Final Report for GNC10-142
We documented native, wild bees of apple orchards, examined their pollination contribution, and observed bee foraging behaviors. We found 80 species of native, wild bees within orchards; bumble bees and mining bees were most abundant. Using managed honey bees did not increase apple fruit set in Wisconsin. However, the number of wild bee species was positively correlated to fruit set. Wild bees showed foraging behaviors of more effective pollinators as compared to honey bees. We conclude that most growers can rely solely on native, wild bees for pollination. Additionally, this research project developed an online guide to identifying Wisconsin’s bees.
Insect-mediated pollination is essential for adequate apple yields (Delaplane and Mayer 2000). Historically many growers have relied on honey bees for apple pollination (Parker et al 1987). However, due to declining numbers of honey bee colonies in the USA (Committee on the Status of Pollinators in North America 2007) and associated increases in hive rental costs, many growers are reducing the numbers of hives they manage or rent (Mallinger, unpublished data). These growers hope they can obtain adequate pollination by relying on the activity of wild bees, but recognize they are incurring risks that yields will be lower without managed pollinators. In general, there is little information on whether wild bees can fully pollinate apples in the Midwest.
Using wild bees may be an effective alternative to honey bees. Because of their behavior and biology, wild bees may be more efficient pollinators of apples compared to honey bees; wild bees intentionally collect pollen while some honey bees collect only nectar, avoiding pollen in a behavior called ‘nectar robbing’ (Delaplane and Mayer 2000). Wild bees are also more active in cooler and inclement weather and deposit more pollen on apple blossoms compared to honey bees (Boyle and Philogene 1983, Vicens and Bosch 2000, Thomson and Goodell 2001). Yet, while guidelines have been developed for the number of honey bee hives per acre of apples needed to achieve full fruit set, no similar information exists on the numbers or types of wild bees needed for sufficient pollination. Growers often have only anecdotal information on what native bees are found in and around their orchards and cannot make an informed decision about the extent of native bee activity on their farms. This project will address the question of whether wild bees can provide adequate pollination of apples and will determine which native pollinators are most effective at pollinating apples. In addition, this project will develop some tools necessary for growers to evaluate the wild bee community on their farms.
Wild bees have been shown to fully pollinate other crops when they are abundant and diverse. Studies in watermelon fields in eastern USA have demonstrated that these crops can receive full pollination by wild bees alone (Winfree et al 2007). However, in Canada, it was determined that wild bees were not abundant enough to provide adequate apple pollination (Scott-Dupree and Winston 1987). As pollinator abundance depends on landscape context as well as local factors such as field size (Kremen et al 2004; Greenleaf and Kremen 2006; Isaacs and Kirk 2010), some orchards may be able to receive full pollination by wild bees while others will need to supplement with managed bees. In addition to the quantities of native bees present in an orchard, pollination services also depend on the pollination efficiencies of individual bee species. That is, having many bees present in an orchard will only go so far if they are generally inefficient pollinators. Previous studies of wild pollinators in both northern Wisconsin and New York found a diversity of bees visiting apple blossoms, though these studies did not examine the pollination efficiency of these bees or whether they could provide full pollination (Watson 2009, Gardner and Ascher 2006). Thus the potential exists for native bees to be significant contributors to apple pollination, but it is not known to what extent they contribute or whether farmers can achieve some level of pollination “insurance” given decreased levels of honey bees.
Previous SARE funded research has focused largely on managing lands to enhance wild bees (LNE07-261, SW08-056, GNC07-086, ONE09-107, ONE09-094, ONE05-045, GS08-077, GW09-018, LNC08-297, FNE02-411) or examined the effects of landscape context on bees (GS10-092). Few studies have examined which bees are efficient pollinators and none have tried to determine whether wild bees alone can provide adequate pollination services to any crop. This proposed research examines the efficiency and contribution to pollination for bees in apples and provides growers with tools to aid in bee identification. By giving growers a way to evaluate pollinators on their lands, they can adjust or enhance conservation practices to increase native bee diversity and abundance and potentially increase crop pollination.
