future research should be designed to examine site-level variation in vegetation management, with sampling designs that are more dispersed throughout the solar facilities, to better understand the biodiversity responses across the entire solar site.

The purpose of our study was to describe the interannual, temporal changes in the insect community in the years following the planting of solar-pollinator habitat. We did not conduct surveys in CRP grassland areas during this study to determine how the insect community in the solar-pollinator habitat compared to offsite reference grasslands. However, several other studies have demonstrated how insect abundance and diversity within restored areas have reached levels comparable to reference grasslands within a relatively short amount of time (<4 years). For example, in a study of restored grasslands across North America, Purvis et al (2020) found wild bee diversity increased sequentially following restoration and approximated bee communities in reference grassland sites after 1–4 years. Similarly, Onuferko et al (2018) found that wild bee abundance and species richness increased rapidly in the first 4 years following grassland restoration to resemble bee communities in reference grassland areas. Onuferko et al (2018) also noted long-term persistence of the bee community in the restored grassland for over 10 years (the length of the study). The rapid increase in insect group abundance and diversity we observed, especially native bees, is similar to these studies and highlights the potential for these plantings to benefit local insect communities. In the only other published field study on insect responses to solar-pollinator habitat in the U.S., Graham et al (2021) recorded the establishment of native pollinator-friendly vegetation at a solar facility within 2 years of planting and described the interactions between vegetation and micro-scale locations within a solar facility on pollinator abundance and diversity. They found insect abundance, diversity, and richness were similar in full sun and partial shade regions of the solar facility where solar-pollinator habitat was planted, both of which were greater than full shade regions of the solar facility. Our study did not include transects in full shade regions directly under the solar panels.

Along with observed annual increases in insect group abundance and diversity within the solar-pollinator habitat transects, we also found positive effects of proximity to solar-pollinator habitat on bee visitation to nearby soybean fields. We found that bee visitation to soybean flowers adjacent to (i.e. within 15 m of) solar-pollinator habitat was comparable to soybean visitation adjacent to CRP grasslands, both of which were greater than bee visitation to soybean field interior and roadside soybean transects (though the difference was only significant for soybean field interior; see figure 4). As bee visitation is often used as an indicator of pollination services (Ricketts et al 2008), these results suggest that solar-pollinator habitat could help improve soybean production, similar to CRP and other conservation grassland systems through the 'spillover effect' (Levenson et al 2022). The rationale behind the spillover effect is that habitat establishment in agricultural areas will increase populations of insect pollinators, which will result in the spillover of these insects into crop fields that will increase crop pollination and help improve crop yields (Morandin and Kremen 2013). Future research focused on the seasonal variation in insect community responses to solar-pollinator habitat will help identify the seed mixes and best management practices that optimize the agricultural services of solar-pollinator habitat by maximizing the spillover of insect pollinators and beneficial insects during the months that these services are most needed.

The insect community responses to solar-pollinator habitat we observed—both within the onsite habitat test plots and visitation to adjacent soybean fields—underscores two important potential implications of solar-pollinator habitat. First, solar-pollinator habitat can play an important role in conserving biodiversity in agricultural landscapes. Given the increasing amount of ground-mounted solar energy that is expected to be developed by 2035 ([DOE] U.S. Department of Energy 2021), solar-pollinator habitat can offset losses of natural areas and restored grasslands in agricultural landscapes to support pollinators and their agricultural services. The CRP program is a nationally recognized grassland restoration program to conserve insect pollinator populations. However, CRP enrollments have been declining in recent years, which could result in insect population declines, and negatively impact agricultural services (Otto et al 2018, Smith et al 2021). Solar-pollinator habitat can offset CRP losses to support insect populations and safeguard their agricultural services. For example, it was estimated that over 3500 km² of agricultural land could benefit from pollination services supported by solar-pollinator habitat across 2244 operating solar facilities in the U.S. in 2016 (Walston et al 2018). The potential agricultural service benefits of solar-pollinator habitat are likely much larger today, as the amount of utility-scale solar energy generation in the U.S. has more than doubled in the past 5 years ([EIA] Energy Information Administration 2023).

Second, like other forms of agrivoltaics, solar-pollinator habitat could help mitigate land use conflicts associated with the conversion of farmland for solar energy production. The United States has lost over 12 million acres of agricultural land since 2015 (USDA 2023b), increasing the pressure on the remaining agricultural lands for food production. Solar energy development may contribute to further declines in farmland, as approximately 80% of future ground-mounted solar energy development could occur on agricultural lands (Sorensen et al 2022). Rather than exacerbate the effects of these land use tradeoffs, these effects can be alleviated through the proper siting of solar energy developments and pairing with solar-pollinator habitat or other agrivoltaic dual land uses. For example, siting future solar energy sites on marginal farmland and pairing these developments with solar-pollinator habitat could preserve prime farmland, improve the productivity of those remaining lands through pollination and pest control services supported by solar-pollinator habitat, and increase the site's ecosystem services potential (Walston et al 2021). In addition, other forms of agrivoltaics such as co-locating solar energy development with crop production or livestock grazing, can ensure that onsite agricultural practices continue which can help balance the nation's needs for food and energy production (Walston et al 2022).

