Inorganically and organically modified mineral clays: a sustainable approach in the control of the olive tree fly pest, Bactrocera oleae

The most relevant phytophagous insect associated with olive tree plantations throughout the Mediterranean area is Bactrocera oleae, the olive tree fly. Innovative and environmentally-friendly methods for the mitigation of such pest have been attracting an ever-increasing attention, especially after 2014, when the whole Italian olive oil production suffered from dramatic losses. Novel prevention strategies should meet four main criteria: toxicologic safety, environmental selectivity and compatibility, pest-control efficiency and economic sustainability. In this aim, two series of solids showing a detrimental effect on the life cycle of Bactrocera oleae have been designed and prepared, i.e. montmorillonite-based clays from mineral origin containing: i) Group 11 transition metal cationic species, such as Cu(II) and Ag(I) or ii) polyphenolic organic compounds derived from olive mill waste waters. The bioactive solid materials have been prepared either by ionic exchange from aqueous solutions of sulfate or nitrate metal precursors at various concentrations or by impregnation of the polyphenol-containing effluents onto the clay support, respectively (Scheme 1). In the case of metal-based solids, by optimising the ion-exchange procedure, a fully adequate bioactive metal content was reached for practical purposes, although reducing by more than 80% the use of precursor salts with respect to the current state of the art (1). These materials contain approx. 5 wt.% of active metal and possess acid sites, which not only, when dispersed on the fruit surface, inhibit the egg-laying activity of flies into the olives, but also catalytically-active metal cation sites with enhanced biocide effect against parasites of several crops (2). In polyphenol-modified solids, on the other hand, amounts of organics in the range of 0.9-2.6 wt.% can be deposited. Thanks to the immobilisation within the interlayer spaces of the clay, lower concentrations of bioactive species may be applied onto the olive tree, thus reducing the potential undesired dispersal of the active molecules into the environment. In order to evaluate the performance of the most promising solids in open-field tests, two sets of experimental campaigns have been carried out on olive tree orchards in Southern Tuscany, in summer 2015 and 2016. Preliminary promising results have been obtained with Cu(II) species and with aqueous-phase olive mill wastewaters deposited onto bentonite-like montmorillonite clays, in terms of reduction of both fly infestation and damages on the olive fruit. These materials have indeed proved to be easily prepared, cost effective, environmentally friendly, stable to rainwater leaching, toxicologically safe and led to a remarkable diminution in the use of bioactive species for on-field applications. References: 1. Baker, H. Desalination, 244 (2009) 48. 2. Belcari, A., Bobbio, E., Informatore Fitopatologico, 12 (1999) 52.