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Action: Pest regulation: Plant or maintain ground cover in orchards or vineyards Mediterranean Farmland

Key messages

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Pest regulation (3 studies): One replicated, randomized, controlled study from the USA found that more leafhopper eggs were parasitized in plots with cover crops, compared to bare fallows, in one of six comparisons. Two replicated, randomized, controlled studies from the USA found inconsistent differences or no differences in the parasitism of leafhopper eggs between plots with or without ground cover.

Crop damage (1 study): One replicated, randomized, controlled study from the USA found that more grapes were damaged by pests in plots with cover crops, compared to bare fallows, in some comparisons.

Ratio of natural enemies to pests (0 studies)

Pest numbers (12 studies)

  • Weeds (2 studies): One replicated, randomized, controlled study in an olive orchard in Spain found fewer weeds in plots with cover crops, compared to bare soil, in one of two comparisons. One replicated, controlled study from a vineyard in the USA found more weeds in plots with cover crops, compared to bare soil, in one of nine comparisons.
    • Implementation options (4 studies): Three studies from vineyards in the USA found different numbers of weeds or weed species in plots with different types of ground cover, in some or all comparisons. One study from the USA found similar numbers of weeds in vine rows with or without cover crops. One replicated, randomized, controlled study from the USA found that plant diversity decreased over time in plots without tillage, but increased in plots with tillage. This study found that tillage had no effects on the number of plant species and had inconsistent effects on plant biomass.
  • Insects (5 studies): Two replicated, controlled studies (one randomized) from the USA found fewer leafhoppers in plots with cover crops, in some comparisons. One replicated, randomized, controlled study from the USA found more leafhoppers, in some comparisons. One replicated, randomized, controlled study from the USA found similar numbers of leafhoppers. One replicated, randomized, controlled study from the USA found more navel orangeworm moths in plots with resident vegetation, compared to tilled soil, in one of two comparisons.
    • Implementation options (2 studies): Two studies from the USA found fewer pests in plots with mown ground cover, compared to unmown ground cover or ground cover before mowing.
  • Mammals (1 study)
    • Implementation options (1 study): One study from the USA found more gophers in plots with clover, compared to other cover crops.

Natural enemy numbers (6 studies): Four replicated, controlled studies (three randomized) from Spain and the USA found more natural enemies in plots with ground cover, compared to plots without ground cover, in some or all comparisons. One replicated, controlled study from the USA found fewer parasitoids in plots with ground cover, in some comparisons. One replicated, randomized, controlled study from the USA found inconsistent differences in the numbers of spiders between plots with or without ground cover. One of these studies found no difference in the number of spider species between plots with or without ground cover, and another one found no difference in the composition of spider communities.

  • Implementation options (1 study): One study from the USA found more natural enemies in plots with mown cover crops, one week after mowing, compared to before mowing.

Supporting evidence from individual studies

1 

A replicated, randomized, controlled study in 1992–1995 in a vineyard in the San Joaquin Valley, California, USA, found similar numbers of spiders and spider species on grape vines with or without cover crops between the vine rows in spring and summer. Natural enemy numbers: Similar numbers of spiders and spider species were found on grape vines with or without cover crops between the vine rows in spring and summer (15.1 vs 13.6 spiders/vine; data on species not reported). More Trachelas pacificus spiders were found on grape vines with cover crops, compared to bare soil, between the vine rows in spring and summer (7.2 vs 4.7 spiders/vine), but fewer Hololena nedra spiders were found (0.8 vs 1.2 spiders/vine). Methods: Cover crops were seeded between the vine rows in autumn 1992–1994 in ten plots (1.4 ha plots; 8 rows x 80 vines). In five treatment plots, the cover crops were mown in March 1993–1995 and allowed to regrow with resident vegetation over the summer, but in five control plots they were tilled and bare soil was maintained with herbicide (1993) or cultivation between rows and ploughing within rows (1994–1995) until mid-August. In July 1995, herbicide was used on all plots. Spiders were collected in May–September 1993–1995 by shaking the grape vines over drop cloths (two samples/plot/month, 15 seconds/sample, 9 x 3 m cloth).

