Action: Leave part of the crop or pasture unharvested or uncut
Natural enemies: We found eight studies from Australia, Germany, Hungary, New Zealand, Switzerland and the USA that tested leaving part of the crop or pasture unharvested or unmown. Three (including one replicated, controlled trial) found an increase in abundance of predatory insects or spiders in the crop field or pasture that was partly uncut, while four (including three replicated, controlled trials) found more predators in the unharvested or unmown area itself. Two studies (one replicated and controlled) found that the ratio of predators to pests was higher in partially cut plots and one replicated, controlled study found the same result in the uncut area. Two replicated, controlled studies found differing effects between species or groups of natural enemies.
Predation and parasitism: One replicated, controlled study from Australia found an increase in predation and parasitism rates of pest eggs in unharvested strips.
Pests: Two studies (including one replicated, controlled study) found a decrease in pest numbers in partially cut plots, one of them only for one species out of two. Two studies (one replicated, the other controlled) found an increase in pest numbers in partially cut plots, and two studies (including one replicated, controlled study) found more pests in uncut areas.
Crops studied were alfalfa and meadow pasture.
This intervention involves harvesting or cutting part of a crop field or pasture, often by leaving uncut strips. In pasture, fodder or perennial crops, these strips may be harvested later in a rotation system. The uncut areas provide a refuge for predators from harvesting itself, as well as providing habitat once the rest of the field is cut. This maintains predator populations and enables them to recolonise the following crop.
Here we present evidence from eight of 12 studies testing this intervention.
Supporting evidence from individual studies
A controlled study in 1972 of two 16.1 ha alfalfa Medicago sativa fields in California, USA (Summers 1976) found that predator and pest numbers were higher in the field with uncut strips than the completely cut field. There were 18,044 individual predators and 16,138 pest (lygus bugs Lygus spp. and pea aphids Acyrthosiphon pisum) individuals in the field with uncut strips and 7,131 predators and 12,557 pests in the completely cut field. Predators included spiders (Araneae), damsel bugs Nabis spp., green lacewing Chrysoperla (Chrysopa) carnea and ladybirds (Coccinellidae). Lygus bugs moved from uncut strips into cut areas, but moved back to uncut strips when cutting occurred. Predatory species showed a similar pattern. Alfalfa hay protein content was slightly higher in the field with uncut strips (18.1-20.7% protein) than the completely cut field (17.1-18.2%) but modified crude protein was slightly lower. One field had banks 1 m-wide and 0.2 m high distributed every 15-25 m. At each mowing period, banks were cut alternately (one alfalfa strip left uncut at every alternate raised strip, the next bank cut). Cuttings were distributed either side of the strip. Invertebrates were sampled on uncut and cut strips and between strips (10 samples/location) using a D-vac suction sampler. The second field was cut completely, and sampled using the same method as in the field with cut strips. Sampling took place one week after strip-cutting began (after the second cut, 7th May) and continued bimonthly until mid-September.
A controlled trial from 1978-1980 at Pukekohe, New Zealand (Cameron et al. 1983) found a higher ratio of predators to aphids Acyrthosiphon spp. in five out of seven periods in a plot where strips of alfalfa Medicago sativa were left uncut than in a fully cut plot. The plot with uncut strips had fewer aphid outbreaks (two aphid outbreaks in seven interharvest periods) than a fully cut plot (four aphid outbreaks). Peak aphid numbers on alfalfa stems were also higher in the fully cut plot (5.5-400.3 aphids/stem) than the plot with uncut strips (0.6-124.6 aphids/stem). Two 40 x 60 m plots were compared: one continuously-cut plot and one strip-cut plot. In the strip-cut plot, two 10 m-wide strips were cut when the continuously-cut plot was mown, the remaining two strips were cut when the previously cut strips were half-grown. Aphids were sampled by sweep netting (50 sweeps/plot) and counting aphids on 10 alfalfa stems at six points along one transect/plot.
