Intra- and intercrop diversification in cereal cropping and effect on pest control

Increasing intrafield plant diversity has been shown to regulate pest populations in various agroecosystems. Polyvarietal mixtures of a crop species (intraspecific diversity) or associations of a crop and a companion plant (interspecific diversity) are both considered as promising agroecological practices for low-input or organic agriculture systems by providing several ecosystem services such as pest, disease and weed control, and nitrogen fertilization. However, combining both diversification practices has not been studied yet in perspective of winter wheat pest control.In organic field experiments over two growing seasons, we combined both practices and examined the direct impact on aphid and natural enemy populations and on wheat production. We also investigated the potential pest regulation service through the assessment of the rate of predation by using sentinel preys.Results show that combining intra- and interspecific diversity did not outperform each practice individually in reducing aphid populations, thus not clearly showing synergetic effects. Taken separately, intercropping tended to have lower aphid infestation, while it the cultivar mixtures was more infested by aphids than the least susceptible cultivar. Yearly variation in climatic conditions strongly impacted wheat and clover development, as well as the appearance of aphid peaks. Wheat yields and grain nitrogen content were reduced in intercropping by 7 to 10%, but not in cultivar mixtures. Functional biodiversity, especially natural enemies such as ground beetles, tended to be positively correlated to the presence of a clover cover in the wheat fields (interspecific diversification), but did not respond to the wheat cultivar mixture (intraspecific diversification). Results varied according to the family of arthropods concerned and their position within the vegetation layer (ground dwelling or foliage dwelling arthropods). The cover of white clover and the field context influenced the community composition of predatory ground dwelling beetles. Rates of predation on sentinel preys were not influenced by any of the diversification practices.Under laboratory conditions, we evaluated how combining wheat and legumes (clover or pea) modifies the behaviour of the cereal aphid Sitobion avenae in terms of host-plant location, and population growth. We observed that aphids’ residence time on wheat was decreased when this host-plant was intercropped with clover. At the population level, wheat-legume intercrops reduced the number of aphids on wheat plants compared to wheat sole crops but if we take into account plant biomass, only intercropping clover with wheat significantly reduced aphid densities on wheat. The species used as non-host plants and their density are important parameters that should be taken into account in studies on intercropping systems and that may explain the large variability in the results observed in the literature.Our findings suggest that intrafield diversification may regulate wheat aphids to some extent, but combining the two diversification practices did not result in an interesting trade-off between pest regulation and wheat production in real farming conditions.