Providing evidence to improve practice

Action: Convert to organic farming

Key messages

This synopsis includes experimental tests of organic farming only. Comparative studies of existing organic systems are not included.

Parasitism and mortality (caused by natural enemies): One of five studies (three replicated, controlled tests and two also randomised) from Europe, North America, Asia and Australasia found that organic farming increased parasitism or natural enemy-induced mortality of pests. Two studies found mixed effects of organic farming and two randomised, replicated, controlled studies found no effect.
Natural enemies: Eight of 12 studies (including six randomised, replicated, controlled tests) from Europe, North America Asia and Australasia found more natural enemies under organic farming, although seven of these found effects varied over time or between natural enemy species or groups and/or crops or management practices. Three studies (one randomised, replicated, controlled) found no or inconsistent effects on natural enemies and one study found a negative effect.
Pests and diseases: One of eight studies (including five randomised, replicated, controlled tests) found that organic farming reduced pests or disease, but two studies found more pests. Three studies found mixed effects and two studies found no effect.
Crop damage: One of seven studies (including five randomised, replicated, controlled tests) found less crop damage in organic fields but two studies found more. One study found a mixed response and three studies found no or inconsistent effects.
Weed seed predation and weed abundance: One randomised, replicated, controlled study from the USA found mixed effects of organic farming on weed seed predation by natural enemies. Three randomised, replicated, controlled studies from the USA found more weeds in organically farmed fields, but in one of these studies this effect varied between crops and years. One study found no effect.
Yield and profit: Six randomised, replicated, controlled studies measured yields and found one positive effect, one negative effect and one mixed effect, plus no or inconsistent effects in three studies. One study found net profit increased if produce received a premium, but otherwise profit decreased. Another study found a negative or no effect on profit.

 

Crops studied were apple, barley, beans, cabbage, carrot, gourd, maize, mixed vegetables, pea, pepper, safflower, soybean, tomato and wheat.

Supporting evidence from individual studies

1 

A randomised, replicated, controlled study in 1989-1993 in northern California, USA (Lanini et al. 1994) (part of the study by Poudel et al. 2001) found 32% of tomato fruitworm Helicoverpa zea eggs were parasitised in organic plots compared to 25-37% in conventional plots of tomato Solanum lycopersicum in 1989. In organic plots, parasitism occurred on 0-9% of leaves with potato aphids Macrosiphum euphorbiae compared to 0-5% in conventional plots, across all years. Weeds were more widespread in organic than conventional plots for some years and crops, for example in maize Zea mays in 1991 (3.5% vs. 0.5-1.8% ground cover). Plant-parasitic nematodes decreased in organic but increased in conventional plots in 1989-1992. Occurrence of disease and mobile pests was similar between treatments, and the authors suggest that pests may have migrated between the small plots. Organic tomato plots had more tomato fruitworm damage in 1989 and more stink bug (Pentatomoidea) damage in 1992 (1.5% and 11% of fruits damaged, respectively) than in conventional plots (0.0-0.5% and 6-7%), but damage was similar in other years. Organic plots were tilled to control weeds and conventional plots were treated with herbicides, insecticides, acaricides, fungicides and tilled. Treatments were replicated four times in 0.12 ha plots.

2 

A randomised, replicated, controlled study in 1990-1991 in Alberta, Canada (Carcamo et al. 1995) found no difference in overall ground beetle (Carabidae) abundance between organic (averaging 194-200 beetles/plot) and conventionally farmed (145 beetles) plots. However, more ground beetles were found in organic (194-344 beetles/plot, across all species) compared to conventional (109-194 beetles) plots when a dominant non-native (and highly mobile) species was excluded from analysis. In 1991, four ground beetle species were more abundant in organic than conventional plots while two species showed the opposite trend. Species richness was also higher in organic (averaging 22.5-24 species) than conventional (16.5-21.5 species) plots in 1991. Organic and conventional regimes were examined in 10 x 25 m plots replicated eight times. Organic plots received mechanical and manual weed control and no synthetic fertilizers or herbicides. Conventional plots received synthetic fertilizer and/or herbicide. Each plot contained either barley Hordeum vulgare, faba bean Vicia faba or a barley-pea Pisum sativum intercrop. Beetles were sampled every two weeks from mid-June to mid-October 1990 and mid-April to mid-October 1991 using two pitfall traps in each plot.

