Inoculation dose, formulation and air temperature shape Metarhizium anisopliae virulence against the oriental fruit fly: lessons for improving on-target control strategies

Entomopathogenic fungi are a promising tool for the biological control of crop pests provided low or no impact on non-target organisms. Selection for host specificity as well as on-target applications open new avenues for more sustainable strategies for pest management. Isolates of Metarhizium anisopliae (Metschn.) Sorokin have been identified as promising for developing innovative entomovection-based strategies for the control of the oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), in Africa. To be effective, this technology requires high strain virulence at a low number of spores, in field conditions, but sufficient incubation time to allow transmission to wild conspecifics. In the present study, we investigated the virulence of the Met69 OD strain against adult flies, depending on the inoculation dose, formulation and air temperature. High pathogenicity was observed at very low inoculation doses (LT50 of 4.85 days with 6100 spores per fly) independently of fly sex. Virulence increased with spore load in a tight range (5600 and 6100 spores per fly) and with air temperature observed in the field (20-28 degree Celsius). Unexpectedly, corn starch used as an adjuvant to increase the carrying capacity of insects decreased the virulence of the pathogen. Results are of utmost importance to improve area-wide control strategies based on contamination of wild flies in mango orchards through auto-inoculation devices or interactions with released mass-reared sterile males coated with fungal spores. ### Competing Interest Statement The authors have declared no competing interest.