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Providing evidence to improve practice

Action: Pollination: Plant hedgerows Mediterranean Farmland

Key messages

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Pollination (1 study): One replicated, paired site comparison from the USA found higher seed-set in canola plants due to flower visitation by native bees in fields next to planted hedgerows, compared to fields next to unplanted edges. However, this study found no difference in seed-set due to flower visitation by honey bees or syrphid flies.

Crop visitation (2 studies): One replicated, paired, site comparison from the USA found higher crop visitation rates by native bees, but not by honey bees or syrphid flies, in fields next to planted hedgerows, compared to fields next to unplanted edges. Another replicated, paired, site comparison from the USA found no difference in flower visitation rates by bees in fields next to planted edges.

Pollinator numbers (6 studies): Five replicated studies from the USA found more bee species in fields with hedgerows, or in hedgerows themselves, compared to fields or field edges without hedgerows. Three of these studies found more syrphid fly species in hedgerows, compared to field edges without hedgerows. One of these studies found similar numbers of syrphid fly species in fields with or without hedgerows. Two of these studies found more native bee and hoverfly individuals or more specialist bees in hedgerows, compared to field edges without hedgerows. One replicated site comparison from the USA found fewer ground-nesting bees, but similar numbers of bee species and flower-visiting bees, in planted hedgerows, compared to unplanted edges.

Implementation options (3 studies): Two replicated site comparisons from the USA found more bee species in old hedgerows, compared to young hedgerows, and one of these studies also found more syrphid fly species. One replicated site comparison from the USA found more bee species on native plants, compared to non-native plants, in old hedgerows, but not in young hedgerows.

Supporting evidence from individual studies

1 

A replicated, paired, site comparison in May–August 2009–2010 in tomato fields in the Sacramento Valley, California, USA, found more individuals and species of flower-visiting bees and syrphid flies in hedgerows than in weedy field edges, and more individuals and species of bees but not syrphid flies in fields with hedgerows than in fields with weedy edges. Pollinator numbers: More individuals (10 m: 1.2 vs 0.3 individuals/sample; 100 m: 0.8 vs 0.3; 200 m: 0.5 vs 0.2) and more species (10 m: 0.63 vs 0.25 species/sample; 100 m: 0.54 vs 0.22; 200 m: 0.39 vs 0.16) of native bees were found 10–100 m, but not 200 m, into fields with hedgerows than in fields with weedy edges. More honey bees were found 10 m into fields with hedgerows, but there was not a significant difference between these fields in honey bees or syrphid flies at greater distances (honey bees: 10 m: 0.50 vs 0.14 individuals/sample; 100 m: 0.13 vs 0.04; 200 m: 0.20 vs 0.17; syrphid flies: 10 m: 0.63 vs 0.60; 100 m: 0.50 vs 0.67; 200 m: 0.20 vs 0.56). Flower-visitor communities had more species and greater diversity in hedgerows than in weedy edges (bees: 5.7 vs 3.6 species; syrphid flies: 2.7 vs 1.8 species). Twenty bee species were found only in hedgerows, not in weedy edges. Uncommon bee species (species represented by <1% of collected individuals) had larger populations in hedgerows than in weedy edges (6 vs 1 individuals), but uncommon syrphid fly species did not (numbers not reported). Honey bee, native bee, and syrphid fly species had larger populations in hedgerows than in weedy edges (numbers of individuals not reported). The number of flower species and the amount of bare ground did not differ significantly between hedgerows and weedy edges (6 vs 4 species; amount of bare ground not reported), but floral cover was higher and there was more dead wood in hedgerows (amounts not reported). Methods: Native perennial shrubs (305–550 x 7 m), bordered by native perennial grasses (3 m), were planted in 1996–2003 on the edges of six fields (hedgerows) and compared to the unplanted edges of six fields (weedy edges). Insects were netted if they touched the reproductive parts of flowers (in field borders) or they were identified landing on flowers or flying through quadrats (1 m3 quadrats; four minutes/quadrat; three quadrats/field-edge; six quadrats/field).