3 

A randomised, replicated, controlled study in 1993 in North Carolina, USA (Hoyt & Walgenbach 1995) found greater weed biomass in organic (1,178-1,265 kg/ha) than conventional (213-422 kg/ha) plots of cabbage Brassica oleracea after harvest (27th August). Damage by moth and butterfly caterpillars (Lepidoptera) was similar between treatments. Damage caused by alternaria leaf spot Alternaria brassicae was lower in organic (score of 2.7) than conventional plots (score of 4) under standard tillage conditions, but was similar between organic and conventional strip-tilled plots (scores of 3.2-4.4). Disease damage was scored from 0 (no disease) to 5 (severe damage). Cabbage weight and the percentage of marketable cabbage heads were similar between organic (0.5-0.8 kg/head and 90-95%, respectively) and conventional (0.6-0.9 kg and 93-95%) plots. Four treatments (organic/standard tillage, organic/strip tillage, conventional/standard tillage, conventional/strip tillage) were tested in plots of 8 x 14 m, replicated four times. Organic plots received soybean Glycine max meal fertilizer and mechanical weed control. Conventional plots received chemical fertilizers and herbicides. All plots received Bacillus thuringiensis insecticide applications. Weed biomass was sampled from 1 m² areas in the centre of plots. Twenty cabbage heads per treatment were examined for insect and disease damage at harvest.

4 

A randomised, replicated, controlled study in 1992-1993 in Ohio, USA (Creamer et al. 1996) found that organic tomato Solanum lycopersicum plots containing a mulch had similar broad-leaved weed and grass biomass (6.6-10.6 g dry weight/m² and 1.3-7.3 g, respectively) to conventional plots treated with herbicide (3.0-6.0 g and 0.7-1.3 g) at 12 weeks after planting. There were no differences in the frequency of insect pests and diseases between the management regimes. Tomato fruit yields were similar between organic (26 t/ha) and conventional (36 t/ha) plots at one site (Columbus), but lower in organic (35 t/ha) than conventional (66 t/ha) plots at a second site (Fremont). Economic return was similar between treatments (organic US$2,029 vs. conventional US$2,068) in Columbus but lower in organic (US$2,743) than conventional (US$4,315) plots in Fremont. Organic plots had a cover crop mechanically killed and left as mulch before being cropped with tomato, and received organic fertilizer and mechanical weed control. Conventional plots were unmulched and received herbicides, insecticides, fungicides and synthetic fertilizer. Treatments were replicated four times and weeds and grasses were collected from four 0.5 m² areas within plots to calculate their biomass. Pests and diseases were scouted for weekly.

5 

A randomised, replicated, controlled study in 1988-1991 in Therwil, Switzerland (Pfiffner & Niggli 1996) reported 1.8-2.2 times more natural enemies in organic and biodynamic plots than in conventional plots of wheat Triticum sp.. More ground beetles (Carabidae) were found in organic and biodynamic plots (averaging 72-75 individuals) than conventional (46 individuals) plots in 1999, but only biodynamic plots had greater abundance (208 individuals) than conventional plots (89 individuals) in 1998 and no differences were found in 2000. Organic and biodynamic plots had more rove beetles (Staphylinidae, 42-58 individuals) than conventional plots (20-33 individuals) in 1998-1999, but there was again no difference in 2000. Spider (Araneae) abundance was similar between treatments in 1998 but was greater in organic and biodynamic (64-89 individuals) than conventional (28-45 individuals) plots in 1999-2000. Organic and biodynamic plots had 4-7.5 more ground beetle species (on average) than conventional plots. Treatments were tested in 10 x 20 m plots replicated four times. Organic and biodynamic plots (farmed identically in this study) were fertilized with farmyard manure and weeds were controlled mechanically. Conventional plots received manure and mineral fertilizer and integrated plant protection. Natural enemies were sampled using 4-8 pitfall traps (of 10 cm diameter) per treatment.