 

2 

A replicated, randomized, controlled study in 1993–1996 in four vineyards in the San Joaquin Valley, California, USA, found fewer pests and more natural enemies in plots with cover crops between the vine rows, compared to plots without cover crops between the vine rows. Pest regulation: No consistent differences were found in the parasitism of Erythroneura spp. leafhopper eggs in plots with or without cover crops between the vine rows (data not reported). Pest numbers: Fewer leafhoppers were found in plots with cover crops, compared to plots without cover crops, in seven of eight comparisons (1–36 vs 5–48 third-generation leafhopper nymphs/20–30 vine leaves). Natural enemy numbers: More spiders were found on grape vines in plots with cover crops, in one of four vineyards (data not reported). Methods: Cover crops were grown between the vine rows in a total of 19 plots (0.05–0.6 ha plots), and no cover crops were grown in a total of 19 plots (which were treated with herbicide, mown, and/or disked between the vine rows), in a total of four vineyards. Leafhoppers were sampled on 20–30 vine leaves/plot, and vines were shaken (15 seconds/sample) to collect spiders on sheets (3 x 7.3 m) or in funnels (0.9 x 0.9 m), in May–October 1993–1996.

 

3 

A replicated, controlled study in 1996–1997 in two vineyards in northern California, USA, found greater pest regulation and fewer pests, but fewer natural enemies, in vine rows with cover crops, compared to vine rows without cover crops. Pest regulation: More parasitized eggs of Erythroneura elegantula western grape leafhoppers were found in vine rows with cover crops, compared to those without cover crops, in one of six comparisons (July 1997: 64% vs 55% parasitism). Pest numbers: Fewer leafhoppers and fewer Frankliniella occidentalis western flower thrips were found in vine rows with cover crops, compared to those without cover crops, in most comparisons (in most of 1996: 6–53 vs 8–75 leafhopper adults/trap; in most of 1997: 60–460 vs 90–690; from 30 May to 9 August 1996: 1–33 vs 3–38 leafhopper nymphs/leaf; from 25 July to 7 August 1997: 9–10 vs 21–22; in 1996: 70–920 vs 110–1,170 thrips/trap; in 1997: 8,200–12,900 vs 11,000–17,200). Natural enemy numbers: Fewer Anagrus epos parasitoids of grape leafhopper eggs were found vine rows with cover crops, compared to those without cover crops, in some comparisons (31 July–28 August 1996: 300–1,750 vs 450–2,200 parasitoids/trap; 24 July–28 August 1997: 400–3,650 vs 400–4,100). Implementation options: More predators and fewer leafhopper nymphs were found in rows with mown cover crops, one week after mowing, compared to before mowing (in 1996: 6 vs 2 predators/trap, 45 vs 53 leafhoppers/trap). Methods: In each of two vineyards, one block of vines had cover crops between the vine rows (in every other vine row), and one block was tilled and had no cover crops between the vine rows. Fagopyrum esculentum buckwheat and Helianthus anuus sunflower were grown as cover crops. Pests and natural enemies were sampled with sticky traps in April–September 1996–1997 (10 yellow and 10 blue traps/row, 10 rows/block). Leafhopper nymphs and parasitized eggs were sampled from 10 vine leaves/row. In one of the two vineyard blocks, three rows of cover crops were mown three times/year. In these rows and three unmown rows, pests and natural enemies were sampled (five sticky traps/row).

