# Intraspecific crop diversity for enhanced crop pollination success. A review

Stan Chabert, Maxime Eeraerts, Lisa W Devetter, Monica Borghi, Rachel E Mallinger

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## To cite this version:

Stan Chabert, Maxime Eeraerts, Lisa W Devetter, Monica Borghi, Rachel E Mallinger. Intraspecific crop diversity for enhanced crop pollination success. A review. Agronomy for Sustainable Development, 2024, 44 (5), pp.50. 10.1007/s13593-024-00984-2. http://dx.doi.org/10.1007/s13593-024-00984-2

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Distributed under a Creative Commons Attribution 4.0 International License
# Intraspecific crop diversity for enhanced crop pollination success.
A review

Stan Chabert<fnref n="1" />,<fnref n="2" /> ID · Maxime Eeraerts<fnref n="3" /> ID · Lisa W. DeVetter<fnref n="4" /> ID · Monica Borghi<fnref n="5" /> ID · Rachel E. Mallinger<fnref n="1" /> ID

Accepted: 4 September 2024 / Published online: 15 October 2024

© The Author(s) 2024

## Abstract

Interspecific crop diversity (e.g., intercropping) has been documented to promote sustainability in agroecological systems with benefits for pollination services and insect pollinators. These benefits may also be extended to intraspecific crop diversity (e.g., cultivation of multiple genotypes or cultivars in a production space), but no review to date has examined the benefits of intraspecific crop diversity for pollination and pollinator communities. While mixing cultivars is necessary and a widespread practice for pollination of self-incompatible or male-sterile crops, it is not as widespread for other crop species. However, many other crops have shown reduced yield quantity or quality with self-fertilization due to partial self-sterility, early acting inbreeding depression, and xenia. These crops could thus experience increased production in diverse cultivar mixtures. Cultivar mixtures could also benefit pollinator communities through providing complementary and temporally consistent floral resources, with cascading effects on pollination services. However, successfully implementing cultivar mixtures requires an understanding of how cultivar identity and arrangement affect successful cross-pollination. In this review, we describe the potential benefits of increased intraspecific crop diversity for optimal pollination and pollinator populations across insect-pollinated crops. Additionally, we explore how research advances in cultivar characteristics and insect pollinator behavior and movement, as well as crop pollen flow, can inform cultivar mixtures and spatial arrangements. We find evidence that mixing cultivars, even in self-compatible crops, improves pollination outcomes and yields. Additionally, given insect pollinator behavior and pollen flow, such mixing must occur at relatively small spatial scales. Furthermore, cultivar diversity could ensure successful pollination and resource production for pollinators under extreme weather events. We also discuss costs and benefits of diverse cultivar mixtures from a grower's perspective and offer suggestions for future research including translating findings within the context of farming systems so that recommendations are practical and achievable.

**Keywords** Crop pollination · Cross-pollination · Cultivar diversity · Entomophilous crops · Extreme weather · Field design · Insect foraging behavior · Intraspecific crop diversity · Pollinator community · Pollen flow

## Contents

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Rachel E. Mallinger

Rachel.mallinger@ufl.edu

1. Introduction

2. Cultivar mixing and cross-pollination enhance pollination success

2.1 Cultivar mixing ensures pollen availability and performance

2.2 Cultivar mixing ensures fertility and enhances seed and fruit quality

3. Implementation of effective cultivar mixtures for pollination success

4. Benefits of cultivar mixtures for pollinators

5. Agronomic considerations for commercial implementation

6. Conclusions

Acknowledgments

References

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<footnote n="1">¹ Department of Entomology and Nematology, University of Florida, 1881 Natural Area Drive, Gainesville, FL 32611, USA</footnote>

<footnote n="2">² Université de Toulouse, INRAE UMR LIPME, 31326 Castanet-Tolosan Cedex, France</footnote>

<footnote n="3">³ Forest & Nature Lab, Department of Environment, Ghent University, 9090 Melle-Gontrode, Belgium</footnote>

<footnote n="4">⁴ Department of Horticulture, Northwestern Washington Research and Extension Center, Washington State University, 16650 WA-536, Mount Vernon, WA 98273, USA</footnote>

<footnote n="5">⁵ Department of Biology, Utah State University, Logan, UT 84322-5305, USA</footnote>
# 1 Introduction

It is well known that biodiversity increases ecological processes and ecosystem functioning (Loreau et al. 2001; Hughes et al. 2008; Bolnick et al. 2011; Rasmussen et al. 2024). Crop diversity can increase the diversity and abundance of beneficial organisms, increase soil health, reduce the need for agrochemical and/or mechanical inputs, and provide greater resilience in the case of unfavorable weather conditions or other disturbances (Tamburini et al. 2020; Magrach et al. 2023). In turn, crop yields can be stabilized over time and space, and total profits increased (Lin 2011; Renard and Tilman 2019; Garibaldi and Pérez-Méndez 2019). In cropping systems, this concept has been well studied at the interspecific level with agricultural diversification practices including intercropping, cover cropping, and polycultures (Andow 1991; Langer et al. 2007; Boudreau 2013; Bedoussac et al. 2015; Brooker et al. 2015; Liebman 2018; Bourke et al. 2021). Intraspecific crop diversity, achieved through the use of multiple plant genotypes, cultivars or varieties, could also boost agroecosystem function and is common in traditional farming (Fig. 1; Harlan 1975; Clawson 1985; Jarvis et al. 2008; Dawson and Goldringer 2012). It has also received recent scientific interest, especially in the context of agricultural intensification and climate change (Howden et al. 2007; Hajjar et al. 2008; Malézieux 2012; Tooker and Frank 2012; Litrico and Violle 2015; Barot et al. 2017; Montazeaud et al. 2020; Snyder et al. 2020; Wuest et al. 2021). However, reported benefits of cultivar mixtures have been inconsistent across studies (Barot et al. 2017; Borg et al. 2018; Montazeaud et al. 2020; Wuest et al. 2021), raising the need to examine the underlying ecological mechanisms that may explain the benefits of cultivar diversity for crop production such as niche complementarity, facilitation or synergy, compensation, sampling effect, and insurance (e.g., stability and resilience against a stress or perturbation) (Creissen et al. 2013). Additionally, to be maximally effective,

diversity, either interspecific or intraspecific, should result in some level of functional diversity (Barot et al. 2017; Borg et al. 2018; Lazzaro et al. 2018; Lowry et al. 2020; Montazeaud et al. 2020; Hauri et al. 2021, 2022; Dubs et al. 2023). Understanding the unique functions of different cultivars, and how they may complement or facilitate one another, is thus important for ensuring that cultivar diversity results in enhanced crop productivity and stability.

Cultivar mixtures and resulting functional diversity have mainly been studied for their effects on pathogen and pest management and potential to decrease pesticide inputs while maintaining yields (Zhu et al. 2000; Lazzaro et al. 2018; Pélissier et al. 2021; see reviews in Wolfe 1985; Finckh et al. 2000; Lin 2011; Tooker and Frank 2012; Kristoffersen et al. 2020; Snyder et al. 2020). However, no recent review has focused on the benefits of mixing cultivars for pollination and insect pollinator communities of entomophilous crops, including for both self-sterile and self-fertile crops (but see Hajjar et al. 2008 for a section on benefits for pollinators). An estimated 75% of the 115 most important global crops rely on animal pollination to some degree (Klein et al. 2007), which adds an estimated $235–577 billion US dollars to global agricultural production (IPBES 2016). Furthermore, animal pollination enhances crop quality (Gazzea et al. 2023) and contributes disproportionately to human nutrition given that animal-pollinated crops contain the majority of vitamins and minerals needed in an optimal human diet (Eilers et al. 2011; Chaplin-Kramer et al. 2014; Smith et al. 2015). While insects are not the only animal pollinators, they are the most ubiquitous and typically most effective pollinators for global crops (Klein et al. 2007), but their populations and the pollination services they provide are compromised by numerous stressors including land-use change and loss of resources (Gallai et al. 2009; Zattara and Aizen 2021). Indeed, suboptimal pollination success, expressed as pollen limitation, has been reported for multiple crops and production regions (Reilly et al. 2020; Olhnuud et al. 2022; Eeraerts et al. 2023a)

<figure><img src="image_5.png" /><figcaption>**Fig. 1** An example of a mixed-cultivar field of northern highbush blueberry *Vaccinium corymbosum*. Photo credit: Rufus Isaacs, Michigan State University.</figcaption></figure>

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<figure><img src="image_7.png" /><figcaption>**Fig. 2** Mechanisms and contexts in which mixing cultivars within a crop field can have positive effects on pollination and on floral resources, with subsequent effects on crop performance and pollinator communities.</figcaption></figure>

and with variation across different cultivars of the same crop (Garratt et al. 2021; Eeraerts et al. 2024). Hence, reducing pollen limitation of pollinator-dependent crops remains a significant and important challenge for increasing global food supply and addressing deficits in human nutrition (Isaacs et al. 2017).