A replicated, controlled study from 1982 to 1986 in a meadow in Germany (Nentwig 1988) found a higher ratio of predatory invertebrates to plant-eating insects in an area consisting of unmown and mown strips (0.66-2.55 predators/prey individual) than in a completely mown area (0.69-2.23). There were more spider (Araneae) species and a faster increase in diversity in the strip-managed area (average 40 species, 21 new spp./year) than the completely mown area (25 spp., 12 new spp./year). There were also more ground beetle (Carabidae) and rove beetle (Staphylinidae) species in strip-managed plots than mown plots (strip-managed: 33 ground beetle spp., 26 rove beetle spp.; mown plots: 24 ground beetle spp., 14 rove beetle spp.). The 44 x 6 m meadow plot was divided into: two 10 x 6 m plots (one mown, one unmown), four 1 x 6 m unmown strips and four 5 x 6 m mown strips. Mown and unmown strips were alternated. Plots were mown approximately every two weeks (5 cm high) during the growing season, cuttings were not removed. Unmown strips were not cut April 1982-autumn 1986. Invertebrates were sampled with six pitfall traps in the two mown/unmown plots and three traps in each strip. Traps were emptied every 10-14 days from June-September in 1982 and 1984-1986. Traps were not set in 1983.
A replicated trial in 1990-1991 on alfalfa Medicago sativa strips in cotton Gossypium hirsutum fields (Godfrey & Leigh 1994) found that strips where each half was cut alternately every 14-17 days had more natural enemies (big-eyed bugs Geocoris spp., minute pirate bugs Orius spp. and damsel bugs Nabis spp.) than a completely cut alfalfa field (alternately-cut strips every 14 days: 50.8-184 individuals/1.9 m²; alternately-cut strips every 17 days: 148.8-181.7; completely-cut field: 39.3-101.5). However, alternately cut strips also had more lygus bugs Lygus hesperus (pest) than completely cut alfalfa (alternately-cut strips every 14 days: 43-66.6 individuals/1.9 m²; alternately-cut strips every 17 days: 38.6-103; completely-cut: 4.6-8). Uncut strips had high numbers of lygus bugs and natural enemies (128.4-191.4 lygus bugs/1.9 m², 87.1-339.4 natural enemies). Alfalfa strips (91.4 x 4.1 m) within the cotton crop or adjacent to it were established November 1989. Eight and twelve strips were studied in 1990 and 1991 respectively. Strips were cut completely on 30 April 1990 and 28 May 1991. There were three cutting treatments in both years, starting two weeks after the first cut: uncut, cut alternately every 28 days (one 2.05 m half strip cut, the other half cut 14 days later) or one alfalfa field cut completely every 28 days. In 1991, a 35 day alternate cutting treatment was also used (half the strip cut every 17 days). There were four replicates. Cuttings were not removed. Arthropods were D-Vac suction sampled weekly from May to August.
A replicated, controlled trial in 1997-1998 in a 4 ha alfalfa Medicago sativa field with strip- and conventional-harvesting in New South Wales, Australia (Hossain et al. 2001) found that predation and parasitism of Helicoverpa spp. (pest) eggs was higher in unharvested (36.7% eggs predated, 3.31% parasitised) than harvested strips (21.7% eggs predated, 0.85% parasitised). Total predator abundance (spiders (Araneae), red and blue beetles Dicranolaius bellulus and transverse ladybird Coccinella transversalis) was higher in the strip-harvested area (average 5.1-9.1 predators/0.4 m²) than the conventionally-cut area (1.2-7.6), and higher in unharvested than harvested strips. Helicoverpa spp. was less abundant in the strip- than conventionally-harvested area (0.1-9.2 individuals/0.4 m² vs. 0.7-27.6) but another pest, lucerne leaf roller Merophyas divulsana had similar numbers in both treatments (0.3-18.4 vs. 0.1-19.0); both pests were more abundant in unharvested than harvested strips. There were eight 200 x 14 m strips, split lengthways; one half cut a week before normal harvesting (harvested strip), one half cut two weeks later (unharvested strip). Subsequently, strips were cut when 10% alfalfa was flowering. Strips were vacuum-sampled five times. Helicoverpa spp. eggs were placed in strips to assess predation and parasitism rates. The 112 x 158 m conventionally-harvested block was cut three times, with three vacuum samples.
A replicated, controlled study in 1995-1997 in an alfalfa Medicago sativa field in Hungary (Samu 2003) found leaving unmown strips increased the number of spiders (Araneae) in the unmown strips but did not increase numbers in adjacent mown strips. Unmown strips had an average of 53% more spiders than control continuously-cut control alfalfa plots. Average spider diversity was similar in controls and mown strips in rotation with unmown strips (control: 1.8 Shannon diversity; mown strips: 1.75). Unmown strips had slightly higher diversity (2.15). One 1.6 ha field was divided into six 50 x 50 m plots. Three plots were strip-managed (each mowing session four 1 m-wide strips were left unmown in each plot, the following mowing session these unmown strips were cut and adjacent 1 m strips left unmown), three plots were cut completely. Alfalfa was sown mid-April 1995. Plots were mown three-four times each year (starting July 1995) when approximately 10% of the alfalfa was flowering. Spiders were sampled 64 times from July 1995-December 1997, using three pitfall traps in control plots, three traps in unmown strips and three traps in mown strips. Suction samples were also taken at the pitfall trap locations.