 

2 

A replicated site comparison in 2009 in farmland in the Sacramento Valley, California, USA, found more bee species on native plants than on exotic plants in mature hedgerows. Implementation options: Species richness of bees was higher on native plants than on exotic plants in mature hedgerows, but not in new hedgerows (mature: 3.7 vs 1 species; new: 4.2 vs 2.8 species). Abundance of bees was higher on native than on exotic plants in both mature and new hedgerows (mature: 17 vs 3 individuals; new: 19 vs 9). Methods: Similar but not identical species of native flowering shrubs and forbs were planted in four mature hedgerows (305–550 m; planted in 1996) and four new hedgerows (350 m x unreported width; planted in 2008). New hedgerows were sampled three times (April–August) and mature hedgerows were sampled four times (May–July). In timed surveys (30 minutes/mature vs 60 minutes/new hedgerow), bees were netted if they touched the reproductive parts of a flower.

 

3 

A replicated, before-and-after study in April–August of 2006–2013 in field borders in the Central Valley, California, USA (same study as (4)), found that flower-visiting insect species were more likely to be present after woody hedgerows were planted than before. Pollinator numbers: Insects that specialize in relatively few flower species (specialists) were more likely to be present six years after planting, compared to the first year after planting (bee species: 0.3 vs 0.0 probability of occurrence/transect; syrphid fly species: 0.1 vs 0.02), and so were generalist syrphid fly species, but not generalist bee species (syrphid fly species: 0.12 vs 0.07; bee species: 0.2 vs 0.2). Methods: Field borders (350 x 3–6 m) were planted with native shrubs and trees in 2007–2008 in five fields, and unplanted borders in ten fields were used as controls. Fields borders had an irrigation ditch or slough. Fields were approximately 80 acres of row crops, vineyards, or orchards. Hedgerows were watered and weeded for three years. At least three times per year, insects were collected from flowers on one-hour transects at each site.

 

4 

A replicated site comparison in April–August of 2006–2013 in field borders in the Central Valley, California, USA (same study as (3)), found more species of bees and syrphid flies in planted hedgerows than in unplanted field borders, but only after several years of hedgerow growth. Pollinator numbers and Implementation options: More species of bees and syrphid flies were estimated to be present in planted hedgerows than in unplanted field borders, 4–6 years after planting (2013: 65 vs 45 species; 2012: 60 vs 40; 2011: 55 vs 40), but not 0–3 years after planting (2010: 50 vs 40 species; 2009–2008: 45 vs 40; 2007: 35 vs 35). Methods: Field borders (350 x 3–6 m) were planted with native shrubs and trees in 2007–2008 in five fields, and unplanted borders in ten fields were used as controls. Fields borders had an irrigation ditch or slough. Fields were approximately 80 acres of row crops, vineyards, or orchards. Hedgerows were watered and weeded for three years. At least three times per year, insects were collected from flowers on one-hour transects at each site.

 

5 

A replicated site comparison in 2007–2013 in farmland in the Central Valley, California, USA, found greater bee diversity in mature hedgerows compared to weedy field edges or immature (“maturing”) hedgerows. Pollinator numbers and Implementation options: Greater bee diversity was found in mature hedgerows compared to weedy field margins or immature hedgerows, but not in immature hedgerows compared to weedy field margins (data was reported as beta-diversity, which is change in the diversity of species between sites). Twenty-eight percent of bee species were found only in hedgerows. Thirteen percent were found only in weedy edges. Methods: Native, perennial shrubs and trees (3–6 x 350 m) were planted 1–10 years (immature hedgerows) or >10 years (mature hedgerows) before bees were collected. Bees were collected if they touched the reproductive parts of flowers, in one-hour samples of 21 hedgerows and 24 weedy edges, 2–5 times/year, in April–August 2007–2013.