In this review, we describe the types of entomophilous crops that may benefit from cultivar mixtures and the mechanisms responsible for such benefits (Fig. 2). We additionally synthesize the existing literature on pollinator behavior and movement, as well as crop pollen flow, to inform optimal cultivar spatial arrangements for cross-pollination. Finally, we discuss cultivar characteristics to consider when selecting cultivar mixtures and costs and benefits of mixed-cultivar fields from a grower's perspective. A glossary of the botanical terms used throughout is provided in Table 1.

# 2 Cultivar mixing and cross-pollination enhance pollination success

## 2.1 Cultivar mixing ensures pollen availability and performance

Mixing cultivars for pollination is common practice in some cropping systems such as those with genotypes that do not

produce pollen, including dioecious and gynodioecious crops, or those that produce pollen only during one stage of flowering, including heterodichogamous crops (Table S1). Examples of these crops include avocado *Persea americana*, kiwifruit *Actinidia chinensis*, watermelon *Citrullus lanatus*, and the production of hybrid seed for numerous vegetables and oilseeds (Table S1). Dioecious crops, such as kiwifruit, require the transport of pollen from the male plants that bear only staminate flowers to the female plants that bear only pistillate flowers. Gynodioecious crops have two distinct genotypes: genotypes that are functionally “male-fertile” and bear hermaphroditic flowers or both staminate and pistillate flowers on the same plant and genotypes that are functionally “male-sterile” and bear only pistillate flowers. These crops can include ones that spontaneously produce male-sterile genotypes (e.g., European chestnut *Castanea sativa*; Goji berry *Lycium barbarum*; Chaudhury et al. 1994; Chen et al. 2018; Larue and Petit 2023; Larue et al. 2023), ones with triploid genotypes that produce pollen with very low viability and are thus functionally male-sterile (e.g., triploid seedless watermelon or triploid common pear *Pyrus communis* genotypes; Takasaki-Yasuda et al. 2013; Wijesinghe et al. 2020; Subasinghe Arachchige et al. 2022), or artificially achieved male-sterile genotypes including those obtained by using plant growth hormones in hybrid seed cropping

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<table><caption>**Table 1** Glossary of botanical terms related to plant mating systems, floral traits, and pollination</caption><thead><tr><td>Term</td><td>Definition</td></tr></thead><tbody><tr><td>Cultivar</td><td>A cultivated plant genotype that exhibits distinct and desirable traits achieved through human selection</td></tr><tr><td>Variety</td><td>A plant genotype that exhibits distinct traits that arose naturally, without human intervention; sometimes interchangeably used with "cultivar"</td></tr><tr><td>Self-pollination</td><td>Pollen transfer from anthers to stigmas within the same flower, between flowers on the same individual plant, or between flowers on different but genetically identical plants</td></tr><tr><td>Cross-pollination</td><td>Pollen transfer from anthers to stigmas between different, and genetically distinct, individual plants</td></tr><tr><td>Self-sterile</td><td>Plant partially or fully unable to self-fertilize or produce mature seeds due to temporal, morphological, biochemical, or genetic barriers</td></tr><tr><td>Self-fertile</td><td>Plant able to be fertilized and produce mature, viable seeds following self-pollination</td></tr><tr><td>Staminate</td><td>Unisexual flower or flower stage that bears only functional stamens; these flowers are functionally male</td></tr><tr><td>Pistillate</td><td>Unisexual flower or flower stage that bears only a functional pistil; these flowers are functionally female</td></tr><tr><td>Dioecy (gynodioecy)</td><td>Male and female reproductive organs on separate individual plants; gynodioecious plant species have morphs that bear only pistillate flowers and morphs that are hermaphroditic</td></tr><tr><td>Monoecy</td><td>Plants bearing separate pistillate and staminate flowers on the same individual</td></tr><tr><td>Dichogamy (heterodichogamy)</td><td>Temporal separation of male and female flowering phases; heterodichogamous plants have two morphs that exhibit complementary and synchronized dichogamy (e.g., one morph opens first in female stage while another morph opens first in male stage)</td></tr><tr><td>Male-sterile</td><td>Plants that do not produce pollen at all or do not produce viable pollen</td></tr><tr><td>Hermaphroditic</td><td>Flowers with both male and female reproductive parts</td></tr><tr><td>Early-acting inbreeding depression (EID)</td><td>Partial (self-)sterility resulting from partial embryo abortion following self-fertilization or fertilization with pollen carrying embryo-lethal alleles</td></tr><tr><td>Self-incompatibility (SI)</td><td>Self-sterility (partial or full) resulting from one of the following genetically controlled mechanisms: GSI, SSI, HSI, LSI</td></tr><tr><td>Homomorphic gametophytic SI (GSI)</td><td>Molecular barrier preventing the elongation of pollen tubes in a non-permissive pistil</td></tr><tr><td>Homomorphic sporophytic SI (SSI)</td><td>Molecular barrier preventing pollen germination and penetration in a non-permissive stigma</td></tr><tr><td>Heteromorphic sporophytic SI (HSI)</td><td>Molecular barrier preventing pollen germination, pollen tube elongation, or fertilization additionally characterized by heterostyly in plants</td></tr><tr><td>Late-acting (ovarian) SI (LSI)</td><td>Molecular barrier preventing self-fertilization or the development of embryo or endosperm from self-fertilized ovules</td></tr><tr><td>Heterostyly</td><td>Differing lengths of anthers and style within the same flower preventing self-pollination</td></tr><tr><td>Xenia</td><td>Effects of pollen identity (self or other) on fruit and seed quality including maternal, embryo, and endosperm tissues</td></tr></tbody></table>

systems (Havey 2004; Kumar and Singh 2004; Chase 2007; Chase et al. 2010; Kempe and Gils 2011; Chen and Liu 2014; Colombo and Galmarini 2017) (see Table S1 for crop list). In all these crops, the male-sterile genotypes require the presence of a male-fertile genotype in their vicinity for pollen supply and seed and fruit production.

Heterodichogamous crops additionally require cross-pollination between morphs or cultivars due to temporal separation between male and female sexual phases and are thus typically planted in mixed-cultivar fields (Renner 2001; Endress 2010). In these crops, all genotypes produce hermaphroditic flowers that exhibit distinct, temporally separate male and female phases, and the opening of such flowers between genotypes can be synchronized to promote cross-pollination. For example, in avocado, flowers of one genotype open in the morning in the female phase and then later produce pollen while in

the male) phase (Endress 2010). Flowers of another genotype open in the afternoon in the female phase and transition to the male phase the following morning. In jujube *Ziziphus jujuba*, the opposite occurs in which flowers open first in the male phase and move later to the female phase (Yao et al. 2015; Tel-Zur 2023). Thus, having two genotypes that vary in anthesis timing interplanted within the same fields ensures the presence of flowers in both the female and male phases at the same time enabling successful cross-pollination (i.e., earlier-flowering genotypes may be in the male phase while later-flowering genotypes are in the female phase or vice versa). Other heterodichogamous crops, such as several wind-pollinated nut crops, have two different morphs that flower at the same time but with one morph producing flowers that open first in the female stage and the other morph producing flowers that open first in the male stage (Renner 2001).