Two replicated, controlled trials from 1998-2000 in twelve 7.5-17 ha alfalfa Medicago sativa fields at 3-4 sites in Iowa, USA (Weiser et al. 2003) found that in more than 50% of whole fields surveyed there were more predatory insects (net-winged insects (Neuroptera), minute pirate bugs (Anthocoridae), and ladybirds (Coccinellidae)) captured by sweep-netting in 3 m-wide uncut strips than cut areas, 1-3 weeks after hay had been collected (numbers not given). Numbers from sticky traps were similar between treatments. The proportion of insect predators to prey was higher in uncut than cut alfalfa in one field in 1998 and four in 1999, 1-4 weeks after cuttings were collected (uncut strips: 0.28-8.65 predators/prey, cut alfalfa: 0.06-0.94). In the plot-scale trial, predator numbers were similar between treatments and sampling periods in 1999. In 2000, uncut strips attracted insect predators in sweep net samples in weeks 1, 3 and 4, however ladybirds caught in sticky traps were more abundant in controls in one plot in week 1. Potato leafhopper Empoasca fabae (pest) numbers were higher in 73% of uncut strips surveyed for 2-3 weeks after harvest in 1998 and 2000. In 1999, leafhopper numbers were generally not higher in uncut than cut strips.
A replicated, controlled study on nine fen meadows in northern Switzerland (Schmidt et al. 2008) found overall spider (Araneae) species richness and abundance were similar between fallow strips and mown strips (fallow: 22.2 species, 75 individuals/m²; control: 19.8 species, 82 individuals/m²). Four out of ten spider families were more abundant in rotational fallows than completely mown plots (orb weavers (Araneidae), sac spiders (Clubionidae), ground spiders (Gnaphosidae) and jumping spiders (Salticidae)), four families had similar abundances (dwarf sheet spiders (Hahniidae), wolf spiders (Lycosidae), tangle-web spiders (Theridiidae) and crab spiders (Thomisidae)) and two had lower abundances in fallow strips (money spiders (Linyphiidae) and long jawed spiders (Tetragnathidae)). Three meadows were chosen in each of three regions. Starting in autumn 2002, in each meadow one plot of three 35-50 x 10 m-wide strips was mown rotationally (each year one of the three strips was not mown), and all three strips in the control plot were mown every year. Plots were mown in September and litter removed. Spiders were sampled March-June 2005 in each meadow using six emergence traps in the unmown fallow strip in the rotational plot and six traps in one mown strip in the control plot.
- Summers C.G. (1976) Population fluctuations of selected arthropods in alfalfa: influence of two harvesting practices. Environmental Entomology, 5, 103-110
- Cameron P.J., Allan D.J., Walker G.P. & Wightman J.A. (1983) Management experiments on aphids (Acyrthosiphon spp.) and beneficial insects in lucerne. New Zealand Journal of Experimental Agriculture, 11, 343-349
- Nentwig W. (1988) Augmentation of beneficial arthropods by strip-management. 1. Succession of predacious arthropods and long-term change in the ratio of phytophagous and predacious arthropods in a meadow. Oecologia, 76, 597-606
- Godfrey L.D. & Leigh T.F. (1994) Alfalfa harvest strategy effect on lygus bug (Hemiptera: Miridae) and insect predator population density: implications for use as trap crop in cotton. Environmental Entomology, 23, 1106-1118
- Hossain Z., Gurr G.M. & Wratten S.D. (2001) Habitat manipulation in lucerne (Medicago sativa L.): strip harvesting to enhance biological control of insect pests. International Journal of Pest Management, 47, 81-88
- Samu F. (2003) Can field-scale habitat diversification enhance the biocontrol potential of spiders? Pest Management Science, 59, 437-442
- Weiser L.A., Obrycki J.J. & Giles K.L. (2003) Within-field manipulation of potato leafhopper (Homoptera: Cicadellidae) and insect predator populations using an uncut alfalfa strip. Journal of Economic Entomology, 96, 1184-1192
- Schmidt M.H., Rocker S., Hanafi J. & Gigon A. (2008) Rotational fallows as overwintering habitat for grassland arthropods: the case of spiders in fen meadows. Biodiversity and Conservation, 17, 3003-3012