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Providing evidence to improve practice

Action: Pest regulation: Plant flowers Mediterranean Farmland

Key messages

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Pest regulation (3 studies): Three replicated studies from Italy and the USA found greater pest reduction or higher proportions of parasitized pests in fields and farms with planted flower strips.

Crop damage (2 studies): One replicated, randomized, controlled study from Italy found more damage by caterpillars, but not by aphids, in tomatoes next to planted flower strips, compared to tomatoes next to bare ground. One replicated, paired, controlled study from Italy found that planted flower strips had inconsistent effects on crop damage by pests.

Pest numbers (2 studies): One replicated, paired, controlled study from Italy found more pests on tomatoes next to planted flower strips, compared to tomatoes next to unplanted field margins. One replicated before-and-after study from the USA found more aphids in fields after flower strips were made available.

Natural enemy numbers (4 studies): Two replicated studies from the USA found more natural enemies in fields with planted flower strips, compared to fields without planted flower strips, in some comparisons. Two replicated, controlled studies from Italy found more natural enemies in planted flower strips than on bare ground, and one of these studies also found more species of natural enemies.

Implementation options (4 studies): Two replicated, controlled studies from the USA and Spain found that some flower species were more attractive to natural enemies than others. Two replicated, controlled studies from Italy found that planting more species of flowers, compared to fewer, had inconsistent effects on pests and pest species, but one of these found less crop damage next to flower strips with more species, compared to fewer, in some comparisons. This study also found more species of natural enemies in flower strips, over time, but did not find more individuals.

Supporting evidence from individual studies

1 

A replicated, randomized, controlled study in 2003 in organic tomato fields in the Sacramento Valley, California, USA, found that a higher percentage of stink-bug eggs were parasitized near planted flower borders than near bare borders. Pest regulation: Parasitism of consperse stink bug Euschistus conspersus eggs was significantly higher near borders planted with sweet alyssum Lobularia maritima than it was near bare borders in September (41 vs 22%), but not in July or August (early July: 9 vs 6%; late July–early August: 47 vs 40%). Natural enemy numbers: There were not significantly more predators near flower borders, compared to bare borders (3 vs 2–5 individuals/sample), except for spined stilt bugs Jalysus wickhami in June (2 vs 0). Methods: In each of four tomato fields, one 23 m border was planted with 60 alyssum plants and one border was tilled and left bare. Eight (3 and 31 July) or ten (5 September) masses of stink bug eggs were placed on the undersides of tomato leaves at each of three distances (0.3, 6, and 15 m) from each border. Yellow sticky traps were placed at four (18 June: 0.3, 1.5, 6, and 15 m) or three (26 August: 0.3, 6, and 15 m) distances, to sample predators. Eggs and cards were collected after 6–7 days.

 

2 

A replicated before-and-after study in 2007 in lettuce fields in the Salinas Valley, California, USA, found more pests, more predators, and different distributions of predators after restoring floral resources compared to before. Pest and natural enemy numbers: After restoring sweet alyssum Lobularia maritima rows, there were more pests (12.5 vs 8.5 currant-lettuce aphids Nasonovia ribisnigri per lettuce) and more hoverfly eggs and larvae (2.3 vs 1.2 eggs/lettuce; 1.4 vs 0.8 larvae/lettuce) than there were before (second sampling dates). After restoration, there were more hoverfly adults near restored flower strips (72 vs 57), but fewer elsewhere in the field (60–76 vs 77–95), than there were before. Methods: Flower strips were planted 48 m apart on four lettuce fields. Access to one flower strip/field was restricted (strips were covered on 31 August) and then restored (covers were removed on 5 September). Insects were sampled (2–5 days after restriction and restoration) by searching lettuce heads or by counting hoverflies on transects.