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Additional crops that would benefit from cultivar mixing include those with genotypes that have low male fertility, including genotypes that produce limited pollen or limited male flowers. In these cases, mixing cultivars within fields can ensure that plants with low male fertility receive adequate pollen from cultivars with high male fertility, though cultivar mixing may not be as common and widespread of a practice as it is for dioecious, gynodioecious, and heterodichogamous crops described above. For instance, the amount of pollen produced per flower can vary by a factor of 2.7 across genotypes in cultivated sunflower *Helianthus annuus* (Prasifka et al. 2023), by a factor of 16.2 in honeysuckle *Lonicera* spp. (Bieniasz et al. 2019), or by a factor of 34.0 in kola nut *Cola nitida* (Nyadanu et al. 2023) (see Table S2 for full list of crop species). In a similar way, for monoecious crops, the ratio of the number of staminate to pistillate flowers can vary across genotypes by a factor of 1.9 in pumpkin *Cucurbita moschata* (Whitaker 1931) and by a factor of 2.6 in other cucurbits such as squash *Cucurbita pepo* and muskmelon *Cucumis melo* (Whitaker 1931; NeSmith et al. 1994; see also Milc et al. 2016). Pollen performance is also highly variable across genotypes, including the percent of viable pollen released from the anthers, the percent of pollen germinating in vitro or on the stigma, and the growth rate of pollen tubes in vitro or through the style to reach the ovary. The effect of genotype on pollen performance is documented for many crop species, e.g., oilseed rape *Brassica napus* (Singh et al. 2008), coconut *Cocos nucifera* (Hebbar et al. 2018), kola nut (Nyadanu et al. 2023), longan *Dimocarpus longan* (Pham et al. 2015), upland cotton *Gossypium hirsutum* (Kakani et al. 2005), honeysuckle (Bieniasz et al. 2019), almond *Prunus dulcis* (Sorkheh et al. 2018), and common pear (Kuroki et al. 2017).

In addition, sexual reproduction is considered to be one of the most vulnerable stages of the plant life cycle against thermal and water-deficit stress, and pollen is especially sensitive to temperature extremes (Hedhly et al. 2009; Zinn et al. 2010; Hedhly 2011; Sage et al. 2015; Rosbakh et al. 2018; Jagadish 2020; Lohani et al. 2020; Chaturvedi et al. 2021; Cohen et al. 2021; Sinha et al. 2021; Tushabe et al. 2023). In particular, pollen germination displays a unimodal relation with temperature; i.e., the percent of pollen grains germinating on the stigma is maximized at an optimum temperature and reaches zero below and above minimum and maximum temperature thresholds, respectively (Coast et al. 2016; Rosbakh and Poschlod 2016). This reaction norm of pollen germination in relation to temperature is genotype specific such that some genotypes may be more tolerant than others to high or low temperatures or adapted to a wider range of temperatures, likely due to variation in pollen antioxidant content (Kakani et al. 2005; Singh et al. 2008; Pham et al. 2015; Coast et al. 2016; Hebbar et al. 2018; Koski and Galloway 2018; Muhlemann et al. 2018; Gan et al. 2020;

Koski et al. 2020). In addition, heat stress can disrupt anther reactive oxygen species (ROS) homeostasis as well as sugar and lipid metabolism, ultimately leading to pollen abortion, reduced pollen viability, and male sterility (Ma et al. 2021; Khan et al. 2023; Zhao et al. 2018a,b, 2023a,b; Xu et al. 2024). Genotypes with high ROS scavenging activity under high temperatures may thus have greater male fertility and ensure pollination under heat stress (Fábián et al. 2019; Santiago et al. 2021; Xu et al. 2024). Additionally, while water-deficit stress has not been shown to affect pollen quantity per flower (Kuppler and Kotowska 2021), it can decrease pollen viability and germination (Fang et al. 2010; Jagadish et al. 2011; Su et al. 2013; Guo et al. 2016; Descamps et al. 2018), with potential variation across cultivars. Therefore, cultivar mixtures could help buffer the negative effects of high- and low-temperature extremes and water-deficit stress on pollen availability and performance and thereby help maintain crop fertility (Fig. 2). As extreme weather events such as heat and drought are expected to increase in frequency and intensity in the present and coming decades due to climate change (IPCC 2021), there is thus a need for insurance and resilience within cropping systems. However, mixing cultivars to achieve such resilience and insurance in pollination may not be common practice unless the crop displays other barriers to self-pollination and fertilization.

## 2.2 Cultivar mixing ensures fertility and enhances seed and fruit quality

In several crops, pollination success depends not only on the quantity of viable conspecific pollen deposited on stigma but also on interactions between the pollen and the pistil, which are mediated by the genetic identity of the pollen and its relation to the pistil (Aizen and Harder 2007). Seed and fruit production is reduced or prevented if pollen from some genotypes including self-pollen is deposited on the stigma. In cases where seed and fruit production is reduced, the crop is “partially self-sterile,” whereas when seed and fruit production is completely prevented, the crop is “fully self-sterile.” These crops include two main types of species, those species displaying early-acting inbreeding depression (EID) and those displaying a system of self-incompatibility (SI).

EID is inbreeding depression expressed at the stage of embryo development and results in partial embryo abortion leading to reduced seed set, fruit set, and size of multi-seeded fruits (Crane 1964; Husband and Schemske 1995, 1996; Keller and Waller 2002; McAtee et al. 2013; Kumar et al. 2014). It is caused by an accumulation of recessive lethal alleles located on loci coding for embryo development, called genetic load. The magnitude of EID, i.e., the proportion of aborted seeds, depends on the inbreeding coefficients of the crossed genotypes and on the relatedness coefficient between these genotypes as seen in blueberry *Vaccinium*

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spp. and alfalfa *Medicago sativa* (Fyfe 1957; Busbice and Wilsie 1966; Busbice 1968; Dessureaux and Gallais 1969; Hellman and Moore 1983; Krebs and Hancock 1988, 1990; Harrison et al. 1993; Keller and Waller 2002; Charlesworth and Willis 2009; Hedrick and Garcia-Dorado 2016). Thus, for crops displaying EID, particular attention should be paid to mixing the least-related genotypes possible to maximize seed and fruit production. EID has been identified in the following entomophilous crops: blueberry (DeVetter et al. 2022), chestnut *Castanea* spp. (McKay 1942; Zhang et al. 2016; Xiong et al. 2019; Larue and Petit 2023; Larue et al. 2023), physic nut *Jatropha curcas* (Fresnedo-Ramírez 2013; Raju and Bahadur 2013; Negussie et al. 2014; Rincón-Rabanales et al. 2016), lychee *Litchi chinensis* (Stern et al. 1993; Degani et al. 1995; Chu et al. 2015; Xie et al. 2019), and alfalfa (Sayers and Murphy 1966; Barnes et al. 1972; Viands et al. 1988; Parajuli et al. 2021). All crops displaying EID are partially self-fertile; i.e., they produce a substantial number of seeds and fruits even following self-pollination or cross-pollination by a highly related pollen donor, and thus mixing cultivars is not necessarily a widespread or universal practice (e.g., blueberry; DeVetter et al. 2022).

