Influence of zeolitite foliar coating on photosynthesis, VOC emission and quality of extra virgin olive oil

Olive fruit fly (Bactrocera oleae) is the most dangerous pest of olive fruits and strongly impairs both quality and quantity of the resulting olive oil. Organic farms have few tools against this pest and are constantly looking for effective and sustainable products; furthermore, for conventional farms the recent ban on dimethoate use in EU, made the defence from B. oleae very difficult. In this context the use of zeolitites, applied as particle films, began to take hold.

Since particle film covers the leaves, the organs responsible for gas exchange, a study on the plant responses to zeolite foliar coating was carried out by measuring photosynthetic rates from July to October (harvest) in two orchards located at San Lazzaro di Savena and Montiano in the Emilia Romagna region (Italy), respectively under organic and conventional farming.

Plant response to foliar treatment was also evaluated by measuring oil quantity in olives fruits. The layer of particle film covering leaves and fruits reduces the attractiveness of visual cues and prevents insects from recognizing and finding the plant parts on which they lay eggs: volatile organic compounds (VOC) emitted from both leaves and olives could act as oviposition promoters and were determined as well. Finally, chemical and sensory analyses on the resulting olive oils were performed. In the San Lazzaro orchard the tested treatments were: natural zeolitite (NZ), natural zeolitite enriched with ammonium (EZ) and Spyntor Fly® (SF), a protein bait based on spinosad for the control of B. oleae. In the Montiano orchard the treatments tested were: Dimethoate (DM), an organophosphate insecticide, natural zeolite with a reduced dose of dimethoate (ZN-DM) and negative control (Test).

Photosynthetic activity of plants treated with EZ was higher than the other two treatments in all dates, while no differences in photosynthetic rate were found between SF and NZ. In the Montiano orchard a slight reduction in photosynthetic rate was found only on the last two dates. The analyses of the VOC emitted by leaves and fruits allowed to identify respectively 35 and 31 different chemical compounds, belonging mainly to the chemical classes aldehydes, alkanes and alcohol, ketones, esters, ethers and terpenes. Chemical and sensory characteristics of oils were influenced by the incidence of olive fruit fly rather than foliar treatment with zeolite. In the Montiano orchard, subjected to a severe B. oleae attack, the effectiveness of the zeolite against the pest was observed, and the oil from untreated plants showed higher chemical parameters associated with secondary oxidation phenomena. In the San Lazzaro orchard, where  a weak B. oleae attack occurred, sensory differences were recorded between treated (NZ and EZ) and untreated plants. According to the results of this study, the use of zeolite film cover on olive tree canopy do not negatively influence the plant physiology and represents a useful tool against olive fruit fly.