6 

A replicated, controlled study in February-November 1997 in Davis, California, USA (Clark 1999) found that average ground beetle (Carabidae) abundance was 0-17 captures/week in organic plots compared with 0-7 captures in conventional plots of tomato Solanum lycopersicum. Abundance was higher in organic (6 captures/week) than conventional (2 captures) plots in June, but statistically similar at other times. The total number of ground beetles caught since February was higher in organic (averaging 33-46 captures) than conventional (18-22 captures) plots from June to November. Ground beetle species richness was higher in organic (averaging 2.3-3.5 species) than conventional (0.3-1.3 species) plots in June, September and November but not in other months. The organic system received no synthetic chemical insecticides, herbicides or fertilizers and included a legume cover crop prior to and after the tomato crop. Tomatoes were harvested in July. Each treatment was replicated four times in 0.12 ha plots.  Beetles were sampled using two pitfall traps placed in the centre of each plot.

7 

A replicated, controlled study in Hawke’s Bay, New Zealand in 1995-1996 (Suckling et al. 1999) found 24-58% of pest woolly apple aphid Eriosoma lanigerum colonies were parasitised by the wasp Aphelinus mali in organic orchards compared to 3-33% in conventional orchards. The wasp Dolichogenidea tasmanica parasitised 28% and 0% of leafrollers (Tortricidae) in organic and conventional apple Malus domestica orchards, respectively. The predatory mirid bug Sejanus albisignata appeared more abundant in organic than conventional orchards, but average numbers of predatory mites were similar (0.05-0.39 predatory mites per leaf overall). Woolly apple aphids were reportedly more frequent in organic (0-132 colonies/minute) than conventional orchards (0-37 colonies). In organic orchards, 59-95% of apples were undamaged compared with 90-99% from conventional orchards. Percentage fruit damage by leafrollers and woolly apple aphid in organic orchards was 1.8-4.5% and 0.002-39.6% respectively, compared with 0.1-1.2% and 0.03-11.5% in conventional orchards. Organic treatments included biological insecticide (Bacillus thuringiensis), organic fungicide, and disease control using minerals (e.g. slaked lime). Conventional orchards received 4-12 organophosphate insecticide applications and regular fungicides. Treatments were tested in 0.3-1.6 ha blocks at each of three sites.

8 

A randomised, replicated, controlled study in 1989-1999 in northern California, USA (Poudel et al. 2001) (same study as Lanini et al. 1994) found that the density of weed seeds doubled (to 10,000 seeds/m²) in organic plots relative to conventional plots over the 10-year study period. Fewer root-knot nematodes Meloidogyne spp. were found in organic than conventional plots in 1994, but the degree of galling on tomato Solanum lycopersicum roots was similar between treatments. Average crop yields were similar in organic versus conventional plots for tomato (67 vs. 77 t/ha), safflower Carthamus tinctorious (2.4 vs. 2.7 t/ha), maize Zea mays (10.5 vs. 11.2 t/ha) and beans Phaseolus vulgaris (1.9 vs. 1.7 t/ha). However, early in the experiment tomato and maize yields were lower in the organic than conventional plots. Organic plots were more profitable (cumulative net income US$6,875/ha by 1999) than conventional plots (US$4,438/ha) when premium prices were applied, but the opposite was true without premiums for organic produce (a loss of US$-1,563/ha vs. a gain of US$4,438/ha). Organic plots were tilled to control weeds and conventional plots were treated with herbicides, pesticides and tilled. Treatments were replicated four times in 0.12 ha plots.

9 

A randomised, replicated, controlled study in 1998-2000 in Iowa, USA (Delate et al. 2003) found similar numbers of natural enemies in organic pepper Capsicum annuum plots (averaging 0.002-0.003 individuals/plant) and conventional plots (0.001-0.004 individuals) in 2000. Natural enemies included seven-spot ladybird Coccinella septempunctata, common green lacewing Chrysoperla carnea and spiders Araneae. In 1999, peppers had 0.25-0.58 natural enemies/plant in organic plots compared with 0.08-0.35 in conventional plots. Numbers of pest corn borer Ostrinia nubilalis larvae were similar between organic (0-0.04 individuals/plant) and conventional (0-0.02 individuals) plots in 1999-2000. Crop damage averaged 3-8.5 blemishes/fruit on organic peppers compared with 8.5-12.5 blemishes on conventional peppers in 2000. Pepper yields were generally similar between organic (7-38 peppers/plant) and conventional (6-47 peppers) plots in 1998-2000. Where significant yield differences were found these depended on the fertilizer regime and were not consistent between years. Organic plots received mechanical weed control and organic (or no) fertilizer. Conventional plots received herbicide and synthetic (or no) fertilizer. Organic management was tested in 16 or 24 plots (1998 and 1999-2000, respectively) and conventional management in 12 plots. Plots were 8 x 3 m and insects (natural enemies and pests) and yield were assessed for 10 plants/plot.