 

4 

A replicated, randomized, controlled study in 1991–1992 in an irrigated vineyard in the San Joaquin Valley, California, USA, found more spiders in plots with cover crops, compared to bare soil, between the vine rows. Pest regulation: Similar percentages of Erythroneura variabilis leafhopper eggs were parasitized in plots with or without cover crops between the vine rows (5–90%). Pest numbers: Similar numbers of leafhoppers were found in plots with or without cover crops between the vine rows (2.9 vs 2.4 nymphs/leaf; 19 vs 18 adults/trap, 9–45 vs 9–31 eggs/leaf). Natural enemy numbers: More spiders were found in plots with cover crops, compared to bare soil, between the vine rows (9 vs 6 spiders/sample), but there was no difference in spider species composition (data not reported). Methods: Cover crops (1.5 m width) were grown between the vine rows (3.7 m width) in three plots, and bare soil was maintained through cultivation between the vine rows in three control plots (two vine rows/plot, 110 m length). The cover crops (Avena sativa oats, Vicia sativa common vetch, and V. benghalensis purple vetch) were seeded in November 1991, mown to 20 cm height in April 1992, tilled in July 1992, and cultivated thereafter. Leafhoppers were sampled every 14–18 days in May–September 1992 (nymphs on 24 grape leaves/plot, adults on three yellow sticky traps/plot, eggs and egg parasitism on five grape leaves/plot). Spiders were sampled every month by shaking the vine canopy for 10 seconds into funnels (0.58 m2 funnels, two samples/plot).

 

5 

A replicated, controlled study in 2001–2003 in an irrigated vineyard in the Salinas Valley, California, USA, found more weeds under grape vines in vine rows with cover crops, compared to vine rows without cover crops. Pest numbers: More weeds were found under grape vines in rows with cover crops, compared to rows without cover crops, in one of nine comparisons (in winter, in cultivated rows: 65–80% vs 35% weed frequency). Methods: There were nine plots (0.045 ha) for each of two cover crops (Secale cereale Merced rye or Triticosecale triticale, in the central 80 cm of the 240 cm between the vine rows, which were disked every year in November, before they were planted, and were mown every year in spring), and there were nine control plots (bare soil between the vine rows, which were disked every month). One-third of the plots were cultivated under the vine rows. Weeds were sampled in summer (June 2002), winter (March 2003), and spring (May 2003) on 30.5 m transects.

 

6 

A replicated, randomized, controlled study in 1996–2000 in an irrigated vineyard in the Sacramento Valley, California, USA, found more pocket gophers in plots that were cover cropped with clovers, compared to other species of cover crops. Implementation options: More Thomomys spp. pocket gophers were found in plots that were cover cropped with clovers, compared to other cover crops (0.9–6.7% vs 0–0.3% of each plot had signs of gophers). Similar numbers of weeds were found in plots with different cover crops (0.15–0.41 t dry weight/planted ha). Methods: There were four plots for each of four cover crops (1.8 m width, between vine rows of 3.4 width), and there were four control plots (periodically disked between the vine rows). Each plot was 10 contiguous vines and two adjacent interrows. The cover crops were Californian native grasses (not tilled, mown), annual clover (not tilled, mown), barley and oats (mown and disked), or legumes and barley (mown and disked in spring and used as a green manure). The Californian native grasses were seeded between the vine rows in autumn 1996. The others were seeded in autumn 1997–1999. All plots were drip irrigated, fertigated (20 kg N/ha/year), and the grass cover crops were also fertilized with urea (45 kg N/ha/year). Herbicide was used under the vines. Weeds were sampled in the cover crops in April 1998–2000 (four samples/plot, 1.0 x 0.5 m quadrats). Gophers were sampled in January, February, and March 1999 (looking for mounds and feeding holes that were less than two days old, throughout the plots).

 

