Reduction of adult olive fruit fly populations by targeting preimaginals in the soil with the entomopathogenic fungus Metarhizium brunneum

The primary research focus of the present work was to evaluate the effectiveness of soil applications of the Metarhizium brunneum EAMa 01/58-Su strain beneath the tree canopy targeting Bactrocera oleae (Rossi) preimaginals to reduce the adult fly population. In a first series of laboratory bioassays, the virulence of the strain against puparia and adults was ascertained, with LC50 values of 1.0 x 10(7) and 7.0 x 10(6) conidia ml(-1), respectively. In a second long-term field experiment, an experimental mycoinsecticide was produced using this strain and evaluated in selected farms in Andalusia (Spain) over 4 years. In each season, the field experiments lasted for 4-5 months, with two applications of the mycoinsecticide beneath the tree canopy at a rate of 1.0 x 10(5)-3.0 x 10(5) conidia g soil(-1), the first one in late fall, when prepupariating larvae typically fall from the olive fruit to the soil, and the second one in early spring, when adult emergence from the soil is expected. After each application, the fungal inoculum reached basal levels varying from 0.8 x 10(2) to 1.1 x 10(3) conidia g soil(-1). During the four seasons, a 50-70 % reduction of adult olive fruit fly populations was observed in the treated plots compared with the untreated ones, which could help in minimising the number of treatments, if any, within an olive fruit fly IPM strategy.