10 

A randomised, replicated, controlled study in 2003-2004 in Fort Pierce, Florida, USA (Chellemi et al. 2005) found that the incidence of fusarium wilt (caused by Fusarium oxysporum fungus) was < 3% in organic plots of tomato Solanum lycopersicum compared to a range of 1-19% in conventional plots. Damage by root-knot nematodes Meloidogyne spp. was low for both types of management, with < 2% of root systems developing galls. Marketable tomato yields were reportedly lower in organic compared to conventional management plots. Organic and conventional regimes were applied for 3-4 years before the study crops were grown in 2003-2004. Organic plots received poultry manure and urban plant debris annually and had contained cover crops (sunn hemp Crotalaria juncea and Japanese millet Echinochloa esculenta) prior to the study. Conventional plots received soil fumigant and herbicides and had been cropped annually with tomato prior to the study. Each treatment was tested in six replicate 0.16 ha plots. Disease sampling methods were not described.

11 

A randomised, replicated, controlled greenhouse trial on a tomato Solanum lycopersicum farm in Florida, USA (Kokalis-Burelle et al. 2005) found root-knot nematode Meloidogyne incognita egg mortality and the number of eggs parasitized by fungi was similar between soils from organic (egg mortality 10-15 eggs, 0.5-2 eggs parasitised) and conventionally-managed treatments (mortality 12 eggs, 0-1 eggs parasitised). The severity of root galls on tomato plants was similar in soil from organic (0-0.92 on a scale of 0-10 where 10 is severe galling) and conventional plots (0.2-0.6). Cucumber Cucumis sativus plants planted after tomato also had similar root gall severity between both treatments. Tomatoes had been grown on the farm for 10 years. Treatments were set out in 0.16 ha plots and replicated six times. Organic plots were established in July 2000: treated with 22 t/ha chicken manure, 67 t/ha partially composted municipal plant waste and sown with two cover crops: sunn hemp Crotalaria juncea in August and Japanese millet Echinochloa crusgalli in March. Conventional plots were treated with pesticides and herbicides. Soil samples were taken in each plot in July 2001, placed in 7.8 l plots in a greenhouse and planted with two tomato seedlings/pot. Nematode eggs were placed in each pot, one or six weeks after planting to assess mortality and fungal parasitism. Tomato roots were assessed for galling 42 days after transplanting. Cucumber plants were transplanted to the pots after the tomato plants were removed.

12 

A randomised, replicated, controlled study in Michigan, USA (Menalled et al. 2007) found that removal of weed seeds by ground beetles (Carabidae) was similar in organic (averaging 9-45% seeds predated/5 days) and conventional (13-40%) plots of soybean Glycine max from mid-August to early September 2000. In early August, 83-84% of weed seeds were predated in organic plots compared with 55-56% in conventional plots. Fewer ground beetles (of all types) were found on the soil surface in organic (57 captures/sampling date on average, 863 individuals in total) than in conventional plots (144 captures on average, 342 individuals in total), but seed predators in particular were similarly abundant between management regimes (averaging 6.2 captures in each).  Organic plots received no external chemical input and conventional controls received applications of fertilizer and herbicide. Each regime was tested in 1 ha plots replicated six times. Ground beetles were sampled using five pitfall traps/plot. Seed predation was assessed by monitoring the removal of weed seeds placed artificially on the soil surface for 5 days. Seeds from common lambsquarters Chenopodium album or fall panicum Panicum dichotomiflorum were placed on a total of 120 pads for each management regime.

