Introduction
Organic Farming and Integrated Pest Management aim to favour and to rely on natural enemies for pest management. The ability of plant diversity to provide the orchard with natural enemies without any induced damage was studied: (1) in hedgerow lined orchards from different areas in order to assess the effect of hedgerows on the orchard arthropod community and (2) in experimental orchards, in order to test tree assemblages designed for the pear orchard, likely to supply the crop with psyllid (Cacopsylla pyri) antagonists.
Materials and methods
Arthropods were sampled fortnightly from April to October 1997 in eight (8) commercial pear orchards (Bartlett cultivar) and from main tree species of their lining hedgerows. For each tree species, 30 shoots (1 shoot per tree) were hit (Fauvel et al., 1981) at each sampling date. Three equal areas were delimited within each orchard: one along (one of) the hedgerow, another one in the middle of the orchard (central area) and the last one opposite (due to strong Northern wind, hedgerows are East-West oriented). For each orchard and each sampling date, 30 shoots were hit in each of these 3 areas. Using the same method, arthropods were sampled during 4 years from two experimental pear orchards (Bartlett cultivar) and from each species of the tested mixed hedgerows.
All the sampled arthropods have been identified, at least at the family level, allowing to give information about the biology of each arthropod (phytophagous, predatory, parasitic, pollinating, detritiphagous,…).
Results and discussion
40 095 arthropods representing 195 taxonomic families were sampled from commercial orchards and associated tree species. Sambucus nigra, Cornus sanguinea, Hedera helix and Coronilla emerus hosted a wide range of beneficial arthropods (generalist predators, parasitic insects) and did not shared major pest with the orchard, unlike Crataegus monogyna (which hosted fireblight), Rubus fructicosus, Populus nigra or Lonicera sp. (which hosted detrimental phytophagous arthropods). Beneficial diversity and numbers were lower for Alnus cordata, Cercis siliquastrum, Cupressus sp., Acer campestre and Robinia pseudacacia. Although they were generally higher in edge areas, orchard arthropod diversity and richness were not significantly different (variance analysis) between inner and edge areas. Pest management is likely to level possible differences within the orchard.
The tested assemblages showed to provide psyllid antagonists (Anthocorid bugs, generalist predators) with food and shelter all year long: late flowering and indeciduous leaf species (Arbutus unedo, Hedera helix) offering wintering shelters; early flowering species (Salix capraea, Corylus avellana, Viburnum tinus) offering pollen; species offering alternate preys and/or flowers (Sambucus nigra, Cornus sanguinea, Fraxinus oxyphylla, Corylus avellana).
Conclusion
This study allowed to provide information about the risk and the benefit of planting different tree species in the agroecosystem. Hedgerow border effect on the orchard arthropods is most probably related to the biology and the ability to move of each arthropod species and needs a specific study; biological and microclimatic effects can therefore cumulate. Further research is needed about the effect of more complex assemblages, including tree species and grass assemblages. Different scale at the landscape level could also be considered.
References
Fauvel G., Rambier A., Balduque-Martin R., 1981 - La technique du battage pour la surveillance des ravageurs en cultures fruitière et florale. Agronomie, 1(2), 105-113
