Are low infestations of Bactrocera oleae in South African olive groves due to parasitoids? End of a legend

In 1913, Silvestri collected wild olives in Wellington (Western Cape, South Africa) from which more parasitoids than olive fruit flies emerged. He hypothesized that the low infestations of Bactrocera oleae observed on cultivated olives in the region were also due to parasitoid activity. Since then, the assumption that South African B. oleae parasitoids are more abundant and effective than Mediterranean ones has persisted. However, adequate studies on infestations and their causes in South African cultivated olives are nearly absent. We compared B. oleae infestations and its parasitization in cultivated and wild olives from Wellington, Somerset West, and Franshhoek (Western Cape, South Africa) and from Trapani province (Sicily, Italy) during growing seasons, with a typical climate for those areas, harvesting in the second month of fall (April for the Western Cape, October for Sicily). Parasitism rates for B. oleae in cultivated olives were low and insufficient both in the Western Cape (6%) and Sicily (17%), as it has been widely recorded in Tephritidae of economic interest. Higher parasitization (27% in the Western Cape, 48% in Sicily) was recorded in wild olives, where the thinner pulp favors specialized parasitoids with a short ovipositor (<2.6 mm). The significant difference in infestation levels observed between olives cultivated in South Africa (3.2% in 2009, 6.3% in 2010) and Sicily (79% in 2009, 82% in 2010) appears to be influenced by contrasting climatic conditions: specifically, the lower relative humidity and greater temperature range in the Western Cape during the last three months of olive production. In fact, years with lower relative humidity and reduced rainfall in final three months of the season, as observed in Sicilian sites, corresponded to halved infestation levels. These findings suggest that climate, rather than braconid parasitism, is the primary environmental factor limiting olive fruit fly infestations.