- Fruits: berries (strawberries)
- Crop Production: beekeeping, pollination, pollinator habitat, pollinator health
- Education and Training: extension, on-farm/ranch research
- Natural Resources/Environment: biodiversity
- Production Systems: organic agriculture
A diverse group of bee species, including both managed honeybees and wild native bees, are necessary to effectively pollinate commercially grown strawberries. Cultivated strawberries (Fragaria × ananassa Duch.) require pollination to produce marketable fruit, and strawberry seed set, fruit size, and fruit shape can be used to quantify pollination services. However, differences in pesticide application intensity may directly affect the pollination services provided by wild native bees.
Conventional strawberry growers routinely make over 20 synthetic pesticide applications annually to control pathogens, insects, and weeds during the season, most of which are preventative fungicide applications. Organic growers generally make fewer pesticides applications, typically less than 10 per growing season, and organically acceptable active ingredients may be less persistent in the enviroment. In order to measure the effects of pesticide exposure on native bees across a gradient of intensity, we will evaluate conventional and organic farms of both high and low pesticide intensity.
Higher pesticide intensity has been shown to reduce species abundance and richness, thereby reducing pollination services. Such reductions may reduce yield or increase the proportion of malformed berries, and may correlate with the gradient of pesticide intensity. Pesticides have been shown to reduce the pollination efficiency of bumble bees, but their effects on solitary bees are unclear. Further, greater pesticide intensity may reduce native bee immunity, increasing the likelihood of infection. We expected that farms with fewer pesticide applications, and organic farms which make fewer applications of less persistent insecticides, will support larger populations of native bees, which will in turn have greater immunity and result in measurably higher pollination services. This information will be used to both inform grower pest management recommendations to reduce impacts on pollinators and shared with policy makers to inform regulations aimed at improving pollinator health.
We identified a total of 22 strawberry farms in North and South Carolina, 7 of which used organic production practices. Sites were visited 3 to 4 times during the 2017 and 2018 growing seasons At each visit, three 50-meter long transects were established. Pan traps were placed along each transect and bees were sampled with a sweep net along the rows adjacent to transects. To relate pollinator abundance and diversity to fruit quality, up to thirty fully ripe berries were sampled at each visit and weight, seed set, symmetry were recorded. Pesticide records and honey bee stocking records were obtained from each site. Landscape composition was quantified using the National Land Cover Database and ArcGIS.
We defined bee health through estimates of census (bee counts) and effective population size (via molecular population genetics), immuno-competence, and pathogen intensity and are measuring these variables using molecular techniques described in detail in our Methods & Results.
Conclusions to date
We have completed field data collection. Molecular analyses will be completed in Fall/Winter 2018/2019 and statistical analyses will be completed in Spring 2019. Site characteristics varied widely between the selected farms. Pesticide applications over the two years ranged from 0 to 24, and six locations made no pesticide applications. Most foliar pesticide applications in conventional fields were fungicides, and fungicide, insecticide/miticide, and herbicide usage ranged from 0-15, 0-9, and 0-2 applications, respectively.
During 2017, Honey bee stocking rates at conventional farms ranged from 0 to 8 hives, and 6 of the 13 farms had no managed honey bees. Organic farms had between 0 to 5 hives, and 3 of the 7 had none. During 2018, honey bee stocking rates ranged from 0-4 hives per acre, including 9 sites which did not stock honey bees.
Landscape surrounding farms varied across our initial four habitat categories, but the dominant surrounding habitat varied by the size of the area assessed, either 500 or 1500 m surrounding the farm center.
|500 m radius||Mostly Agricultural||Mostly Forested||Mostly Natural, Non Forested||Mostly Urban|
|1500 m radius||Mostly Agricultural||Mostly Forested||Mostly Natural, Non Forested||Mostly Urban|
In general, bee densities were low in southeastern strawberry fields. Nearly all bee samples were collected via targeted sweep netting during transect walks, and very few bees were collected in pan traps. Pan traps were more effective at capturing flies, which were more common than bees on strawberry flowers during the first half the fruiting period. In 2017, we collected a total of 692 honey bees (Apis mellifera), 33 punative Augochlorella spp. bees, 106 punative Andrena spp. bees, 420 punative Lassioglosssum spp. bees, and 39 bees belong to other genera via sweep net. Honey bees, Augochlorella, Andrena, and Lasioglossum bees were collected at 20, 11, 14, and 18 locations, respectively. Honey bees were also collected directly from 34 hives at 11 locations just after the bloom period and 28 hives at 8 locations in September 2017. In 2018, we collected 442 honey bees (Apis mellifera), 26 putative Augochlorella spp. bees, 107 putative Andrena spp. bees, 439 putative Lasioglossum spp. bees, and 27 bees belonging to other genera via sweep net. Honey bees, Augochlorella, Andrena, and Lasioglossum bees were collected at 18, 6, 15, and 17 locations, respectively. Honey bees were also collected directly from 22 hives at 8 locations just after the bloom period and will be sampled directly from the hive again in September 2018. Additionally, syrphid flies from all 18 locations were collected and stored on dry ice. Recent work internationally has determined that bee viruses can be detected in some species of syrphid flies, so we will use a subsample of our specimens to assess if bee viruses are present in common syrphid flies in our region. A total of 1380 ripe berry samples were collected from multiple visits at each location and weight, symmetry, and seed set have already been recorded for all berries.
While the overall goal of our project remained the same, our objectives shifted in response to the greater diversity and lower abundance of native bee species observed after the first season. Further, we developed alternative molecular approaches that are more suitable to our field collected specimens and are still assessing pollinator health in the context of farm management. Specifically, our objectives are:
Objective 1. Assess the impact of a gradient of pesticide use applications on wild and managed bee health.
Hypothesis 1. Sites with higher pesticide application frequency or more persistent active ingredients (conventional vs organic) will have smaller and less diverse native bee populations.
Objective 2. Evaluate the impact of honey bee stocking densities on managed and wild bee pollinator health.
Hypothesis 2. Sites with higher densities of honeybees will have smaller and less diverse native bee populations. These sites will also have greater prevalence of bee pathogens, Deformed wing virus and/or Black Queen Cell Virus.
Objective 3. Estimate the impact of landscape context on the overall health of bee populations.
Hypothesis 3. Sites with less natural land coverage or greater urbanization will host smaller and less diverse native bee populations.
Objective 4 (new): Identify potential strawberry pollinating species of flies and determine if these pollinators are impacted by above factors.
Hypothesis 4: Flies contribute a meaningful amount of pollination early in the strawberry fruiting period.