Crops with a self-incompatibility (SI) system can be categorized according to one of the four following mechanisms of SI: (1) homomorphic gametophytic SI (GSI), (2) homomorphic sporophytic SI (SSI), (3) heteromorphic sporophytic SI (HSI), and (4) late-acting (ovarian) SI (LSI). All these systems correspond to varied molecular barriers that limit or entirely prevent self-pollination. These mechanisms include preventing pollen grains from incompatible pollen donors from germinating on the stigma (for SSI), preventing incompatible pollen tubes from elongating through the transmitting tract (for GSI), or preventing double fertilization to occur or fertilized ovules from growing into mature seeds (for LSI) (de Nettancourt 1997; Newbigin et al. 1997; Allen and Hiscock 2008; Gibbs 2014). For HSI, incompatibility occurs before fertilization (prezygotic barrier) in the stigma, the style, or the ovary (de Nettancourt 1997). For GSI, SSI, and HSI, the incompatibility reaction is genetically determined by allelic variance at the *S-locus* (Hiscock and Tabah 2003; McClure and Franklin-Tong 2006; Allen and Hiscock 2008; Gibbs 2014; Kappel et al. 2017; Barrett 2019; Goring et al. 2023). For LSI, the genetic and physiological mechanisms are not yet understood, but they are hypothesized to be determined by multiple loci (Sage et al. 1994; Allen and Hiscock 2008). Crops displaying GSI, SSI, or LSI can be fully or partially self-sterile depending on the species. Alternatively, entomophilous crops displaying the HSI system are fully self-sterile and additionally display heterostyly (Barrett and Shore 2008; Barrett 2019). The flowers of these crops are composed of two morphs: morphs with long style and short stamens, called L-morphs or pin flowers, and morphs with short style and long stamens, called S-morphs or thrum

flowers. This reciprocal heterostyly or herkogamy prevents pollen from being wasted on incompatible stigmas.

While there are only two insect-pollinated crops known to display HSI, star fruit *Averrhoa carambola* (Knight 1965, 1982; Wong et al. 1994) and buckwheat *Fagopyrum esculentum* (Cawoy et al. 2009; Miljuš-Dukić et al. 2010; Matsui and Yasui 2020a,b), the GSI, SSI, and LSI systems each include several entomophilous crops. SSI entomophilous crops include those in the Asteraceae (chicory *Cichorium intybus*; niger *Guizotia abyssinica*; silflower *Silphium integrifolium*; Geleta and Bryngelsson 2010; Price et al. 2022; Palumbo et al. 2023), Brassicaceae (cabbage *Brassica oleracea* Capitata Group; turnip *Brassica rapa*; radish *Raphanus sativus*; white mustard *Sinapis alba*; Okamoto et al. 2004; Zeng and Cheng 2014), Convolvulaceae (sweet potato *Ipomoea batatas*; Kowyama et al. 2008), and Passifloraceae families (passion fruit *Passiflora edulis*; Suassuna et al. 2003), all these crops being fully self-sterile with the exception of turnip and silflower, which are partially self-sterile. GSI includes entomophilous crops in the Cactaceae, Fabaceae, Rosacea, Rubiaceae, Rutaceae, and Solanaceae families (Gibbs 2014; Liang et al. 2020; Ramanauskas and Igić 2021). Finally, LSI includes entomophilous crops in the Anacardiaceae, Fabaceae, Fagaceae, Lardizabalaceae, Malvaceae, Myrtaceae, Sapindaceae, and Theaceae families (Page et al. 2010; Gibbs 2014; Liao et al. 2014; Ramírez and Kallarackal 2017; Rangappa Thimmaiah et al. 2018).

In addition to EID and SI systems, there is another phenomenon called xenia. The crops displaying this effect have enhanced seed and fruit quality under cross-pollination even if the quantity or yield of the crop is unaffected (Denney 1992). In these cases, the genotype of the pollen donor has direct effects on the characteristics of the seed and fruit, including the embryo, endosperm, and the maternal tissues. These characteristics include the size, color, shape, chemical composition, firmness, and developmental timing of seeds and fruits. The mechanisms underlying these effects have not been elucidated but may include (i) double fertilization of the egg cell; (ii) maternally and paternally inherited transposons; (iii) phytohormones secreted by both the embryo and endosperm, which spread out into the maternal tissues of the seed and fruit; or (iv) the release of mRNAs and small RNAs by the growing pollen tube, which spread out into the maternal tissues (Denney 1992; Liu 2008, 2018). Twenty-one animal-pollinated crops were identified displaying xenia (reviewed in Denney 1992; Liu 2018), but as this effect is largely overlooked, it is possible that many more crops benefit from xenia. Indeed, it is possible that for many self-compatible and fully self-fertile crops, cross-pollination and mixing cultivars within fields can benefit fruit and seed quality through xenia.

While it is common to interplant cultivars of fully self-sterile crops, it is not often the case for partially self-sterile

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crops including those with EID, or for crops that display xenia. For example, northern highbush blueberry Vaccinium corymbosum and macadamia Macadamia integrifolia continue to be planted in single-cultivar plots, though cross-pollination increases the yield outcomes for both crops (True-man 2013; Kämper et al. 2021a, b; DeVetter et al. 2022; Trueman et al. 2022). Additionally, these practices can vary across regions; for example, in highbush blueberry systems in the USA, large monocultivar fields are standard practices in Washington (Eeraerts et al. 2023b), while smaller monocultivar fields are standard practice in Michigan (Isaacs and Kirk 2010), and fields with multiple cultivars are more common in Florida (Mallinger et al. 2021). Understanding the pollination requirements of crops and the mechanisms underlying partial or full self-sterility can inform not only the decision to mix cultivars but also cultivar selection. For the SI systems, whether GSI, SSI, HSI, or LSI, the pollen donor cultivars, called “pollinizers,” need to be compatible with the pollen recipient cultivars, i.e., not sharing the same alleles for the S-locus. For crops displaying EID, the pollinizers should theoretically share the lowest relatedness coefficients with the pollen recipients. For xenia on the other hand, as the genetic and physiological mechanisms are not yet elucidated, there is no recommendation for the pollinizer choice other than that it be non-self.

## 3 Implementation of effective cultivar mixtures for pollination success

The selection and spatial planting arrangement of cultivars influence the degree to which cross-pollination occurs and thus need to be considered when mixing cultivars. Additionally, the biology of insect pollinators and their interactions with crop plants, including pollinator foraging distance and foraging preferences, should inform cultivar selection and planting arrangements (MacInnis and Forest 2019; Hung et al. 2023). Insect pollinators visit flowers for floral resources, also called “rewards” or “primary attractants,” including nectar and pollen (Armbruster 2012; Shivanna 2014). Their foraging behaviors are also influenced by floral cues (or signals) such as volatile organic compounds, flower size, flower morphology, flower color, and flower electric field, also called “secondary attractants,” which can signal floral resource status (Wright and Schiestl 2009; Armbruster 2012; Clarke et al. 2013; Shivanna 2014; Knauer and Schiestl 2015; Ortiz et al. 2021). Cultivars sharing similar floral cues or resources may thus receive greater cross-pollination through promoting even visitation rates and cultivar switching among pollinators. For example, pollinator visitation rates are lower to certain cultivars of blueberry, carrot Daucus carota, Japanese pear Pyrus pyrifolia, onion Allium cepa, strawberry Fragaria × ananassa,

and sunflower due to lower amounts of nectar and pollen per flower (reviewed in Prasifka et al. 2018), restricted access to nectar (Courcelles et al. 2013; Mallinger and Prasifka 2017; Portlas et al. 2018), nectar with compounds reducing palatability and bee learning (Hernández et al. 2019), nectar with lower sucrose amounts (Pham-Delègue et al. 1990, 1994; see also Percival 1961; Quinet et al. 2016; but see Mallinger and Prasifka 2017; Soto et al. 2021), pollen with lower protein content (Mori et al. 2024), or floral volatile blends including innate repellant compounds (Klatt et al. 2013; Ceuppens et al. 2015; Soto et al. 2015; Quarrell et al. 2023). Furthermore, increasing dimorphisms or differences in nectar sucrose amounts between sunflower genotypes led to reduced cross-movement or switching between male-fertile and male-sterile lines by bee pollinators, thereby reducing pollination and yields (Pham-Delègue et al. 1990, 1994; Susic Martin and Farina 2016). These studies suggest that maximizing similarities in floral traits or resources could lead to overall more even visitation rates across co-flowering cultivars and potentially greater cross-pollination.

