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Providing evidence to improve practice

Action: Pest regulation: Plant hedgerows Mediterranean Farmland

Key messages

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Pest regulation (1 study): One replicated, paired, site comparison from the USA found that a higher proportion of pest eggs were parasitized in tomato fields with hedgerows, compared to fields with weedy edges, but only up to 100 m into the crop.

Crop damage (1 study): One replicated, paired, site comparison from the USA found that pest damage to tomatoes was no different in fields with hedgerows than it was in fields with weedy edges.

Ratio of natural enemies to pests (2 studies): Of two replicated site comparisons from the USA, one paired study found a greater ratio of natural enemies to pests in hedgerows, compared to weedy edges, but one unpaired study did not. The unpaired study also found no difference in the ratio of natural enemies to pests between fields with hedgerows and fields with weedy edges.

Pest numbers (1 study): One replicated, paired, site comparison from the USA found fewer pests in fields or field edges with hedgerows, compared to fields or field edges without hedgerows.

Natural enemy numbers (1 study): One replicated, paired, site comparison from the USA found more natural enemies in fields with hedgerows, compared to fields with weedy edges, and in hedgerows themselves, compared to weedy edges, in some comparisons.

Supporting evidence from individual studies

1 

A replicated, paired, site comparison in 1999–2000 in mixed cropland in Yolo County, California, USA, found more natural enemies than pests in hedgerow shrubs, and more pests than natural enemies in weedy field edges. Ratio of natural enemies to pests: On hedgerow shrubs, natural enemies were more abundant than pests (1–3 vs 0.2–1.0 insects/m2). In weedy edges, pests were more abundant than natural enemies in summer (15 vs 8 insects/sample), but were not significantly different in spring (6 vs 4) or fall (9 vs 4). A higher proportion of insects were natural enemies in hedgerow shrubs than in weedy edges (0.81–0.88 vs 0.32–0.46). Methods: On the edges of four crop fields, native shrubs (hedgerow shrubs), bordered by native grasses (hedgerow grasses), were planted in 1996 (305–550 m), and compared to the weedy edges of the same fields every two weeks in March–November 1999–2000. Insects were observed on hedgerow shrubs (four minutes/shrub species), collected from shrubs by shaking, and collected from hedgerow grasses and weedy edges with sweep nets (10 sweeps/sample; four samples each).

 

2 

A replicated site comparison in 2005–2006 on organic vegetable farms on the Central Coast, California, USA, found similar ratios of natural enemies to pests in hedgerows compared to weedy edges, and in fields with hedgerows compared to fields with weedy edges. Ratio of natural enemies to pests: The ratio of natural enemies to pests (2005: 11:1 enemies:pests; 2006: 15:1) was not significantly different between fields with hedgerows and fields with weedy edges, either at the edge (30:1 vs 6:1 enemies:pests), or 50–100 m into the field (3:1 enemies:pests). Different plant species in hedgerows had different ratios (from 4:1 enemies:pests on toyon Heteromeles arbutifolia in 2005 to 43:1 on coyote bush Baccharis pilularis in 2006). Methods: Two fields with hedgerows (>2 years old) and two fields with weedy edges were compared. Insects were sampled using yellow sticky cards (2005: five cards each at 0, 50, and 100 m into fields, collected after three days) and vacuums (2005: 30 seconds/plant in hedgerows; 2006: 60 seconds/plant).

 

3 

A replicated, paired, site comparison in May–August 2009–2010 in tomato fields in the Sacramento Valley, California, USA, found that a higher proportion of pest egg were parasitized in fields with hedgerows, compared to fields with weedy edges, but only up to 100 m into the crop. Similar levels of fruit damage were found in fields with and without hedgerows. Fewer pests were found in fields or field edges with planted hedgerows, compared to fields or field edges without hedgerows. Pest regulation: Parasitism of stink-bug Euschistus conspersus eggs (a tomato pest) was higher in fields with hedgerows than in fields with weedy edges, 0–100 m but not 200 m into the crop (0 m: 0.19 vs 0.11 proportion of eggs parasitized; 10 m: 0.30 vs 0.18; 100 m: 0.20 vs 0.10; 200 m: 0.15 vs 0.11). Crop damage: Similar amounts of fruit damage by pests were found in fields with hedgerows or weedy edges (amounts of damage not reported). Pest and natural enemy numbers: In sweep-net samples, fewer pests, but not significantly fewer predators or parasitoids, were found in hedgerows than in weedy edges (pests: 2 vs 20 individuals/sample; predators: 6 vs 6; parasitoids: 6 vs 2). In shake samples, more predators (10 m: 0.25 vs 0.05 predators/sample; 100 m: 0.15 vs 0.05; 200 m: 0.30 vs 0) and fewer aphids (10 m: 0.21 vs 0.32 proportion of leaves with aphids; 100 m: 0.14 vs 0.23; 200 m: 0.11 vs 0.21) were found in fields with hedgerows than in fields with weedy edges. In sticky-card samples, more parasitoids, but not more predators, and fewer pests were found in hedgerows than in weedy edges; more parasitoids were found in fields with hedgerows than in fields with weedy edges, up to 100 m into the crop, and fewer pests were found in fields with hedgerows than in fields with weedy edges, up to 10 m into the crop (number of individuals 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). Invertebrates were sampled four times/year using sweep nets (40 cm diameter; six sweeps/edge) and sticky cards (7.6 × 12.7 cm; six cards/edge and six cards/crop), and by shaking plants (late May only). Stink-bug egg masses were exposed for five days in early July on the undersides of leaves.

 

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.