Ground cover management with mixtures of flowering plants to enhance insect pollinators and natural enemies of pests in olive groves

Abstract

The potential of ground cover management with mixtures of selected plants to provide habitats for pollinating insects and natural enemies of pests in an olive grove in southern Greece, was studied over a period of 3 years (2011–2013). The management consisted of the establishment of 3 m2 patches of sown plant species or spontaneous natural vegetation between trees along tree lines. The sown plant species in the mixtures were Sinapis alba L. (Brassicaceae), Glebionis segetum (L.) Fourr. and Glebionis coronaria (L.) Cass. ex Spach (Asteraceae), Vicia sativa L. and Pisum sativum L. (Fabaceae), Borago officinalis L. (Boraginaceae), Coriandrum sativum L. and Pimpinella anisum L. (Apiaceae). Sowing was performed in autumn or spring and the peak flowering period occurred at the end of March to end of April, and at the end of May to end of June, respectively. Visual estimation of flower cover, counts of pollinator landings on flowers and presence of beneficial arthropods (suction sampling) were performed in three 7–10 day intervals during peak flowering, for each experimental year. Overall, patches with sown plant mixtures attracted higher numbers of pollinating Hymenoptera compared to native vegetation, especially mining bees and honey bees (Apis mellifera) as well as megachilids and bumble bees (Bombus spp.). Sinapis alba, present in both sown and native vegetation patches, attracted mainly mining bees and honey bees. The flowering mixture with C. sativum and B. officinalis was more attractive to honey bees than the one with G. coronaria as main flowering species but they were both equally attractive to mining bees, although the species composition may well have been different. Hymenopterous parasitoids, primarily Braconidae and Chalcidoidea, were sampled from the patches and the olive fruit fly parasitoid Opius concolor (Hymenoptera: Braconidae) was recorded on olive trees adjacent to the flowering patches. Large numbers of predators, namely Orius sp. (Hemiptera: Miridae) and lacewings (Neuroptera: Chrysopidae), were recorded in the patches, principally in those with the mixture containing mostly S. alba. Our results suggest that ground cover in patches with suitable flowering species could be part of a sustainable olive crop management system, providing food and refuge for pollinating insects and beneficial arthropods.