While similarities in floral resources or cues may help promote cross-pollination behaviors, overall greater differences in genetic makeup between cultivars may maximize the benefits of cross-pollination, particularly for crops that are subject to early-acting inbreeding depression. In these crops, cultivars that are more distantly related will likely produce the greatest quantity and quality of fruits and seeds due to cross-pollination benefits. For self-incompatible crops, interplanted cultivars should be unrelated to the extent that they do not share the same alleles on the S-locus. Additionally, for both dioecious and monoecious crops with imperfect flowers, selecting cultivars that are complementary in the flowering time of male and female flowers will result in optimal cross-pollination. In monoecious crops with imperfect flowers like cucurbits, ratios of male to female flowers produced per plant can vary across cultivars and over time (Whitaker 1931; NeSmith et al. 1994; Milc et al. 2016; Mallinger and Liburd 2021). Selecting cultivars that have complementary ratios of male and female flowers at different stages during the flowering period could optimize pollination and yields. Likewise, for kiwifruit, a dioecious crop, selection of cultivars to ensure overlapping flowering time between male and female plants and to optimize the number of female flowers per field is one of the most important factors predicting yield (Peace et al. 2020; Broussard and Pattemore 2023). Thus, an understanding of the crop's pollination requirements, combined with information on cultivar traits including flowering time and relative attractiveness to pollinators, could help select cultivars that both promote cross-pollination and receive maximum benefits from cross-pollination.

Following cultivar selection, their arrangement in the field to optimize pollen flow and cross-pollination rates
will furthermore determine the benefits of cultivar diversity for pollination success. Studies in different cropping systems including nine orchard crops and six field crops have measured the effect of distance between cultivars on cross-pollination rates and yield outcomes to inform optimal planting design (Table 2). For studies in orchard crops that included some measure of yield, greater spacing between cultivars resulted in 59–100% less fruit set in apples (Free 1962; Maggs et al. 1971; Milutinovic et al. 1996), 86% lower yields in chestnut (Nishio et al. 2019), up to 69% lower yields in lychee (Raz et al. 2022), 47% lower yields in macadamia (Kämper et al. 2021a, b), up to 42% lower fruit weight in mandarin (Wallace et al. 2002), and 26% lower yields in mango (Kämper et al. 2023) (Table 2). For field crops, increased distance between cultivars reduced yields in hybrid seed sunflower by up to 30% (DeGrandi-Hoffman and Martin 1993), by 88% in hybrid seed cotton (Vaissière 1991), and by 93% in triploid watermelon (Nesmith and Duval 2001) (Table 2). For other crops, however, including avocado and strawberry, distance between cultivars had no effect on cross-pollination rates or yield outcomes highlighting the fact that results will vary based on the crop as well as the gradient in distance examined (Table 2). The distance that pollen travels across cultivars for pollinator-dependent crops is determined primarily by pollinator behavior, which is in turn influenced by intrinsic factors (e.g., body size) as well as environmental factors (e.g., the pollinator community, weather, and forage landscape) (Greenleaf et al. 2007; Osborne et al. 2008; Danner et al. 2016). While intrinsic factors may remain more constant across farms of a given crop, extrinsic factors (e.g., weather, forage landscape) likely vary across farms and regions. Furthermore, studies to date are concentrated in a small number of temperate (e.g., apple and almond) and a few tropical (e.g., mango and avocado) crops that represent a fraction of entomophilous crops (Klein et al. 2007). The effect of spacing and distance is generally understudied for entomophilous tropical crops grown on a smaller scale globally (e.g., rambutan and sugar apple, see Table S1) and for entomophilous temperate crops that are self-compatible but display early-acting inbreeding depression or xenia (e.g., blueberry).

In addition to distance between cultivars, the arrangement of cultivars can influence pollen flow and cross-pollination rates (Fig. 3). For example, mixing cultivars within the same row may enhance cross-pollination by bees given that many bees including honey bees *Apis mellifera* frequently forage in a linear pattern known as traplining (Free and Spencer-Booth 1963; Jackson and Clarke 1991; Cresswell et al. 1995; Cresswell 1997; Lihoreau et al. 2013). Such linear foraging behaviors, combined with preferences and floral constancy to certain cultivars, have been shown to limit cross-pollination in crops like blueberries (Estravis-Barcala et al. 2021) and almonds (Jackson 1996). In apples, simulation models

show that mixing cultivars within rows, as opposed to across rows, results in significantly greater cross-pollination success (Sáez et al. 2018), while Hung et al. (2023) highlighted that the effects of spatial arrangement on pollination performance vary across different types of bees (i.e., honey bees, bumble bees Bombus spp., and solitary bees). Similarly, a field study in common pear showed very infrequent row switching by insect pollinators, which resulted in less cross-pollination and overall lower pollination success when cultivars were mixed across rows as compared to within rows (Quinet and Jacquemart 2017). However, other than these few studies in temperate orchard crops, there is little information on the relative benefits of mixing within versus across rows for pollination success and outcomes.

The extent to which bees display directionality in their movement can vary across bee species and with factors including the level of plant aggregation and the density and diversity of other pollinators (Schmid-Hempel 1984; Cresswell 1997; Stern et al. 2001, 2004; Eeraerts et al. 2020a; Miñarro and Gárcia 2021). Because having a high diversity and density of pollinators can result in greater cross-row movement by honey bees and other linear foragers, the use of multiple managed pollinators or the use of managed plus wild pollinators could increase the cross-pollination contributions of honey bees (DeGrandi-Hoffman and Watkins 2000; Greenleaf and Kremen 2006; Brittain et al. 2013; Sapir et al. 2017; Eeraerts et al. 2020b). Finally, ratios of different cultivars can affect rates of cross-pollination through altering ratios of pollen donors and recipients (MacInnis and Forrest 2020). For example, the density of pollinizer (pollen donor) cultivars had a significant effect on the seed set of the focal (recipient) cultivar in apples, especially within a 20-m radius (NeSmith and Duval 2001; Carisio et al. 2020).

## 4 Benefits of cultivar mixtures for pollinators

Bees and other pollinating insects require an abundant and continuous supply of floral resources to survive, reproduce, and maintain stable populations. In addition, a diversity of floral resources is necessary to support diverse pollinator communities, as different pollinator species have different needs. Land-use change, and the subsequent loss of floral resources, is a leading cause of the documented decline in bee abundance and diversity worldwide, resulting in impaired pollination services (Goulson et al. 2015; Zattara and Aizen 2021; Millard et al. 2023). Diversifying cultivars may be one way to increase floral resource availability for bees and other pollinators within agricultural landscapes. Past studies have demonstrated how the presence of diverse and complementary habitat types close to one another enhances floral resource diversity and continuity (Mandelik