7 

A replicated, randomized, controlled study in 2002–2005 in an irrigated vineyard in the Napa Valley, California, USA, found similar numbers of weeds under grape vines, but more weeds between vine rows, in vine rows with seeded cover crops, compared to resident vegetation. Plant diversity between the vine rows decreased over time without tillage, but increased over time with conventional tillage. Tillage had inconsistent effects on plant biomass between the vine rows. Implementation options: Similar numbers of weeds were found under the vines in rows with or without seeded cover crops (2–32 g weed biomass/m2). More weeds were found in interrows with seeded cover crops, compared to interrows with resident vegetation, in three of nine comparisons (cover crops with no tillage: 22–158 vs 1–2 g weed biomass/m2). Plant diversity between the vine rows decreased over time in rows with no tillage, and increased over time in rows with conventional tillage (data reported as the Shannon index), but similar numbers of species were found (3–6 species). Less plant biomass was found between vine rows, in rows with no tillage, compared to conventional tillage, for two plant species (Sonchus aster spiny sowthistle: 0.03–0.09% vs 2.69–2.76% of weed biomass/sample; Anagallis arvensis scarlet pimpernel: 0–0.05% vs 0.12–2.65%), but more biomass was found for one species (Medicago polymorpha California burclover: 2–25% vs 4–8%), and inconsistent biomass was found for three species. Methods: No tillage or conventional tillage was used on eight plots each, between the vine rows (three vine rows/plot). A disk plough was used for conventional tillage (15 cm depth, once/year in April–June). Four plots with conventional tillage had annual cover crops (seeded in October 2002–2004) and four plots had resident vegetation. Four plots with no tillage had annual cover crops (seeded in October 2002–2004), and four had perennial cover crops (seeded in October 2002). All plots were drip irrigated in July–October (85 kl/ha/week). Weeds were sampled under the vines and between the rows (four quadrats/plot in each location, 25 x 40 cm quadrats), when the vines were in full bloom (June 2003, May 2004, and May 2005). Herbicide was used under the vine rows (Glyphosate, twice/year), but not between the rows.

 

8 

A replicated, randomized, controlled study in 2002 in a pistachio orchard in the San Joaquin Valley, California, USA, found more pests in plots with ground cover, compared to plots with tilled soils. Pest numbers: More Amyelois transitella navel orangeworm moths were found in plots with ground cover (without tillage in the drive rows between rows of trees), compared to plots with tilled soils, in one of two comparisons (with unmown ground cover: 7 vs 1). Implementation options: Fewer moths were found in plots with ground cover that was mown, compared to unmown (9 vs 2). Methods: There were six plots (11 square feet/plot) for each of two treatments (ground cover in the drive rows, with or without mowing), and there were six control plots (tillage between the drive rows with a disk plough; depth not reported). The ground cover was resident vegetation. Before mowing or disking, two hundred pistachio nuts, infested with navel orangeworm larvae, were placed in each plot (about 71 larvae/plot). The plots were then covered with cloth, and moths were counted every week, after they emerged from the nuts.

 

9 

A replicated, randomized study in 2001–2006 in an irrigated vineyard in the Central Coast, California, USA, found fewer weeds in plots that were cover cropped with rye, compared to trios, between the vine rows. Implementation options: Fewer weeds were found in plots that were cover cropped with Secale cereale rye, compared to Triticale x Triosecale Trios, in two of six comparisons (3–20 vs 60–177 g weed biomass/m2). Methods: There were six plots for each of two cover crops (Secale cereale rye or Triticale x Triosecale Trios, sown between the vine rows in autumn, mown in spring). All plots were tilled in autumn. The plots were each 84 x 1.8 m, between two vine rows. Weed samples were collected every 2–3 weeks in November 2005–2006 (1 x 0.5 m quadrats; three quadrats/plot).

10 

A replicated, randomized, controlled study in 2002–2004 in a rainfed olive grove in Córdoba, Spain, found fewer weeds in plots with winter cover crops, compared to bare soil. Pest numbers: In summer, fewer weeds were found in plots with winter cover crops, compared to bare soil in winter, in one of two years (69 days after mowing, in 2004: 60% fewer weeds; 100 vs 250 weeds/m2). Methods: Cover crops were grown on 16 treatment plots, and bare soil was maintained on 16 control plots, from mid-October to mid-April, when the cover crops were mown and chopped (3 x 3 m plots). Weed seeds were broadcasted over all plots, in January. Half of the plots were then rototilled (depth not reported), to incorporate the cover crop residues into the soil, and half were not tilled (but the residues were retained as mulch). Common mustard Sinapis alba subsp. mairei was used as a cover crop. Weeds were sampled in five quadrats/plot (31 x 62 cm, every week, 20–69 days after mowing). Bare soil was maintained with tillage or herbicide.