 

3 

A replicated, randomized, controlled study in 2009 on an organic farm in Sonoma County, California, USA, found that natural enemies and pests preferred different flower species planted in flower strips surrounding kale plots. Implementation options: More hoverflies were found on sweet alyssum Lobularia maritima (4–5.8 visits/sample), compared to other flower species in peak bloom (0.2–1.4), except buckwheat Fagopyrum esculentum (3.3 from 24 June to 10 July). More predatory bugs were found on alyssum (3.6 individuals/sample), compared to other species (0.1–1.1), except buckwheat (1.6) and wild mustard Brassica sp. (1.6). More parasitic wasps were found on cosmos Cosmos sulphureus (3.5 individuals/sample), compared to other species (0.3–0.7), except alyssum (1.4), wild arugula Diplotaxis muralis (1.0), and kale Brassica oleracea (3.9). Numbers of spiders did not differ significantly between species (0.1–1.0 individual/sample). More chrysomelid beetles were found on arugula (1.7 individuals/sample), compared to other species (0–0.6), except white borage Borago officinalis and alyssum (0.6–1.0). More cicadellid bugs were found on tansy phacelia Phacelia tanacetifolia (25 vs 1–10 individuals/sample), and more lygaeid bugs were found on arugula (23 vs 0–3), compared to other species. Aphid numbers did not differ significantly between flower-strip species, but more aphids were found on kale than on flower-strip species (480 vs 0–0.9 individuals/sample). Methods: Two rows of kale, between two strips (four rows) of one flower species, were planted on 16 May in each plot (3 x 6 m). There were five replicate plots for each of nine flower species. Invertebrates were sampled by counting flower visitors (6 minutes/plot, every 5–9 days) or vacuuming plants (15 seconds/plot, 25 x 50 cm area, every 12–15 days).

 

4 

A replicated site comparison in 2008–2009 in broccoli fields in the Salinas Valley, California, USA, found that cabbage aphids Brevicoryne brassicae were better controlled by natural enemies on complex than on simple farms. Pest regulation: In August, the proportional reduction in aphid densities (PRD) was higher on complex than on simple farms, if farms were surrounded by low amounts of natural habitat (data were reported as the negative log of PRD), but not if surrounded by high amounts of natural habitat. In June, PRD was not significantly different between complex and simple farms. Methods: Eight farms were compared in 2008 and 10 farms were compared in 2009. Flowers for beneficial insects were planted on complex but not on simple farms. Complex and simple farms also differed in field size (1.2–4 vs 6–12 ha) and crop composition (polyculture vs monoculture). Potted broccoli plants were inoculated with 50 aphids each, placed in fields for 12 days, and either caged (to exclude natural enemies) or uncaged. Farms were surrounded by high (>50%) or low (<10%) amounts of natural habitat (0.5–3 km). It was not clear whether these results were a direct effect of planting flowers, field size, or crop composition.

 

5 

A replicated, randomized, controlled study in 2009–2010 in a winter barley field in the Jarama River basin, Spain, found that aphid-eating hoverflies visited some planted flower species more than others. Implementation options: In 2009, more hoverflies visited the flowers of Calendula arvensis (3.6 visits/minute) and Coriandrum sativum (3.5) than Pimpinella anisum (0.6), but other differences between flower species were not significant in 2009 or 2010 (0.6–5.8). Most visits were made by Sphaerophoria sp. (88–96% of visits). Methods: Six flower species were studied. There was one species/plot (1.5 x 1.5 m), with three (2009) or four (2010) blocks, and six plots/block. Hoverflies that touched the reproductive parts of flowers were counted, in the centres of plots (0.5 x 0.5 m), for six minutes/plot, every 2 to 11 days, from 7 May to 26 June 2009, and nine minutes/plot, two times/week, from 27 April to 1 July 2010.

 

6 

A replicated, paired, controlled study in 2011–2012 in tomato fields in Tuscany, Italy, found higher rates of aphid parasitism, but more aphids, and different amounts of fruit damage by different pests, in tomatoes next to planted flower strips compared to tomatoes next to unplanted field margins. Pest regulation: Aphid parasitism rate was higher on tomatoes next to flower strips than on tomatoes next to unplanted margins (parasitism rates not reported). Crop damage: Less fruit damage by sucking bugs, but more leaf damage by other pests, was found on tomatoes next to flower strips, compared to tomatoes next to unplanted margins (amounts of damage not reported). Similar amounts of fruit damage by noctuid pests and Tuta absoluta were found on tomatoes next to flower strips and unplanted margins. Pest numbers: More aphids were found on tomatoes next to flower strips, compared to tomatoes next to unplanted margins (numbers of individuals not reported). Methods: In each of eight tomato fields, 150 fruits and 60 leaves were sampled for crop damage, aphid mummies (parasitism), and aphids, on transects from each of two field margins (one with flower strips, one without; 3 x 25 m each). The flower strips were planted 7–14 days after the tomatoes were planted. Samples began when the flower strips began to bloom, and continued every 15 days until harvest.