<figure><img src="image_12.png" /></figure>
<table><caption>**Table 2** Studies examining how the distance among cultivars in a field affects cross-pollination rates and yields. Included for each study are the crop, response metric and distance(s) the metric(s) was examined over, nature of response, effect on yield when a measure of yield was included, and reference</caption><thead><tr><th>Crop</th><th>Metric examined</th><th>Distance measure</th><th>Response</th><th>Reference</th></tr></thead><tbody><tr><td>Orchard crops</td><td></td><td></td><td></td><td></td></tr><tr><td>Almond <i>Prunus amygdalus</i></td><td>Isozyme paternity and gene flow</td><td>Distance from the nearest parent pollen source</td><td>Leptokurtic distribution, max gene flow generally within 15 m</td><td>Vezvaei and Jackson 1997</td></tr><tr><td>Apple <i>Malus domestica</i></td><td>Fruit set, seeds per fruit</td><td>Distance from pollenizer branch, 1–6 m</td><td>Decrease with distance from ~ 12 to 2.5% fruit set</td><td>Free 1962</td></tr><tr><td>Apple <i>Malus domestica</i></td><td>Fruit set, seeds per fruit</td><td>Distance from pollenizer, 0–12 m</td><td>Decrease with distance from ~ 45 to 0 fruits/tree</td><td>Maggs et al. 1971</td></tr><tr><td>Apple <i>Malus domestica</i></td><td>Fruit set, number of seeds per fruit</td><td>Distance from pollenizer, 5–80 m</td><td>Decrease with distance for 2 of 3 cultivars from ~ 33 to 0 and ~ 22 to 9% fruit set</td><td>Milutinovic et al. 1996</td></tr><tr><td>Apple <i>Malus domestica</i></td><td>Cross-pollination rates using allozyme markers</td><td>Distance from pollen donor measured in rows</td><td>Decrease with distance; within 14.5 m is optimal</td><td>Kron et al. 2001a</td></tr><tr><td>Apple <i>Malus domestica</i></td><td>Cross-pollination rates using allozyme markers</td><td>Distance from pollen donor measured in trees, rows, and meters</td><td>Decrease with distance for most cultivars; optimal is within 4–31 m</td><td>Kron et al. 2001b</td></tr><tr><td>Apple <i>Malus domestica</i></td><td>Proportion fruit/seed produced by cross-pollen</td><td>Distance from pollenizer, 1–15 m</td><td>Decrease with distance; within 10 m is optimal</td><td>Matsumoto et al. 2008</td></tr><tr><td>Avocado <i>Persea americana</i></td><td>Honey bee mobility</td><td>Number of rows crossed up to 15 rows</td><td>Decrease with distance; within ~ 8 rows optimal</td><td>Ish-Am and Eisikowitch 1998</td></tr><tr><td>Avocado <i>Persea americana</i></td><td>Outcrossing rate and yield</td><td>Distance from pollen donors, three treatments</td><td>Decrease in outcrossing rate with distance but no effect on yield</td><td>Alcaraz and Hormaza 2011</td></tr><tr><td>Avocado <i>Persea americana</i></td><td>Outcrossing rates</td><td>Distance from pollen donors, 3–91.5 m</td><td>Decrease with distance of 30 m or greater; within 30 m is optimal</td><td>Ying et al. 2009</td></tr><tr><td>Avocado <i>Persea americana</i></td><td>Outcrossing rates, fruit size, nutritional quality</td><td>Distance from pollen donor from 10–140 m</td><td>Outcrossing decreased w/ distance, fruit size and quality were not affected</td><td>Kämper et al. 2021a, b</td></tr><tr><td>Chestnut <i>Castanea crenata</i></td><td>Bur set and yield</td><td>Distance from pollenizer, 4–16 m</td><td>Decrease with distance from ~ 66 to 25% bur set and ~ 7 to 1 kg per tree</td><td>Nishio et al. 2019</td></tr><tr><td>Lychee <i>Lichi chinensis</i></td><td>Fruit set and yield</td><td>Distance from pollenizer, 6–66 m</td><td>Decrease with distance from ~ 10 to 4 or ~ 12 to 5% fruit set and from ~ 52 to 20, 62 to 21, or 48 to 15 kg/tree</td><td>Raz et al. 2022</td></tr><tr><td>Macadamia <i>Macadamia integrifolia</i></td><td>Number and size of nuts</td><td>Distance from pollenizer, 8–184 m</td><td>Decrease in number of nuts with distance in wide but not narrow block orchard from ~ 85 to 45 nuts/tree</td><td>Kämper et al. 2021a, b</td></tr><tr><td>Macadamia <i>Macadamia integrifolia</i></td><td>Degree of cross-pollen limitation</td><td>Distance to pollenizer, 8 or 200 m</td><td>No cross-pollen limitation at 8 m, but significant limitation at 200 m</td><td>Trueman et al. 2022</td></tr><tr><td>Mandarin <i>Citrus reticulata</i></td><td>Fruit weight and seed number</td><td>Degree of isolation, number of rows</td><td>Decrease from ~ 107 to 80 and ~ 100 to 58 g/fruit</td><td>Wallace et al. 2002</td></tr><tr><td>Mango <i>Mangifera indica</i></td><td>Yield and income, incidence of nubbins</td><td>Increasing distance from pollenizer cultivar at 10, 40, and 50 m</td><td>Decrease from ~ 68 to 50 kg/tree and ~ $7000 to $5000/ha</td><td>Lucas-Garcia et al. 2021</td></tr><tr><td>Mango <i>Mangifera indica</i></td><td>Percentage of cross-pollinated fruit</td><td>Distance from pollen source in rows and meters</td><td>Decrease for 1 of 2 cultivars at and beyond 45 m</td><td>Kämper et al. 2023</td></tr></tbody></table>

<figure><img src="image_13.png" /></figure>
<table><caption>**Table 2 (continued)**</caption><thead><tr><th>Crop</th><th>Metric examined</th><th>Distance measure</th><th>Response</th><th>Reference</th></tr></thead><tbody><tr><td>Plum <i>Prunus</i> sp.</td><td>Fruit set</td><td>Number of trees away from pollinator, 1–16 trees</td><td>Decrease with distance from ~ 8 to 0.5% fruit set</td><td>Free 1962</td></tr><tr><td>Field crops</td><td></td><td></td><td></td><td></td></tr><tr><td>Buckwheat <i>Fagopyrum esculentum</i></td><td>Outcrossing rates</td><td>Distance from pollen donor, 0–105 m</td><td>Decrease with distance; within 3 m is optimal</td><td>Adhikari and Campbell 1998</td></tr><tr><td>Cotton <i>Gossypium hirsutum</i></td><td>Yield</td><td>Distance from pollen donor in rows and meters</td><td>Decrease with distance from 125 to 15 g seed/plot; little seed beyond 7 m</td><td>Vaissière 1991</td></tr><tr><td>Muskmelon <i>Cucumis melo</i></td><td>Percent hybrid seed</td><td>Increasing distance from pollen source, 1.5–10.5 m</td><td>Decrease with distance; as close as possible is optimal</td><td>Foster and Levin 1967</td></tr><tr><td>Strawberry <i>Fragaria x ananassa</i></td><td>Proportion cross-pollinated fruits and seeds</td><td>Distance from pollen donor, 1–20 m</td><td>No effect on proportion cross-pollinated fruits/seeds</td><td>Kämper et al. 2022</td></tr><tr><td>Sunflower <i>Helianthus annuus</i></td><td>Seed set</td><td>Number of rows away from male fertile rows</td><td>Decrease from ~ 76 to 52 and 69 to 52% set</td><td>DeGrandi-Hoffman and Martin 1993</td></tr><tr><td>Watermelon <i>Citrullus lanatus</i></td><td>Yield</td><td>Distance from pollenizer, 0–9 m</td><td>Decrease with distance from ~ 1.5 to 0.1 fruit/m²</td><td>NeSmith and Duval 2001</td></tr><tr><td>Watermelon <i>Citrullus lanatus</i></td><td>Percent outcrossed seedlings</td><td>Distance across rows, 0–15 m</td><td>Decrease with distance; as close as possible is optimal</td><td>Walters and Schultheis 2009</td></tr></tbody></table>

et al. 2012; Mallinger et al. 2016; Timberlake et al. 2019). Additionally, different crop species, and even different cultivars of the same crop, can complement one another to provide continuous floral resources, with complementary floral traits (Adamson et al. 2012; Martins et al. 2018; Ferguson et al. 2021; Eeraerts 2022). Increasing cultivar diversity on farms may therefore benefit pollinators by providing prolonged floral resources over time or by providing flowers with variable morphology and reward composition to serve different pollinator species and/or provide a complementary and diverse diet for individuals. However, the effects of increased crop diversity in the landscape on pollinators and pollination services are mixed, likely due to differing responses across pollinator taxa and crops (Hass et al. 2018; Sirami et al. 2019; Pisman et al. 2022; Magrach et al. 2023).