 

11 

A replicated, controlled study in 2010–2011 in an olive grove in southern Spain found more natural enemies in plots with ground cover, compared to bare soil, between the olive rows. Natural enemy numbers: More spiders and parasitoids were found in plots with ground cover, compared to bare soil (45 vs 32 spiders/plot; 109 vs 0 parasitoids/plot), but similar numbers of predatory bugs and ants were found (data not reported). Methods: The olive grove was divided into four subzones (two with ground cover, two without). In the subzones without ground cover, herbicides were used in early spring 2010–2011. In the subzones with ground cover, no herbicides were used, and herbaceous vegetation was allowed to grow. There were three plots (4,900 m2 each) in each subzone. In each plot, natural enemies were sampled from the canopies of olive trees in one random sub-plot (1,600 m2), every 10 days, with vacuum samplers (16 trees/sub-plot, two minutes/tree).

 

12 

A replicated, randomized, controlled study in 2008 in an irrigated vineyard in southern California, USA, found more crop damage, more pests (leafhoppers), and more natural enemies in plots with cover crops, compared to bare fallows. Crop damage: More grapes were damaged by bees or wasps (2% vs 0% of grapes were broken) or thrips (in one of four comparisons: 28% vs 18% of grapes were scarred) in plots with cover crops, compared to bare fallows. Pest numbers: On sticky traps, similar numbers of pests were found in plots with cover crops or bare fallows (110–220 vs 110–140 combined pest insects/side). On grape leaves, more leafhoppers were found in plots with cover crops, compared to bare fallows, in one of four comparisons (22 vs 7 insects/leaf). Natural enemy numbers: On sticky traps, more natural enemies (predators and parasitoids) were found in plots with cover crops, compared to bare fallows, in one of eight comparisons (620 vs 310 combined beneficial insects/side). On grape leaves, more predators were found in plots with cover crops, compared to bare fallows, in two of four comparisons (23 vs 1–2 insects/leaf). Methods: Cover crops (Fagopyrum esculentum buckwheat) were sown between the vine rows in four plots, in summer 2008, and the cover crops were irrigated throughout the summer (sprinklers: 10 sprinklers/plot, 45 litre/hour, two hours after sowing and six hours every 7–10 days; tree sprayer: 60.5 litres/plot, thrice/week). This irrigation system was also used on three plots that did not have cover crops. Conventional management was used on six plots (bare fallows were maintained between the vine rows through cultivation and no irrigation). The plots had two vine rows each (28.7 x 6 m plots). Pests and their natural enemies were sampled with transparent sticky traps (two traps/plot, 16.7 x 13.2 cm, 145 cm above the ground, collected and replaced every week, 10 June–19 August 2008) and by observing grape leaves (five leaves/plot, observed every two weeks, 5 June–2 August 2008). Grapes were harvested in September 2008 (10 clusters from 3 m in the centre of each plot).

 

13 

A replicated, randomized, controlled study in 2008–2010 in an irrigated vineyard in the San Joaquin Valley, California, USA, found less weed diversity in plots with cover crops between the vine rows, compared to resident vegetation. Implementation options: Less weed diversity was found in plots with cover crops between the vine rows, compared to resident vegetation (6–7 vs 10 species; other data on diversity reported as indices). Similar weed diversity was found in plots with different mixtures of cover crops (6–7 species). Methods: Cover crops (2.5 m width) were grown in the alleys between the vine rows (3.1 m width) on 16 plots (two alleys/plot, 190 vines/row), and resident vegetation was allowed to grow on 8 plots, over the winter. There were two combinations of cover crops (oats only, or oats and legumes, seeded in November, on 8 plots each). All plots were mown in spring and tilled (15–20 cm depth) in spring, summer, and autumn. Herbicide was used to control weeds in the vine rows (50 cm width), but not in the alleys. Weeds were sampled in the alleys, in April each year, at 4 m intervals on 40 m transects.

 

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.