For the total arthropod fauna and beneficial complex, the presence of a grassy ground cover within the orchard increases
or not the diversity of beneficial arthropods. Responses in terms of pest control vary widely according to the pests, and most of the studied plant covers address one or a few key pests. Many studies in Northern America focus on the effect of understorey covers on mite populations in orchards. Information is provided on the conditions for optimal biological control of spider mites by predatory mites in terms of surface area to be covered by grass, distance to fruit trees, and composition of the plant assemblage. In pear orchards, both an increase in Anthocorid numbers and a decrease in C. pyri prey are assessed when a grassy ground cover is sown in the alleys compared with bare ground. Flower strips in apple orchards enhance D. plantaginea control. By providing a within-orchard higher density of preys they contribute to maintaining a high density of spiders and generalist predators, which prey on immigrating aphids in autumn, and
can survive on, and control, low densities of preys. In spring, flower strips are also expected to favour aphidiphagous syrphids, but this effect is not displayed in a second experiment, most probably because of a delayed bloom in a Northern region and the mowing of the strips in winter. Lastly, very few studies address all the processes involved in the tri-trophic system targeted by plant manipulations: food preference of each orchard pest, effect of companion plants on the
abundance and the fitness of the natural enemies to promote, predation or parasitism rates in the orchard and interactions
with other natural enemies. The plant ground cover is therefore a plant component of the orchard which is easily manipu-
lated and experimented on. A wide range of plant assemblages targeting various pests has already been tested with promising results. As for hedgerows, we notice that empirical, rather than scientific, knowledge is involved in most cases. Consequently, beyond adverse effects due to the management of the plant cover, failure or success cases in pest control cannot be explained and results are not always reproducible. Further research is needed to identify occurring processes and the ability of both pests and beneficial arthropod species to exploit both understorey and arboreal resources.