Cultivars can vary in floral traits and resources, with implications for pollinator recruitment, pollination, and even pollinator fitness (Courcelles et al. 2013; Sampson et al. 2013; Ceuppens et al. 2015; Ferguson et al. 2021; Prasifka et al. 2018; and references in Table S2). For example, corolla length and width can vary across flowers within a crop and influence which pollinators can access floral resources (Courcelles et al. 2013; Sampson et al. 2013). Variability in flower morphology between genotypes including flower size, flower shape, and corolla depth has been described in at least 12 entomophilous crops (Table S2). Floral volatile organic compounds used by pollinators as honest signals of resource status additionally can vary across cultivars of a single crop, as highlighted in at least eight entomophilous crops (Table S2). Similarly, variabilities in flower resource quantity and composition including flower number per plant or per unit area, nectar amount per flower, nectar sugar composition, and pollen amount per flower and pollen chemical composition have been found to vary among cultivars of at least 27 entomophilous crops (Table S2). In general, variable nectar and pollen composition could increase diet diversity for individual pollinators and pollinator populations, with positive subsequent effects on fitness, while varying floral traits including flower morphology could result in different pollinator species visiting different cultivars (e.g., Courcelles et al. 2013; Sampson et al. 2013; Ceuppens et al. 2015; Ferguson et al. 2021; reviewed in Prasifka et al. 2018).

Cultivars can also vary in their tolerance to heat stress and its effects on the quantity of pollen produced per flower (Santiago et al. 2021; Xu et al. 2024; see section 2.1). Additionally, water stress can affect the number of flowers produced per plant and nectar quantity per flower (Phillips et al. 2018; Kuppler and Kotowska 2021), but sensitivity to water stress can vary across plant genotypes (Boose 1997; Leiss and Klinkhamer 2005; Suni et al. 2020). Finally, nectar secretion has been shown to decrease at both high and low extreme temperatures (Villarreal and Freeman 1990; Nicolson 1995; Petanidou and Smets 1996; Descamps

<figure><img src="image_14.png" /></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. 3** Schematic representation of different ways in which cultivars can be mixed including within rows, across rows, and randomly and at high and low densities of the pollenizer cultivar relative to the focal cultivar. Black dots represent the pollen donor cultivar (= pollinator), while grey dots represent the pollen recipient cultivar (= focal cultivar). Mixing cultivars within the same row may facilitate cross-</figcaption></figure>

pollination by traplining pollinators that forage along linear structures (e.g., bumble bees and honey bees), though mixing cultivars across rows may be more compatible with management and harvesting when these are done differently across cultivars. Random mixing may be more feasible for plantings from seed.

et al. 2018, 2020, 2021); although this has not yet been shown to vary across cultivars, photosynthetic responses to temperature are variable across cultivars, and nectar secretion is often directly derived from photosynthesis (Pacini et al. 2003; Yamori et al. 2014; e.g., Pettigrew 2016). Therefore, having a variety of cultivars in a crop field may increase the availability and quality of floral resources including nectar and pollen provided by the crop under extreme weather events.

Extended flowering periods from sequentially blooming cultivars within a field could also benefit pollinators by increasing the amount of nectar and pollen available over time and within a small area (Ferguson et al. 2021; Eeraerts 2022; Fig. 4). Generalist pollinator species such as bumble bees and many sweat bees (Halictidae) with long foraging periods will benefit from an extended flowering period, and pollinators with short foraging ranges will particularly benefit from having floral resources continuously available within a small area. Additionally, more specialized pollinator species that have shorter foraging periods (e.g., squash bees *Peponapis* spp. and *Xenoglossa* spp.) could benefit from cultivar diversity if it provides resources earlier or later in the year to offer maximum overlap with their foraging periods. Furthermore, a prolonged flowering period may benefit pollinator diversity by providing floral resources to different species with differing foraging periods. And, as the pollinator community

can shift throughout the flowering period of a crop, it is important to have a diverse pollinator community so that different pollinators can provide pollination services throughout the flowering period across different cultivars (Eeraerts 2022; Samnegård et al. 2023; Sáez et al. 2023). As such, sequential flowering cultivars that provide temporally continuous resources might be able to support long-term functional complementarity within the pollinator community that can safeguard long-term pollination services (Blüthgen and Klein 2011; Senapathi et al. 2021). However, while prolonging bloom may benefit pollinator health and diversity, cultivar flowering periods should overlap to some degree to enhance cross-pollination and thus maximum benefits for crop production (MacInnis and Forrest 2020; Anders et al. 2023). Hence, having more than two cultivars may allow for overlapping flowering of at least two cultivars while also prolonging the flowering period for pollinators (Fig. 4).

It is well established that different cultivars with variable flowering periods, flower morphology, and floral resources can receive variable flower visitation rates from different pollinator species (e.g., Courcelles et al. 2013; Mallinger and Prasifka 2017; Cole et al. 2022; review in Prasifka et al. 2018). However, studies examining pollinator communities, in addition to pollinator visitation rates, are needed to assess how cultivar diversity may benefit pollinator diversity. For example, both Ferguson

<figure><img src="image_16.png" /></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. 4** Effect of mixing two or three cultivars with partially overlapping and sequential bloom timing in one field on flowering phenology and subsequent cross-pollination, yields, and floral resources for pollinators as compared to a field planted with a single cultivar. While mixing two cultivars is common in self-incompatible, dioecious, or heterodichogamous crops (e.g., apples, almonds, avocados), mixing</figcaption></figure>

et al. (2021) and Eeraerts (2022) found that unique and complementary pollinator communities visited different cultivars of sunflower and sweet cherry (Prunus avium), respectively. Additionally, modular and specialized cultivar-pollinator networks were observed visiting different blueberry cultivars (Cortés-Rivas et al. 2022). Additional examples beyond these three systems are needed to determine how pollinator communities respond to cultivar diversity. Finally, while increasing cultivar diversity could support pollinator diversity and pollination in agroecosystems (Tamburini et al. 2020; Magrach et al. 2023), it needs to be emphasized that conserving semi-natural habitat at the landscape scale is also essential for pollinator conservation (Sirami et al. 2019; Eeraerts 2023). While cultivar mixtures could enhance the availability of floral resources, these resources will be limited to crop plants that may not support specialist pollinators, and furthermore, cultivar mixing does not enhance nesting sites for pollinators. Nesting sites are mainly provided by semi-natural habitat in the wider landscape, further supporting

more than two cultivars is less common, and cultivar mixtures are not as widespread for partially self-fertile crops or crops displaying xenia that still benefit from cross-pollination for optimal yields and crop quality (e.g., blueberry, macadamia). Continuous floral resources within a field may be particularly beneficial for wild pollinators with short foraging ranges and long foraging periods.

the conservation of these non-crop habitats for pollinators (Eeraerts et al. 2021; Eeraerts and Isaacs 2023).

# 5 Agronomic considerations for commercial implementation

While the potential benefits of intraspecific crop diversity are many, management and economic implications will be key considerations for successful implementation at the farm level. Several studies have emerged highlighting the value of broad agricultural diversification and ecological intensification (i.e., intercropping, agroforestry) for increasing yields and stabilizing food production (Kleijn et al. 2019; Garbaldi and Pérez-Méndez 2019; Renard and Tilman 2019; Tamburini et al. 2020). Agricultural diversification refers to the intentional addition of functional biodiversity via implementation of practices that aim to recreate ecosystem services that promote yield, including increasing crop species diversity among other practices (Kremen et al. 2012)

<figure><img src="image_18.png" /></figure>
but may be broadened to include enhancing intraspecific
crop diversity. Ecological intensification, in contrast, aims
to partially replace or complement external farm inputs with
ecological processes and services (Malézieux 2012; Kleijn
et al. 2019). Despite the reported benefits of agricultural
diversification and ecological intensification, farmers often
remain skeptical whether these benefits would translate to
their unique production systems and prefer to make simple
changes in landscape features compared to incorporating
more complex, on-farm enhancement practices even when
there is subsidy support (Bailey et al. 2015; Kleijn et al.
2019). While meta-analyses show that there are usually ben-
efits to diversification in agricultural systems, trade-offs do
occur and demonstrate context dependency often linked to
scale (Gabriel et al. 2010; German et al. 2017; Tamburini
et al. 2020). An important advantage of increasing cultivar
diversity is that this would be a measure that does not require
reducing the amount of land in production. Furthermore,
diversification within a crop species (i.e., cultivar mixtures)
may be more feasible for monoculture systems, but research
has traditionally focused on interspecific diversification in
crop- and non-crop species (e.g., Tamburini et al. 2020),
highlighting the need for more research on within-crop
diversification at a scale relatable to farmers. Determination
of the required density and placement of pollinizer cultivars
within specific crop systems can further aid guidelines for
implementing cultivar mixtures.