13 

A randomised, replicated, controlled study in 1998-1999 in southern Sri Lanka (Rajapakse & Ratnasekera 2007) found more predatory weaver ants Oecophylla smaragdina in organic plots (averaging 4.4 ants/plant) than in conventional treated plots (0 ants) and similar numbers in organic and untreated (3.8 ants) plots. Mortality of pest fruit flies (Tephritidae) averaged 90-100% in organic plots compared to 80-100% in conventional plots at 14 days after flowering, and was higher in organic than untreated (30-50%) plots. Mortality of leaf beetles Aulacaphora spp. was similar in organic (71-81%) and conventional (71-82%) plots in three seasons, but lower in organic (80%) than conventional (100%) plots in the wet season, 1998. Leaf beetle mortality was higher in organic than untreated (21-42%) plots in all years and seasons. Yields of bitter gourd Mormordica charantia and snake gourd Trichosanthes cucumerina (combined) averaged 20-25 t/ha in organic plots compared with 15-20 t/ha in conventional and 5-10 t/ha in untreated plots. Organic, conventional and untreated regimes were replicated eight times across two sites using plots of 1.5 x 1.5 m. Organic plots received a insecticidal neem Azadirachta indica preparation. Conventional plots received carbaryl granules and sprays of fenthion insecticide.

14 

A site comparison study in Drôme, France (Simon et al. 2007) reported more insect, spider and mite natural enemies in apple Malus domestica trees in an organic orchard (132 individuals in 2002, 181 in 2003) than in a conventional orchard (70 and 125 individuals). However, in 2001, 35 and 43 individuals were found in the organic and conventional orchards, respectively. In the grass beneath trees, natural enemies were typically more common in the organic than the conventional orchard. The combined number of pest rosy apple aphid Dysaphis plantaginea, green apple aphid Aphis pomi and European red mite Panonychus ulmi in trees totalled 15,568 and 3,350 individuals in organic orchards (2002 and 2003), compared with 2,771 and 1,924 in the conventional orchard. In 2001, 94 and 237 pests were found in the organic and conventional orchards, respectively. Fruit damage from codling moth Cydia pomonella averaged 9.2% and 1.3% in the organic and conventional orchards, respectively. An organic orchard (0.25 ha) receiving granulosis virus and mineral fungicide applications was compared with a conventional orchard (0.2 ha) receiving insecticides, fungicides and herbicides. Treatments took place for two years before the study began. Invertebrates in trees were sampled by beating 50 branches per orchard.

15 

A randomised, replicated, controlled study in 2005-2006 in Northumberland, UK (Eyre et al. 2009) found that the response of natural enemies to organic farming varied between invertebrate families and crop types. In general, ground beetles (Carabidae) and spiders of the Lycosidae family were more abundant in organically fertilized (averaging 341-671 beetles and 11-37 spiders) than conventionally fertilized plots (285-429 beetles and 3-18 spiders), but not for all tested crop types. Rove beetles (Staphylinidae), money spiders (Linyphiidae) and parasitoid wasps (Braconidae) were typically less abundant in organic (61-125, 53-298 and 13-16 individuals, respectively) than conventionally fertilized plots (114-280, 85-413 and 18-23 individuals). Other natural enemies showed an inconsistent or no response to fertilization regime. Natural enemy abundance showed few or inconsistent differences between organic versus conventional crop protection (pest and weed control). Plots of 48 x 12 m were given organic (mechanical control and mineral applications) or conventional crop protection (synthetic herbicide and pesticide), then further divided into organic (compost or none) and conventional (inorganic) fertilization treatments. Each treatment was replicated 64 times with one of five crop types grown in each plot. Invertebrates were sampled using five pitfall traps (8.5 cm diameter) and three one-minute suction samples per plot.

