High quality extra virgin olive oil from olives attacked by the olive fruit fly, Bactrocera oleae (Rossi) (Diptera Tephritidae): which is the tolerable limit? Data from experimental ‘Nocellara del Belice’ and ‘Cerasuola’ olive groves in Sicily

Abstract

The infestation due to the olive fruit fly, Bactrocera oleae (Rossi), the key pest in most of world olive groves, has been monitored in six years, from 2004 to 2008 and in 2014, in eight coastal Sicilian olive groves consisting of ‘Cerasuola’ and ‘Nocellara del Belice’ cultivars. Infestation was recorded following the classical sampling method based on olive collection and dissection, in order to count live olive fruit fly instars (eggs, larvae and pupae) and exit holes. Four different infestation indexes, all of them calculated using data recorded at harvest, have been used to assess their relationship with the main three quality parameters of the olive oil obtained from the same olive samples processed within 24 hours by quality oil mills: free acidity (% oleic acid), peroxides (mEq O2 /kg of oil) and total phenols (mg·kg−1 oil). Each year and in each olive grove up to five plots were differently treated with different products allowed in organic agriculture, stopping sprays at least one month before harvest, obtaining different infestation levels. In this study a total of 43 theses were tested, 10 of them involving ‘Cerasuola’ and 33 ‘Nocellara del Belice’. The two indexes based on the exit holes produced by mature larvae in olives resulted strictly and positively correlated to the lowering of oil quality. Moreover, among the two infestation indexes based on the occurrence of exit holes at harvest, percentage of olives with exit holes and No. of exit holes per 100 olives, the latter is more sensitive in presence of high infestation levels. No significant relation between infestation indexes and total phenols resulted in our analyses. Nevertheless, when olives are harvested since the end of October to mid-November, and processed by quality oil mills, olives bearing up to 45 % of exit holes produced high quality extra virgin olive oil. Furthermore, all olives bearing up to 62 % of exit holes still produced extra virgin olive oil. Our results, in spite of the widespread prejudices involving an overestimation of damages due to low olive fly infestation, are very close to recent studies on correlation between olive fly attacks and olive oil quality. Late harvesting can lead to worse results at the same olive fly exit holes levels, confirming that timing and quality procedures of harvesting and oil extraction are important almost as much as olive fly control.