 

7 

A replicated, controlled study in 2011 in an organic tomato field near Pisa, Italy (same study as (8)), found more natural enemies in flower plots than on bare ground. Plots with the most flower species had the fewest tomato pests but the most generalist pests, and plots with different numbers of flower species had similar numbers of natural enemies. Natural enemy numbers: More ground-dwelling predators were found in flower plots than on bare ground (carabid beetles/plot: 0–28.7 vs 0–1.2; staphylinid beetles/plot 0.5–7.4 vs 0–0.4; spiders/plot: 0.4–7.1 vs 0.2–1.5). Implementation options: Fewer tomato pests (sap-sucking bugs), but more generalist pests (Lygus sp. and Nezara viridula), were found on flowers in plots with nine flower species, compared to plots with three flower species (numbers of individuals not reported). Similar numbers of natural enemies were found on flowers in all plots (numbers of individuals not reported). On flowers, predatory beetles Hippodamia variegata and parasitic wasps increased over time (beetles: minimum: 0 individuals/plot, on the first day of flowering; maximum: 2.7 individuals/plot, 38 days after flowering; wasps: minimum: 0 individuals/plot, on the first day after flowering; maximum: 36.5 individuals/plot, 21 days after flowering), but formicid ants decreased over time (numbers of individuals not reported). On the ground, carabid beetles increased over time (minimum: 0 individuals/plot, nine days after flowering; maximum: 28.7 individuals/plot, 37 days after flowering), but staphylinid beetles and spiders did not. Methods: Four treatments were compared: three, six, or nine flower species/plot, and a control with no flowers. Five plots/treatment were sown with flower seeds on 6 and 21 June. Each flower plot (2 x 10 m) was next to a tomato plot (4 x 10 m). Ground-dwelling predators were sampled with pitfall traps every 7 days, and natural enemies on flowers were sampled with aspirators every 14 days, after flowering began.

 

8 

A replicated, randomized, controlled study in 2011–2012 in an organic tomato field near Pisa, Italy (same study as (7)), found more damage by caterpillars, but not aphids, in tomatoes grown next to flower strips, compared to bare ground. It also found more individuals and species of natural enemies in flower strips, compared to bare ground. Crop damage: More damage by caterpillars, but not aphids, was found in tomatoes grown next to flower strips, compared to bare ground (amounts of damage not reported). Natural enemy numbers: More individuals and species of ground-dwelling predators were found in flower strips than on bare ground (16–19 vs 4 individuals; numbers of species not reported). Implementation options: Less damage by caterpillars was found in tomatoes grown next to flower strips with more compared to fewer flower species (six or nine species vs three, in 2012, in plots with high numbers of fruit/plant; amounts of damage not reported). Fruit damage varied with the number of flower species/strip, but not all differences were significant in both years or in both varieties (Roma and Perfect Peel; amounts of damage not reported). Fewer tomato pests (sap-sucking bugs), but more generalist pests (Lygus sp. and Nezara viridula), were found in strips with nine flower species, compared to three (numbers of individuals not reported; same results as (7)). Flower strips with more, compared to fewer, flower species (six or nine vs three species) did not have significantly more natural enemies (ground-dwelling: 16–19 individuals/strip; flower-visiting parasitoids: 12–19; flower-visiting predators: 4–7; same results as (7)). Strips with six flower species had more species of flower-visiting natural enemies than strips with three or nine flower species, but did not have significantly more species of ground-dwelling natural enemies (numbers of species not reported). The diversity of flower-visiting natural enemies increased over time, but the number of individuals did not (numbers of individuals and species not reported). Methods: Four treatments were compared: three flower species/strip (Apiaceae species), six species/strip (three Apiaceae and three Fabaceae), nine species/strip (three Apiaceae, three Fabaceae, and three others), and a control strip with no flowers. Three strips/treatment were sown with flower seeds (2011: 6 and 21 June; 2012: 13 and 17 June). Each flower strip (2 x 4 m) was positioned between two tomato plots (4 x 10 m/plot). Ground-dwelling predators were sampled with pitfall traps every 7 days, and natural enemies on flowers were sampled with aspirators every 14 days, after flowering began. Damage by pests was assessed for 30 fruits/plot and 12 leaves/plot.

 

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.