16 

A replicated study in Pennsylvania, USA (Jabbour and Barbercheck 2009) found no consistent trend in the occurrence of Metarhizium anisopliae, an insect-parasitoid fungus, during a three-year transition to organic farming. An experiment in 2003-2006 found more fungi in year one (2.3 samples with fungus, out of 3 samples/plot) than in year three (1.3-2 samples), but a second experiment in 2004-2007 found the opposite trend (1.8-2.1 samples in year one vs. 2.3-2.4 samples in year three). Different crops were grown in each of the three years. The authors suggest variation between years and within seasons may explain the contradictory patterns in fungus numbers. Organic management practices were applied at one farm for a sequence of cover crops, soybean Glycine max and maize Zea mays (first, second and third years, respectively). Fungi presence was assessed using three soil samples at each of 24 randomly selected locations. Sampling took place on four dates between May and October each year.

17 

A replicated, controlled study in 2005-2006 in Davis, California, USA (Sanchez-Moreno et al. 2009) found similar numbers of predatory mites (Prostigmata and Mesostigmata) in organically farmed (7 mites/100 g soil) and conventionally farmed (5 mites) plots receiving standard tillage. Numbers of predatory mites were also similar between organic (12 mites/100 g soil) and conventional (8 mites) plots receiving reduced tillage. Organic plots with reduced or no tillage had more predatory mites (12-14 mites/100 g soil) than conventional plots with standard tillage (5 mites). Tomato Solanum lycopersicum and maize Zea mays were grown (in 2005 and 2006, respectively) in 0.4 ha plots. Organic management included compost fertilizer application and legume cover crops during winter. Conventional management included mineral fertilizer and bare fallow in winter. Subplots of standard and reduced tillage were tested under both management systems, and no-tillage was also tested in organic plots. Each treatment was replicated three times. Three soil samples were taken per plot at eight sampling dates in 2005-2006.

18 

A controlled, replicated study in 2007-2009 in Årslev, Denmark (Meyling et al. 2010) (the same study as Meyling et al. 2011) found 3-4 times more money spiders (Linyphiidae) in organic than conventional plots of white cabbage Brassica oleracea in July 2008. Other natural enemy numbers varied with the method of organic farming. Organic plots with bare soil around crops had 2-4 times more small (<8 mm length) predatory beetle (Coleoptera) activity in May and July 2008 than in other organic and conventional treatments (where numbers were similar). Ground beetles (Carabidae) were most frequent in green-manured organic plots (occurring in 50-70% of traps in May 2007 and 2008) but other organic treatments had similar or lower occurrence (5-30%) than the conventional control (25-30%). Cabbage root fly Delia radicum pupae were 2-3 times scarcer in organic than conventional plots, but numbers of fly eggs were similar between treatments. Cabbage was grown under four types of management: high-input organic, low-input organic with green manure incorporated before cropping (creating bare soil around crops), low-input organic with green manure strips conserved between crop rows, and a conventional control. Pitfall traps were used to sample natural enemies during the root fly egg-laying season.

19 

A controlled, replicated study in 2006-2008 in Årslev, Denmark (Meyling et al. 2011) (the same study as Meyling et al. 2010) found fewer insect carcasses (including pests) infected by parasitoid fungi (Ascomycota: Hypocreales) in organically farmed plots with a green manure (1-11 carcasses/treatment) than in conventionally managed plots (14-24 carcasses), in September 2007 and 2008. Another organic treatment, alternating conserved strips of green manure between vegetable rows, had fewer infected insects (averaging 1.3 carcasses/plot) than conventional plots (7 carcasses) in September 2008. There were no differences between organic and conventional plots in other months (June to August) in 2007-2008. Only 17-28% of carcasses were of plant-eating pests, while 47-63% of carcasses were insect predators. White cabbage Brassica oleracea and carrot Daucus carota were grown in three replicate fields, each containing two organically managed treatments with undersowing (receiving different methods of green manuring) and a conventionally managed control. Treatments were applied to eight plots of 10 x 12.5 m in each field. Insect carcasses were sampled along nine 10 x 0.25 m transects per treatment per field, surveyed monthly from May to September in 2007 and 2008.

Referenced papers

Please cite as:

Wright, H.L., Ashpole, J.E., Dicks, L.V., Hutchison, J., McCormack, C.G. & Sutherland, W.J. (2017) Some Aspects of Enhancing Natural Pest Control. Pages 359-381 in: W.J. Sutherland, L.V. Dicks, N. Ockendon & R.K. Smith (eds) What Works in Conservation 2017. Open Book Publishers, Cambridge, UK