The research will have three main objectives: (1) assess which wild bees are most efficient in pollinating apples, (2) determine whether wild bees alone, and in what numbers, can provide full pollination for apples, and (3) develop a field ID guide to effective apple pollinators.
The overall aim of this research is to help growers determine the contributions of wild bees to pollination of apples, thereby decreasing their dependence on honey bees. The primary audience for this research is apple growers in Wisconsin and throughout the North Central Midwest where bee communities are similar. Additional audiences include growers of other pollinator-dependent fruit, and organizations and researchers that study wild bees. Ultimately, the research developed in this proposal can help growers understand to what extent native bees provide free pollination services and help them evaluate and monitor their own pollinator community the same way that they may scout and monitor pest activity.
The short-term outcomes will include increased grower knowledge of which wild bees are the most efficient pollinators of apples and how these wild bees compare to honey bees. Our research will also determine if orchards without managed honey bees can obtain pollination levels comparable to orchards with honey bees. Growers will furthermore have an increased understanding of the numbers and diversity of wild bees needed for adequate pollination and will have resources to identify wild bees in their orchards.
An intermediate outcome will be that growers can assess the wild bee community in their orchards and monitor over time. With information on the relative efficacy of different bees, growers can make management decisions to conserve these efficient pollinators by providing specific floral and nesting resources. In the long-term, growers can make more informed decisions on whether they need to rent or manage bees to augment their wild bee community. Researchers can use the information generated from this study to work towards developing a threshold for wild bees above which growers can expect to receive adequate pollination. Growers with a wild bee community large and diverse enough to supply pollination will save money by not renting honey bees. Additionally, an increased reliance on wild bees could result in increased participation in bee conservation efforts. Many growers may not currently manage for native bees because they are unaware of their significance or rely largely on honey bees. Finally, a reduced dependency on honey bees may make apples a more sustainable and profitable crop.
Obj. 1: To assess which wild bees are most efficient in pollinating apples
To meet this objective, we first determined the most frequent wild bee taxa visiting apple blossoms. We observed wild bee visitation rates to apple flowers at seven different orchards in southern Wisconsin. We recorded the number of visits to a blooming apple tree in a 10-min observation period, and recorded the identity of all bee visitors to the morphospecies level. Morphospecies categories included: large dark mining bees Andrena spp., dark striped bees, including Andrena spp., Lasiogossum spp. and Halictus spp., small dark sweat bees Lasioglossum spp., metallic green sweat bees, including Agapostemon spp., Augochlora spp., and Augochlorella spp., bumble bees Bombus spp., small carpenter bees Ceratina spp., and mason bees Osmia spp. We conducted a total of 84 10-min observations between 9 AM – 4 PM in sunny conditions with temperatures at or above 15º C.
Next, we compared foraging behaviors between common wild bees and honey bees in order to assess which bees are better apple pollinators. In particular, we examined three aspects of foraging behaviors that influence apple pollination: floral preferences, resource collection (nectar only vs. pollen), and cross-tree movement. To assess floral preferences, we observed visitation rates to both apples and dandelions, which bloomed at the same time, within five orchards in south-central Wisconsin. All orchards in this study minimally managed weeds in the understory, and thus dandelions were blooming in abundance within the orchards throughout the spring. During apple bloom of 2014 (mid-late May), we randomly placed 1 m x 1 m quadrats on the orchard floor between rows of trees and counted the number of dandelions within the quadrat. We then observed visits to these dandelions for 10-minute periods, recording the number of bees and identifying them to the following taxonomic groups: honey bee Apis mellifera, bumble bee Bombus spp., sweat bee family Halictidae, and mining bee Andrena spp. We also conducted observations on apples by marking off a 1 m x 1 m section of apple blossoms in the outer, lower canopy of a blooming tree. We recorded the number of open apple blossoms within our selected area and observed visits to these blossoms for a 10-minute period, recording bees to the same taxonomic groups. We conducted 45 ten-minute observations of apples and 45 ten-minute observations of dandelions.