As a relatively new land management practice, there are several information gaps and considerations associated with solar-pollinator habitat that need to be addressed; many of these are summarized in Macknick et al (2022). Vegetation considerations include the feasibility of establishing solar-pollinator habitat in different geographic regions, the availability of seed mixes required to result in increased pollinator abundance and diversity, and vegetation management considerations to control weed establishment. Research on the ecological effectiveness of various seed mixes, mowing frequencies, and grazing strategies is ongoing in the U.S. (e.g. U.S. Department of Energy InSPIRE Study, https://openei.org/wiki/InSPIRE; U.S. Department of Energy PHASE Study, https://rightofway.erc.uic.edu/phase). Although we observed relatively rapid establishment of solar-pollinator habitat and associated responses of the native insect community in Minnesota, solar-pollinator habitat establishment might be slower in arid regions that are less productive and receive little precipitation (Grodsky et al 2021, Walston et al 2022). Another consideration is the influence of solar energy technological factors on solar-pollinator habitat establishment and management. Most notably, technological designs related to panel height and spacing limit what plant species can be grown at the site so as not to shade the panels and impede energy production. Although the solar panels in our study had a leading edge of approximately 0.9 m at the lowest point, other solar facilities may have panels with lower leading edges, limiting what seed mixes can be planted at those sites. Vegetation management may also be unique at solar energy facilities where smaller specialized mowing equipment may be required due to the spacing of the solar panels. In addition, shading and microclimates created by solar panels may influence the establishment of certain plant species and their blooming phenologies (e.g., Tanner et al 2020, Graham et al 2021). The interactions of these technology-related factors and the ecological performance of solar-pollinator habitat are poorly understood. As a result, future research should focus on methods to optimize habitat establishment within these unique environments to improve the ecological compatibility of solar energy developments.

The establishment of solar-pollinator habitat can be a low impact approach to improve the ecological compatibility of utility-scale solar energy. However, the ecological effectiveness of this practice depends on a variety of factors including solar facility design, geographic region, and the role of solar-pollinator habitat in the ecological mitigation hierarchy (Arlidge et al 2018). Solar-pollinator habitat is unlikely to completely offset the residual ecological impacts of solar developments poorly sited in areas with high ecological value. In this context, solar-pollinator habitat may have the greatest potential for ecological benefit for solar energy facilities sited in areas that have been previously ecologically compromised, such as marginal farmland, former industrial or mine lands, and other disturbed sites. In these situations, solar-pollinator habitat may be able to provide net biodiversity benefits. Given the design and operations of utility-scale PV facilities, solar-pollinator habitat could become a novel ecosystem made up of unique plant and animal assemblages based on the compatibility of seed mixes to be planted and unique site-level vegetation management practices. Additional research is needed to understand the feasibility of solar-pollinator habitat across different regions and to meet different ecological goals (e.g. to conserve target insect or wildlife species) that will optimize the ecological compatibility of these novel renewable energy land uses.

Funding was provided by the U.S. Department of Energy (DOE) Solar Energy Technologies Office under Award Number 38642. This article was developed by Argonne National Laboratory, operated for the DOE under Contract No. DE-AC02-06CH11357. Additional support was provided by NREL, operated for DOE under Contract No. DE-AC36-08GO28308. The views expressed in the article do not necessarily represent the views of the DOE or the US Government. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. We appreciate the constructive comments from Juan Botero and two anonymous reviewers on previous drafts of this article. We are grateful to Katherine Szoldatits, Irene Hogstrom, William Vinikour, Yudi Li, and Meredith Walston for field assistance, data analyses, and/or logistical support. We appreciate Enel Green Power North America and several landowners (C Block, P Bowe, S Bergo, R Bohnert) that permitted site access for this study. Finally, we are thankful to Aaron Hanson and the University of Minnesota's Sustainability Corps Program through the support of the following undergraduate field research interns: Natalie Narvaez, Adam Brodsky, and Gretchen North.

The data that support the findings of this study are available upon reasonable request from the authors.

The authors declare no conflicts of interest.

L W, H H, J M, and J M designed the research. L W, H H, L F, and J J conducted the field work. L W and L J conducted data analyses. All authors participated in writing the paper.

The article does not contain any studies involving human participants. There was no handling of insects or other organisms in this study.