The olive fruit fly Bactrocera oleae (Rossi) (OLF) is a major pest, which causes severe yield losses and quality decay of olive oil. The widespread
use of chemical insecticides to control this pest is a major concern for the resulting environmental impact and food safety issues. Therefore, the
development of sustainable control strategies is highly desirable. The primary endosymbiotic bacterium of the OLF, “Candidatus Erwinia dacicola”,
is essential for successful larval development in unripe olive fruits. Then, targeting this endosymbiont with antimicrobial compounds may exert a
control action against OLF. Here we evaluate the impact on OLF endosymbiont of Copper Oxychloride (CO) and the fungal metabolites Viridiol and
Harzianic Acid (HA) produced by two biocontrol strains of Trichoderma spp. Laboratory bioassays were carried out on OLF wild populations to assess
the effect of the oral administration of these compounds on mortality and fecundity of adult flies, and on larval development of their progeny in unripe
olive fruits. Treated females were processed by qPCR to measure the endosymbiont load in the oesophageal bulb and in the midgut. Exposure to
Viridiol and HA had a strong negative impact on endosymbiont load and OLF larval survival, while CO negatively affected both adults and larval
stages, showing a combined toxic action and an anti-symbiotic effect, which was dose-dependent. These results provide new insights on the symbiotic
control of the OLF and pave the way for developing new strategies based on the use of natural compounds with antimicrobial activity
