Organic farming supports lower pest infestation, but less natural enemies than flower strips

1.    Agricultural intensification has led to dramatic losses of species and associated ecosystem services. In the European Union, agri-environment schemes (AES) have been developed to mitigate these challenges. There are two opposing AES strategies, targeting either production or non-production areas. Organic farming focuses on production and conservation on the same land-use area, whereas flower strips adjacent to crop fields are in favour of intensified conventional production combined with conservation strips outside the cultivated area.
2.    We investigated pest and natural enemy abundances of organic farming vs. establishing flower strip in ten agricultural landscapes in Central Germany along a gradient of mean field size (1.24-6.78 ha). We focused on three winter wheat fields per landscape: conventional field (control), conventional field with adjacent flower strip and organic field. We sampled crop pests such as cereal leaf beetles (CLB) and cereal aphids as well as their natural enemies.
3.    Our results indicate that the abundance of CLB larvae was more than two times higher in conventional farming with and without flower strip than in organic farming. The abundance of natural enemies was supported by landscapes with small mean field size, i.e. their numbers increased ca. threefold when field size decreased from 7 to 2 ha.
4.    Aphid abundance was lower in organic fields and conventional control fields than in conventional fields with flower strips suggesting a potential disservice of flower strips. Parasitoids and natural enemies benefited from flower strips, but they were obviously not able to control the aphids.
5.    Synthesis and applications. The major pests in cereals, aphids and cereal leaf beetles, infested organic farming less than flower strips along conventional fields. However, abundance of natural enemies of pests benefited from flower strips and, in addition, from decreasing field size in agricultural landscapes. Hence, enhancing predator populations for more effective biological pest control may be best with decreasing field sizes combined with organic farming and flower strips. Altogether, organic farming might contribute much more to low pest damage than a conventional farming strategy with flower strips.