We calculated bee visitation rates as the average number of bee visits per 100 flowers per 10-min period (total visits in 10-min period/number of flowers per observation plot*100) for each observation period. To assess floral preferences, we used mixed effects models with the average visitation rate as the response variable, a fixed effect of flower observed (dandelion vs. apple), and orchard as a random effect in order to account for variation in bee abundance among orchard sites. We ran these models combining all bee visits, visits by only honey bees, and visits by only wild bees.
We evaluated resource collection by sampling wild bees and honey bees that were foraging on both apple and dandelion flowers within the same five orchards used for the above objective. For each individual bee collected, we recorded whether the bee was carrying pollen in its scopa (an indication that it is collecting pollen) or not (an indication that it is collecting only nectar). We caught and recorded 26 honey bees and 33 wild bees, approximately half of which were visiting apples and half visiting dandelions. We identified bees to the following taxonomic groups: honey bee Apis mellifera, bumble bee Bombus spp., mining bee Andrena spp., and sweat bee, family Halictidae. We compared the ratio of nectar foragers to pollen foragers using a Chi-Squared test with fixed effects of bee type (honey vs. wild), the forage plant on which the bee was found (dandelion vs. apple), and an interaction between bee type and forage plant.
Finally, as apples require the transfer of pollen from one variety to another in order to set fruit, thus requiring movement of bees across trees, we examined the number of sequential visits to flowers on the same tree made by honey bees and wild bees. Bees that make multiple sequential visits to the same tree are less likely to cross-pollinate compared to bees that move frequently among trees. We observed bee foraging patterns among apple trees at the Peninsula Agriculture Research Station in northeastern Wisconsin, which maintains several small blocks, 0.5 ha, of mixed apple varieties. During peak bloom 2014, we observed visits to apple blossoms within two blocks of dwarf apple trees. Observing the outer, mid-canopy, we waited for an initial visit to an open apple blossom by a bee. We identified bees to the following taxonomic categories: honey bee, bumble bee, sweat bee, and mining bee. After an initial visit, we followed the bee and recorded the number of sequential visits made to the same tree or to an immediately adjacent tree within the same row. In all cases, adjacent trees within the same row were of the same variety as the initial tree. We ended our observations of an individual bee after 10 visits, or when the bee flew away from a tree and did not move to forage on a neighboring tree, at which point we were unable to follow the bee to its next foraging location. We compared the number of sequential visits to the same tree, and to the same tree plus its neighbors, among bee taxa using an ANOVA followed by post-hoc comparisons among taxa using Tukey’s honest significant difference test.
Objective 2: To determine whether wild bees alone, and in what numbers, can provide full pollination for apples
Prior to apple bloom in each of three years, 2011-2013, we identified apple growers that planned to have honey bees on the farm, or would not have honey bees and were unaware of any neighbouring hives. In 2011, we sampled five orchards with and six orchards without honey bees, in 2012, we sampled nine orchards with and eight orchards without honey bees, and in 2013, we sampled 12 orchards with and seven orchards without honey bees for a total of 26 samples from orchards with honey bees and 21 samples from orchards without honey bees. Orchard study sites were located in southern Wisconsin, USA (between 42.5 ºN – 43.75 ºN, and 87.75º W – 91.5 ºW), and were at least 5 km from any other study site in order to ensure independence of measurements.
We measured fruit set as the proportion of flowers per tree that became fruit on each of 10 trees per study orchard in each year. In early spring, when flowers were in the “pink tip” (pink flower buds) or “popcorn” (loose petal clusters) stage, we counted the exact number of flowers (app. 500) on a marked section of each apple tree. Then, three-four weeks after petal fall and during the calyx stage of development, we counted the number of fruits resulting from the ~500 marked flowers. Fruit set at this stage is the best indication of pollination as most unfertilized or inadequately fertilized ovaries would have fallen, and additional fruit drop due to competition, weather, injury, or over ripening (i.e. non-pollination dependent factors) is unlikely to have occurred (Bekey, Burgett & Fischer 1981; McArtney et al. 2004).
