As additional plant strata to productive trees, the plant cover in the alleys is generally composed of grasses (Poaceae),
mixed with weeds and sometimes with other sown species such as leguminous plants. The presence of an understorey
cover generally provides a benefit for the orchard pest control. A three-species plant cover sown in the alleys provides
a higher richness and diversity of the pear canopy entomocenosis compared with a bare ground. The beneficial aphidiphagous complex of the apple orchard is favoured by flowering strips to the benefit of aphid control. Plant covers are only reported to be detrimental compared with a bare ground in peach orchards where they favour leafhoppers and phytophagous mites. The co-existence of different strata creates a diversity of habitats and resources for animals: shelters, e.g. resting, diapause or hibernating sites, reproduction areas and refuge areas to escape disruptive agricultural practices, as well as food, e.g. alternate preys and hosts, nectar and pollen. Arthropod communities exploiting the soil,
the grass and the canopy cohabit within the orchard and contribute to its richness. Some species are likely to exploit more than one of these strata and are therefore likely to stay and multiply through higher levels of resources. As high levels of beneficial arthropod richness are displayed within the grass cover, whereas low levels of predation and pest control are observed within the arboreal strata (Simon et al., 2007b), strong interactions among strata are not always established in field experiments. The hypothesis of a structural rather than a functional assemblage is promoted by some authors. It cannot be excluded that the beneficial complex of the fruit tree canopy does not benefit the grass cover richness or diversity. Although the intrinsic complexity of fruit trees and the diversity in resources provided by the orchard plant design are high, the resulting arthropod diversity is not always highly functional for pest control.