Related to scale and less well described in the literature
is the relationship between scale and marketing strategies
that may be favorable or constraining for the implemen-
tation of inter- and intraspecific crop diversification (De
Roest et al. 2018). For example, small-scale farmers that
sell their product(s) within local or regional foodsheds may
be less challenged by incorporating intraspecific crop diver-
sity, particularly if their customer base is not interested in
purchasing specific cultivars and/or wants to enhance sus-
tainable agriculture in their own community. Large-scale,
specialized producers in contrast may be more challenged

as some buyers desire certain cultivars for fresh sales or
processing given their specific fruit, vegetable, or seed/nut
quality attributes. Operating at larger scales can also entail
use of mechanization, which adds another layer of complex-
ity especially if farmers need to segregate management and
harvest across cultivars.

Production system nuances also present bridges and barriers to adoption of intraspecific cultivar diversity. Tree fruit systems such as apple, sweet cherry, and almond frequently have different cultivars within the farm to act as pollinizers and pollen recipients (Socias et al. 2004; Eeraerts 2022). Dioecious crops like kiwifruit also provide tangible examples of how intraspecific diversity can be achieved across commercial farm settings and scales and provide optimal strategies for implementation (Peace et al. 2020; Broussard et al. 2022). The key will be to find the balance between cultivar mixing that maximizes pollination benefits while minimizing further complexity to the farmer. One possibility is to arrange different cultivars into small blocks or zones so cultivar-specific management (e.g., fertility, irrigation, pesticide application, harvesting) can be implemented when necessary (see Fig. 3, middle), but blocks or zones would need to be of ecologically relevant sizes for cross-pollination to occur (see Table 2). Informing this approach should entail farmer surveys or focus groups to gain an understanding of what cultivar mixing is feasible at the farm level and combining this with relevant information on optimal distances and spatial arrangements across cultivars to achieve maximum cross-pollination (Table 2). Incentive structures, such as “farm-friendly” policies, increased sales prices, and/or ability to use eco-labeling that can offset perceived costs or yield losses, can further motivate farmers to adopt practices that promote pollinator health, which could include intraspecific crop diversification (Dimitri and Oberholtzer 2009; Kleijn et al. 2019).

To disseminate recommendations for mixed-cultivar plant-
ings, planting guidelines should be created but need to be data-
driven and paired with clear instructions on how to implement

<table><caption>**Table 3** Farm-level management considerations for the incorporation of cultivar mixtures.</caption>
   <thead>
    <tr>
     <td>
      Considerations
     </td>
    </tr>
   </thead>
   <tbody>
    <tr>
     <td>
      1. Genetic compatibility—cultivars are compatible (for crops with a self-incompatibility system) or not closely related (for crops displaying early-acting inbreeding depression or xenia)
     </td>
    </tr>
    <tr>
     <td>
      2. Phenology—cultivars overlap in flowering
     </td>
    </tr>
    <tr>
     <td>
      3. High yielding and marketable—cultivars have similar yield potential and economic value
     </td>
    </tr>
    <tr>
     <td>
      4. Management (e.g., irrigation, fertilization, pesticide use) of the cultivars is the same or similar
     </td>
    </tr>
    <tr>
     <td>
      5. Susceptibility to pests or diseases is the same or similar such that one cultivar does not increase pest or disease pressure for another
     </td>
    </tr>
    <tr>
     <td>
      6. Spacing requirements of the various cultivars is the same or similar
     </td>
    </tr>
    <tr>
     <td>
      7. Cultivars are similar in their attractiveness to pollinators
     </td>
    </tr>
    <tr>
     <td>
      8. Cultivars are compatible with on-farm equipment/machinery
     </td>
    </tr>
    <tr>
     <td>
      9. Plant material (e.g., seed, scion, transplants) for all cultivars is accessible
     </td>
    </tr>
   </tbody>
  </table>

<figure><img src="image_19.png" /></figure>
cultivar mixtures successfully. Additionally, these recommen-
dations should include any drawbacks associated with utilizing a certain cultivar (e.g., increased disease susceptibility, differing horticultural needs). Various aspects that a farmer or crop advisor would need to consider are listed in Table 3. Such guidelines will also need to be updated regularly to keep pace with the release of new cultivars, requiring collaboration among horticulturists, public- and private-sector breeders, and entomologists that specialize in pollination.

# 6 Conclusions

In this review, we synthesized the literature on crop pol-
lination requirements and pollinator behavior and move-
ment, as well as pollen flow, to show how intraspecific
crop diversity can benefit crop pollination and pollinators
and how cultivars can be selected and spatially arranged to
optimize cross-pollination. For fully self-sterile crops that
require cross-pollination to set fruit or seed, mixing culti-
vars at close distances is a common practice. However, we
show that other types of cropping systems can benefit from
cross-pollination and that there are additional contexts in
which cultivar mixing could ensure pollination success
and yields. Specifically, fruit and seed size and quality can
be improved with cross-pollination for a range of crops
exhibiting early-acting inbreeding depression or xenia, but
because of partial to full self-fertility, they are not always
planted in mixed cultivar fields. Additionally, cultivar mix-
ing may ensure pollination under extreme temperatures
and drought by increasing the likelihood of viable pollen
production in the field. Finally, cultivar mixing can ensure
pollen availability for crops in which pollen quantity and
performance vary across cultivars and can furthermore
ensure both nectar and pollen availability for pollinators
over an extended period of time. While for many crops
there is limited information on optimal spacing and spatial
arrangement of cultivars to promote cross-pollination, we
show that information on pollinator movement and behav-
ior, as well as pollen flow, can inform planting design.
We found that in previously studied crops, most cross-
pollination occurs at relatively small spatial scales (within
1–30 m), though for some crops, cross-pollination occurs
over larger distances. We also found evidence that mix-
ing cultivars within rows may promote cross-pollination
more than mixing across rows due to linear bee foraging
behaviors. However, the costs of mixing cultivars at small
spatial scales and within rows must be weighed against
the relative benefits of cross-pollination, which can vary
across cultivars within a crop, especially for partially self-
fertile crops and crops displaying early-acting inbreeding
depression or xenia. Information on the benefits of cross-
pollination for specific crops and crop cultivars, along

with tables on cultivar traits that aid in creating optimal
cultivar mixtures, can make this practice more achievable
for farmers particularly if paired with policy or marketing
incentives.

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1007/s13593-024-00984-2.

**Acknowledgements** The authors are thankful to the editors and two anonymous reviewers for their useful feedback. We thank Andrew Mack for designing Figures 3 and 4. We also thank Jenna Walters for fruitful discussions which provided some additional ideas for Section 2 of this review.

**Authors' contributions** S.C., R.E.M., L.W.D., M.E., and M.B. contributed to conceptualization. R.E.M., S.C., L.W.D., and M.E. contributed to methodology. R.E.M., S.C., and M.E. contributed to visualization. R.E.M. and L.W.D. contributed to funding acquisition. All authors contributed to writing and editing.

**Funding** This work is supported by Specialty Crops Research Initiative Award #2020-02627 from the USDA National Institute of Food and Agriculture. 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 US Department of Agriculture. ME was supported as an FWO postdoctoral fellow (grant no. 1210723N).

**Data availability** Data sharing not applicable as no primary data were generated or analyzed for this review.

**Code availability** Not applicable.

# Declarations

**Ethics approval** Not applicable.

**Consent to participate** Not applicable.

**Consent for publication** Not applicable.

**Conflict of interest** The authors declare no competing interests.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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