Additionally, in order to assess the dependence of apples on animal pollinators, we measured fruit set within fine mesh bags made from bridal veil, which is impermeable to even the smallest insects. On each of the 10 trees per orchard, we covered one branch with approximately 30 flowers. We covered branches at the pink tip or popcorn stage, and counted the exact number of flower buds prior to bagging. Observations made during bloom confirmed that flowers inside the mesh bag had fully opened. At petal fall we removed bags in order to avoid any effects of the bags on fruit development, and marked the branches with flagging tape. Approximately three to four weeks after petal fall, we counted the fruit on these marked branches. Fruit set on closed branches was calculated for each orchard using the proportion of flowers that became fruit across all ten trees.
We sampled bees during the apple bloom period in 2012 and 2013. Bee traps were 355-ml white plastic cups (Solo Cup Co., Urbana, IL) painted fluorescent blue, fluorescent yellow, or left white, and filled up to 2 cm from the top with a soapy solution containing approximately 1 ml unscented dish soap per litre of water. At each site, we hung ten traps of alternating colours from stakes 1.5 m above ground, approximately mid-canopy height, so as to sample wild bees that are flying in the apple canopy and likely to be contributing to apple pollination. After one week in the field, we emptied the contents of bee traps and added new soap solution for each consecutive week of sampling. All bees were stored in alcohol until processed and identified with the assistance of expert taxonomists.
Objective 3: Develop a field ID guide to effective apple pollinators.
With the results obtained from objective 1, we constructed a list of relatively frequent wild bee visitors to apple blossoms. We took high resolution, high magnification photographs of these wild bee taxa using collected specimens and a camera attached to a microscope. We then wrote two identification guides to these wild bee taxa, one in the format of a dichotomous key, and one in the format of picture matching. With the assistance of web design experts, we put together an interactive website that allows visitors to identify common wild bees of the Midwest during spring and early summer using one or both of the guides. Additionally, we converted the interactive, online guides into a downloadable, hard copy guide that can be accessed and printed from the website.
Results Obj. 1: To assess which wild bees are most efficient in pollinating apples
Our observations of bee visitations recorded 220 visits to apple blossoms by wild bees, and 37 visits to apple blossoms by honey bees Apis mellifera. The most common wild bee visitors were bumble bees Bombus spp. (43% of wild bee visits), dark striped bees, including Andrena spp., Halictus spp., and Lasioglossum spp. (40%), and large dark bees Andrena spp. (14%).
Examining bees’ floral preferences for apple flowers versus competing dandelions, honey bees had a significantly higher visitation rate to apples (9.4 ± 2.3 visits/100 flowers/10-min period) compared to dandelions (2.4 ± 1.2 visits/100 flowers/10-min period) (F1,83 =9.6, P=0.003), while wild bees showed a higher visitation rate to dandelions (9.9 ± 2.8) than to apples (3.9 ± 0.7) (F1,83 =4.4, P=0.04, Fig. 1). These trends were driven by sweat bees Lasioglossum spp. (33 visits) and mining bees Andrena spp. (189 visits), as bumble bees Bombus spp. were relatively uncommon and only visited apples (4 visits), and no other bee taxonomic groups were recorded on the flowers.
Honey bees were significantly more likely to forage for nectar only (60% of specimens) as compared to wild bees (23% of specimens) (Chi Square = 7.85, df = 1, P = 0.005). Among separate taxonomic groups, honey bees were most likely to forage for nectar only (60%), followed by bumble bees Bombus spp. (40%), while mining bees Andrena spp. were the least likely to forage for nectar only (17%) (Chi Square = 10.2, df = 3, P = 0.02). The sample size for sweat bees Lasioglossum spp. (3 specimens) was too small to be included in this analysis.
