Ground cover management with mixtures of flowering plants to enhance insect pollinators and natural enemies of pests in olive groves
Introduction
Expansion of agricultural land and intensification of farming has been associated with the loss of biological diversity and delivery of ecosystem services, such as biological control of pests and pollination in agroecosystems, partly through the reduction of suitable habitats for foraging and nesting sites; as a consequence sustainability of agroecosystems is undermined (Matson et al., 1997; Stoate et al., 2001; Robinson and Sutherland, 2002; Tillman et al. (2002); Carvell et al. (2006); Klein‚ et al. (2007); Wade et al., 2008; Potts et al., 2010; Bretagnolle and Gaba, 2015).
Sustainable agricultural practices in conjuction with ecological restoration methods for the management of field margins, uncultivated areas (set aside), headlands, hedges and mid-field strips within a farming unit can create suitable habitats providing food sources and shelter to pollinators or natural enemies of insect pests in disturbed agro-ecosystems (Carvell et al., 2004; Gurr et al., 2004; Pywell et al., 2005a, b; Cullen et al., 2008; Lu et al., 2014; Nicholls and Altieri, 2014; Bretagnolle and Gaba, 2015). In particular, the introduction of non-crop vegetation in agricultural land has been tested in many studies (Heimpel and Jervis, 2005; Tscharntke et al., 2007; Thomson et al., 2010; Wäckers and van Rijn, 2012; Lu et al., 2014; Campbell et al., 2017).
Implementation of the so-called ‘shotgun’ approach, through the introduction of a species-rich wildflower seed mixture (Carreck and Williams, 2002; Pfiffner and Wyss, 2004) or through the natural regeneration of species from the seed bank (Pywell et al., 2005b) requires a good understanding of the food-web theory. This is significant not only for the successful establishment of habitats for desired insect communities, but also to avoid the spread and possible dominance of a single or only a few plant species or insect-pollinator species in the target area (Pywell et al., 2005b). A good example of the importance of the seasonal flowering phenology and longevity of a plant mixture to support a great range of pollinators is the pilot project ‘Environmental Stewardship’ targeting to enchance bumble bee populations in the U.K. (Carvell et al., 2007). A legume-based ‘pollen and nectar flower mix’ could quickly provide a highly attractive forage resource for bumble bees, including rare long-tongued species, whereas a diverse mixture of native wild flowers attracted more of the shorter-tongued Bombus spp. and provided greater continuity of forage resources, especially early in the season (Carvell et al., 2007). Nevertheless, the established plant species could possibly attract herbivorous pests for the crop, higher-order predators/hyperparasitoids or plant diseases (Stephens et al., 1998). Thus, multiple criteria should be taken into consideration regarding the selection of plant species, such as soil/climatic requirements, growth habit, flowering period, nectar and pollen capacity and flower structure, possible threats to the crops and native plant species etc.
There is an increasing interest in the floral diversity in olive groves in Southern Europe, however their functional role for pollinators and natural enemies of pests is poorly understood (Potts et al., 2006; Solomou and Sfougaris, 2011; Paredes et al., 2013; Nave et al., 2017). Studies by Solomou and Sfougaris (2011) in organic olive groves in central Greece (Magnesia Prefecture) showed a rising trend for the diversity of herbaceous plant species in each site, species composition among sites for both herbaceous and woody plants, as well as density and cover of woody plants, in comparison with conventional ones. Potts et al. (2006) also concluded that olive groves in stands managed by ploughing on the island of Lesvos had an understory of many annual flowers supporting habitats of great value for plant-pollinator communities and provision of the healthiest pollination services in adjacent agricultural areas. These managed olives were superior to olive groves in abandoned stands where the vegetation regenerated to include many perennial species found in the frygana.
Messara plain is one of the most intensively cultivated areas with olives in Crete where decreased biodiversity problems are reported in relation to cultivation practices on the island (Stobbelaar et al., 2000; Allen et al., 2006). Among the common cultivation practices is the clearance of ground flora with chemical and mechanical control methods, applied mainly at the end of the crop’s dormant season in early spring and if needed in late autumn-early winter to facilitate harvest. In addition, olive growers maintain the winter ground cover of Oxalis pes-caprae (Oxalidaceae), an invasive alien species in Southern Europe which is abundant in the area (Damanakis and Markaki, 1990), because of its smothering capacity against other weeds and protection of the soil surface from erosion. As a result, O. pes-caprae has dominated the olive herbaceous flora and excluded other species. Invation of olive groves’ groundcover by O. pes-caprae on Lesvos island supressesed significantly plant species richness, compared to natural vegetation in uninvaded areas, turning it into monospecific in a few months and indicating that the species reduction was not due to seed bank depletion but to the competitive nature of this invading weed (Petsikos et al., 2007).
The present study was conducted in the frame of the pilot project ‘Operation Pollinator’ targeting at creating habitats for pollinators in the E.U. by enhancing the abundance and diversity of flowering resources in field margins. The study aimed at managing the ground cover in an Oxalis pes-caprae dominated olive grove in the locality of Messara plain, in order to i) unveil and increase the underlying floral biodiversity in O. pes-caprae invaded areas and ii) measure the capacity of mixtures of selected plants to support/attract desirable insect communities of the next trophic levels of the ecosystem, especially hymenopterous pollinating insects and natural enemies of insect pests. The aim of this study is in compliance with the principles of the EU policy for sustainable agriculture and the Directive (2009/128/EC) for the sustainable use of plant protection products, as it supports the development of best practices for the enhancement of biodiversity (both flora and fauna).
Section snippets
Experimental design
All experiments were performed in a 0.4 ha olive grove in Messara plain (Crete, Greece) (35.034379, 24.843804) with 25-year old trees of cv. Koroneiki, during three consecutive growing seasons (2011–2013). Soil type was clay and the ground cover during winter was mainly by O. pes-caprae. The trees were arranged in 6 × 13 rows with 7 m x 7 m planting distance. The experimental design was randomised complete block with three treatments (two sown mixtures and the control with native vegetation)
Plant mixture establishment and flowering – Presence of pollinators
Eight out of the 18 annual broadleaf species used in the plant mixtures germinated and reached flowering. The remaining 10 species germinated in small numbers but did not reach flowering or did not germinate. The species S. alba, B. officinalis, V. sativa and C. sativum germinated and reached flowering every year, regardless of sowing time. Of these, S. alba demonstrated a strong capacity to establish in large numbers and become dominant. Overall, the establishment of ground cover with sown or
Discussion
The establishment of selected plant mixtures in 3 m2 patches at an intra-row set-up in the olive grove dominated by O. pes-caprae, increased plant diversity and the potential habitats of hymenopterous pollinating insects and natural enemies of olive insect pests.
Conclusion
Overall, the findings support that establishment of patches with selected flowering plants in an olive grove, by sowing or maintaining (from natural vegetation) selected species (e.g. S. alba, C. sativum, B. officinalis, G. coronaria), is a sustainable practice that can provide habitats for Hymenoptera pollinators as well as parasitoid wasps against the olive fruit fly and scale pests. We also suggest that this practice can be implemented in O. pes-caprae infested olive groves to increase plant
Acknowledgements
The present work was funded under the biodiversity project Operation Pollinator of Syngenta (Syngenta Hellas, Greece, contract 15-11-2010 . We would like to thank Syngenta for the financial support of this research and Ms Voula Kaliakaki, Head of Regulatory Affairs and Stewardship, and Mr Fotis Andrinopoulos, Stewardship Manager and Regulatory Specialist, of Syngenta Hellas for the collaboration. We are also indebted to Dr Geoff Coates for the scientific and technical advice throughout this
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