Coli et al. (1994) did not relate high densities of predatory mites hosted by bush species to mite populations of the adjacent orchard. As an adverse effect, codling moth abundance was observed to be the highest along the
hedgerow. Lastly, no significant correlation was displayed between predator abundance due to plant environment and C. pyri control in a survey of 8 commercial pear orchards. Patterns of pest distribution and patterns of predator densities introduced by the plant environment are thus not always correlated. Such discrepancies may be explained by the thermal and biological requirements of the considered arthropod species and by the climatic effect of the hedgerow on the distribution of both pests and natural enemies (Debras et al., 2008; Ricci et al., 2009). Lastly, the age and the cultural management of these hedgerows may alter their structural and plant diversity, therefore altering their functionality. The benefit of the increase in the abundance and diversity of natural enemies induced by plant manipulation is, however, seldom measured; the benefit, if any, can be slight, and not sufficient to avoid pesticides against the most noxious pests. We noticed that only pests such as mites and psyllids, which can be tolerated at high population levels in the orchards, may benefit from such manipulations (Tab. II). Beyond short-term pest control, the recolonisation of the orchard by natural enemies issuing from adjacent plant assemblages can contribute to the restoration of the community structure and to a more stable system.