Honey bees made significantly more sequential visits to flowers on the same tree (7.8 ± 0.6) as compared to wild bees (4.2 ± 0.5) (t78 = -4.66, P < 0.0001, Fig. 2a). Honey bees also made significantly more sequential visits to flowers on the same tree and adjacent trees within the same row (8.4 ± 0.6) as compared to wild bees (4.4 ± 0.5) (t78 = -4.66, P < 0.0001, Fig. 2b). Among wild bees, bumble bees Bombus spp. made the most sequential visits to the same tree (6.0 ± 1.2), and neighboring trees (6.8 ± 1.5), followed by mining bees Andrena spp. (4.3 ± 0.6, 4.5 ± 0.6, respectively), with sweat bees Lasioglossum spp. making the fewest sequential visits to flowers on the same tree (1.3 ± 0.18), and neighboring trees (1.3 ± 0.2) (F3,76 = 10.6, P < 0.0001; F3,76 =11, P < 0.0001, Fig. 2a,b).
Results Obj. 2: To determine whether wild bees alone, and in what numbers, can provide full pollination for apples
Total wild bee abundance per orchard during bloom averaged 112 individuals in 2012 (range 14–330), and was significantly lower in 2013 with an average of 24 individuals (range 7–62) (F1,21 = 16.2, P < 0.001). Across the entire study period, 78 different species of wild bees were found in apple orchards during bloom. Species richness per site in a given year ranged from 5 to 23, and was significantly higher in 2012 compared to 2013 (F1,19 = 10.5, P = 0.004).
Average fruit set was not significantly different between orchards that used managed honey bees (12.5 ± 0.02) versus those that did not have honey bees (13.8 ± 0.02) (F1,28 = 0.09, P = 0.77). The best-fit multiple regression model (R2 = 0.38) found that proportion fruit set was significantly affected by only two measured variables: frost damage significantly decreased fruit set (b = -0.18 ± 0.05, P = 0.001), and wild bee species richness significantly increased fruit set (b = 0.008 ± 0.002, P = 0.003, Fig. 3). No other factors significantly affected fruit set including the abundance of honey bees (P = 0.13), the abundance of wild bees (per week: P = 0.30 or per bloom period: P = 0.46), orchard size (P = 0.47), or year (P = 0.52).
Results Obj. 3: Develop a field ID guide to effective apple pollinators.
The interactive, online identification guide can be found at: https://energy.wisc.edu/bee-guide/. The downloadable, hard copy guide can also be found on this website.
The most frequent visitors to apple blossoms were mining bees Andrena spp. and bumble bees Bombus spp. These wild bees were more common than honey bees, even within orchards that managed or rented honey bee hives. These results suggest that among the various wild bees, the mining bees and bumble bees will have the greatest contribution to apple pollination due to their high abundances within orchards and high visitation rates to apples.
Wild bees, in particular mining bees, showed foraging behaviors of effective apple pollinators. Specifically, mining bees were more likely to forage for pollen as compared to honey bees, the majority of which foraged only for nectar. Bees foraging for pollen are more likely to remove and deposit pollen as they make regular contact with the flowers’ reproductive organs, while bees collecting only apple nectar have been found to avoid the flowers’ reproductive organs and thereby contribute little to no pollination. Mining bees additionally made fewer sequential visits to flowers on a single tree as compared to honey bees. Bees making sequential visits to a tree will transfer mainly incompatible pollen of the same variety, whereas bees moving among trees are more likely to transfer compatible, mixed variety pollen. Therefore, wild bees, in particular mining bees, may be more effective apple pollinators since they reliably forage for pollen, and are more apt to move between trees, thereby transfering compatible pollen.
However, wild bees, including mining bees and sweat bees, preferred to forage on dandelions, the dominant flowering weed in the orchard understory, over apples. Honey bees, on the other hand, showed a preference for apple flowers. Orchard management could potentially enhance the pollination efficacy of wild bees by manipulating foraging decisions. Because dandelions distract wild bees from foraging on apples, mowing or selectively using herbicides in the orchard understory could potentially increase wild bees' visits to apple flowers.