 

6 

A replicated, paired site comparison in 2012–2013 in sunflower fields in the Central Valley, California, USA, found more bees, more sunflower-specialist bees, fewer generalist bees, and more bee species in hedgerows than in bare/weedy edges. Crop visitation: Visitation rates to sunflowers were not significantly different in fields with hedgerows than in fields with bare/weedy edges (rates not reported). Pollinator numbers: Bee abundance and species richness were higher in hedgerows than in bare/weedy edges (abundance: 17 vs 6 individuals/sample; richness: 5 vs 2 species/sample). More sunflower-specialist bees, but fewer generalist bees, were found in hedgerows than in bare/weedy edges (specialists: 0.6 vs 0.1 relative abundance; generalists: 0.0 vs 0.3). Methods: In field edges, when >90% of sunflower heads were blooming in adjacent fields, bees were netted for 16 minutes/field (2012: 10 fields; 2013: 8 fields), and bees that touched the reproductive parts of flowers were counted for 2 minutes/plot in 8 plots/field (visitation rates). Half of fields had bare/weedy edges (managed by burning, scraping, or herbicides). Half had hedge rows (3–6 x 250–300 m, 5–12 years old). Sunflower specialists and generalists were netted in 26 hedgerows and 21 bare/weedy edges (one hour/sample; five samples in April–August 2012–2013).

 

7 

A replicated, paired site comparison in 2009–2011 in tomato fields in Yolo County, California, USA, found more native bees, and higher seed-set due to native bees, in fields next to planted hedgerows, compared to fields next to conventional edges. Pollination: Seed-set in canola plants, due to flower visitation by native bees, was higher in fields next to hedgerows, compared to fields next to unplanted edges (21% higher estimated seed yields). Seed-set, as a result of flower visitation by honey bees or syrphid flies, was similar in fields next to hedgerows or unplanted edges (data not reported). Crop visitation: More native bees were found on canola flowers in fields next to hedgerows, compared to fields next to unplanted edges (4.2 vs 1.0 visitors/observation), but similar numbers of honey bees (1.4 vs 2.6), syrphid flies (2.9 vs 3.5), or total visitors (8.4 vs 7.1) were found. Methods: Hedgerows (300–350 m length) were planted along the edges of four treatment fields, but not four control fields, about 10 years before this study began. The edges of control fields were mown, disked, or sprayed with herbicide. Tomatoes were grown in all fields, but pollination was measured in clusters of potted canola plants, placed at four distances from the edges (0, 10, 100, and 200 m), in 2010 and 2011. Flower visitors were observed for four minutes/cluster (one observation period in 2010 and four in 2011). Pollination deficits were measured by comparing seed-set in open-pollinated and hand-pollinated canola flowers.

 

8 

A replicated site comparison in farmland in the Central Valley, California, USA (years of study not reported), found fewer ground-nesting bees in planted hedgerows, compared to unplanted field edges. Pollinator numbers: Fewer ground-nesting bees were found in planted hedgerows, compared to unplanted edges (13 vs 33 individuals/site), but there were similar numbers of flower-visiting bees (data reported as statistical results), and similar numbers of bee species (2.9 vs 3.2 rarified species richness). Indicators of ground-nesting bee habitat did not differ between planted hedgerows and unplanted edges (data on bare ground, soil compaction, particle size, and surface heterogeneity reported as statistical results). Methods: Eight field edges with planted hedgerows (mostly Californian native shrubs and forbs, at least five years after planting) were compared to eight field edges without planted hedgerows. Ground-nesting bees were sampled with emergence traps (0.6 m2, 30 traps/site/sample, three samples in two years, in May–August). Foraging bees were netted on inflorescences (one hour/site/sample, within 10 days of emergence samples). Nesting indicators were assessed using soil samples (0–10 cm depth, two samples/site) and visual estimates.

 

Referenced papers

Please cite as:

Shackelford, G. E., Kelsey, R., Robertson, R. J., Williams, D. R. & Dicks, L. V. (2017) Sustainable Agriculture in California and Mediterranean Climates: Evidence for the effects of selected interventions. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.