Furthermore, we found that wild bees alone were able to achieve an adequate fruit set comparable to that at orchards using managed honey bees. Surprisingly, the use of managed honey bees in southern Wisconsin did not result in greater apple fruit set. Furthermore, after accounting for frost damage, the majority of orchards, including those without honey bees, received a proportion fruit set of 10% or greater. This proportion fruit set is the upper threshold commonly cited as adequate pollination for apples. Our results therefore suggest that the wild bee community in southern Wisconsin is sufficient enough to achieve acceptable crop yields without the help of managed honey bees. As the availability of honey bees is decreasing and rental costs are increasing in North America and Europe, apple farmers could see economic benefits in relying solely on wild bees for pollination.
The only measured factors that were related to fruit set were frost damage and the species richness of wild bees during crop bloom. After accounting for frost damage, each additional wild bee species resulted in a 0.8% increase in proportion fruit set. An increase of nearly 1% per species is economically significant considering that growers typically aim for a proportion fruit set of around 10%. Our results highlight the critical role that wild bees play in apple pollination, and show that bee species richness is important for crop productivity. While we were unable to determine the exact mechanism for the relationship between bee species richness and apple fruit set, other studies have found that facilitation, niche partitioning, and redundancy among pollinator species increases pollination.
Educational & Outreach Activities
Mallinger, R.E. & Gratton, C. (2014) Species richness of wild bees, but not the use of managed honeybees, increases fruit set of a pollinator-dependent crop. Journal of Applied Ecology, Early View: http://onlinelibrary.wiley.com/doi/10.1111/1365-2664.12377/abstract
Mallinger, R. & Gratton, C. In prep. Honey bees and wild bees display different foraging behaviors within apple orchards that have implications for their efficacy as crop pollinators. In preparation for submission to Journal of Pollination Ecology.
Mallinger, R. In prep. Cultivating alternative apple pollinators: Examining the contribution of wild bees to crop pollination and the factors that influence their abundance and diversity within Wisconsin Orchards. Doctoral Dissertation. University of Wisconsin-Madison.
Online bee identification guide: https://energy.wisc.edu/bee-guide/
Integrating broad-scale landscape perspectives with bees, floral resources, and fruit crop yields. Annual Meeting of the Entomological Society of America. Portland, OR. November 19, 2014.
Pollination services provided by wild and managed pollinators to apple crops of the Midwest. Annual Meeting of the Entomological Society of America, Austin TX. November 11, 2013.
The role of wild bees in fruit pollination. Peninsula Agriculture Research Station Fruit School, Sturgeon Bay, WI. April 8, 2014.
The role of wild and managed bees in apple pollination. Wisconsin Fresh Fruit and Vegetable Growers Annual Meeting, Wisconsin Dells, WI. January 20, 2014.
Can wild bees meet the pollination requirements of apples in Wisconsin? Wisconsin Fresh Fruit and Vegetable Growers Annual Meeting, Wisconsin Dells, WI. January 22, 2013.
Wild bees of Wisconsin: Biology, importance, and conservation. Kemp Natural Resources Station, Woodruff, WI. July 28, 2014.
Wisconsin’s wild bees: Who are they, what do they do, and why should we conserve them? Wednesday Nite @ the Lab, University of Wisconsin-Madison. March 26, 2014.
Native bees of WI: Conservation and importance. Door County Wild Ones Chapter Meeting, Sturgeon Bay, WI. May 28, 2011 and March 18, 2012
This research shows the overall contribution of wild bees to crop pollination, suggests that farmers in Wisconsin can rely solely on wild bees for adequate apple fruit set, and illustrates the importance of bee diversity, not just abundance, for pollination. We expect that in the future, this research will influence farmers’ decisions to use managed honey bees. Farmers could save costs by relying solely on wild bees for pollination. Furthermore, costs associated with managing or renting honey bees could instead be redirected towards the conservation of wild bees. We also expect that this research will generate greater interest in, and commitment to, wild bee conservation.
This project has produced one peer-reviewed publication (see below) and another publication in preparation for peer-review (see below). The peer-reviewed publication has been widely shared, with 74 tweets from 66 accounts and an upper estimate of 96,014 combined followers, putting it in the 98th percentile in terms of attention received compared to other tracked articles. This includes international attention (78% outside of the United States), as well as attention by media users identifying themselves as members of the public (74%), scientists (19%), and science communicators (6%).
Research has been shared with farmers in a variety of settings, including presentations at fruit grower conferences with an estimated total attendance of 500+ growers (see below). Additionally, research will be shared in an article written for Fresh Magazine, a publication to Wisconsin specialty crop growers with distribution to over 1,000 farmers. This article is scheduled to be published in the winter 2015 issue of Fresh Magazine.
The online identification guide has received 46,440 visits to date, making it the most popular webpage associated with and tracked by the Wisconsin Energy Institute (home and developer of the guide). This guide has also been featured in a variety of news stories including:
What's the buzz? Online bee guide features Wisconsin pollinators (2013, November 15) retrieved 23 January 2015 from http://phys.org/news/2013-11-online-bee-features-wisconsin-pollinators.html
Online guide provides the A-Bee-C’s of bee identification (2013, November 21) retrieved 23 January 2015 from http://www.thegrower.com/news/Online-guide-provides-the-A-Bee-Cs-of-bee-identification-232879041.html
This study did not propose to monitor farmer practices or changes in farmer practices. The study length was not long enough to document extensive changes. For example, results on the role of managed honey bees and wild bees in apple pollination were shared with growers just in the past 12 months. However, after sharing such results, many growers expressed interest in wild bee conservation and in eliminating the use of managed honey bees for pollination. In particular, we have received 3 requests to do on-farm assessments of wild bee populations, possible conservation actions for wild bees, and orchard design to enhance pollination. We were able to conduct 2 of these assessments, and expect that these growers will take our recommendations into consideration in the future. Additionally, 3 of our study sites have put in designated pollinator habitat in recent years, including artificial nest sites and floral plantings, and many other growers manage their orchard understory to provide flowers for pollinators outside of apple bloom. We expect that our research results will motivate additional growers to do such conservation activities. Finally, we are aware of at least 3 growers that are increasingly relying on wild bees for pollination, without the help of managed honey bees, due in part to our research results. We also expect that more growers will increasingly rely on wild bees, eliminating the use of managed honey bees, and thereby save on production costs.
Areas needing additional study
- How does bee species richness increase apple fruit set? We found that orchards with higher wild bee species richness had a higher fruit set, but we were unable to determine the mechanism for this finding. We hypothesize that it is one of the following three 1) resource complementarity or niche partitioning, whereby different bee species have different foraging behaviors and preferences including apple variety, location within the apple canopy (upper, lower, inner, outer), foraging time of day, and foraging period during the growing season, 2) facilitation whereby different bee species interact to increase rates of cross-pollination among apple varieties or 3) a sampling effect whereby species rich or diverse communities contain the most effective pollinator species.
- How does orchard design affect pollination rates? Since apples require cross-pollination among trees of different varieties to set fruit, the planting design of different varieties within blocks or rows can affect how frequently successful cross-pollination occurs. Research into this area could inform the design of new orchards, or new plantings within established orchards, for maximum pollination.
- Effects of fungicide use on wild bees. Fungicides are the primary pesticide applied in Wisconsin apple orchards. While fungicides applied alone and in low doses are not highly lethal to bees, we do not know as much about the sublethal effects of fungicides on bees, including effects on bee behavior, reproductive rates, or immune systems. Furthermore, recent research suggests that fungicides interact with insecticides to have synergistic, negative effects on bees. Investigating these sublethal effects and interactions should be a priority given the heavy use of fungicides in orchards.