Background Entomopathogenic fungi are primary pathogens that naturally affect insect pests by suppressing their populations and considered as an ecofriendly agents. The present study aimed to evaluate in vitro activity of different isolates of Beauveria bassiana and Metarhizium anisopliae against the development of larval stages of the Cucurbit fruit fly, Zeugodacus cucurbitae (Coquillett) (Diptera: Tephritidae). Results Larval mortality was significantly high with B. bassiana isolate Bb337 (5.82–21.70%) and with the lowest in M. anisopliae isolate MaD (1.49–6.33%). Pupal mortality rate was comparatively higher with more than 50%. The cadavers of all host instars produced conidia (sporulation). Sporulated dead larvae were significantly higher in Bb337 (61.10%) than at the least in MaD (18.60%) at 105 conidia/ml. At 108 conidia/ml, MaD induced the highest pupal cadavers with mycosis (32.42%). Regardless of applied fungal species, host instars mortality significantly increased with increasing concentration of B. bassiana isolates, suggesting a concentration-dependent response of Z. cucurbitae. Conclusion The tested isolates demonstrated their pathogenicity through vertical transmission of mycosis from one instar to another, regardless of the concentrations used.

The efficacy of field application of two isolates of Metarhizium anisopliae (MaD and ICIPE-69) and two isolates of Beauveria bassiana (Bb337, Bb338) was compared to that of synthetic insecticide, K-Optimal (Lambda-Cyhalothrin and Acetamiprid), for the control of major pests infesting watermelon (Citrullus lanatus), Zeugodacus cucurbitae and Dacus vertebratus. Both biopesticides and chemicals were applied twice a week from 3 weeks after sowing to 2 weeks before harvest. Experiments were conducted during the dry and rainy seasons at the garden of the International Institute of Tropical Agriculture (IITA) in Benin Republic. Significant damage was caused to watermelon, primarily in young and immature stages. During the dry season, the highest infestation rate (63.62 pupae/kg) was recorded in the control, with the lowest on plots treated with MaD (11.88 pupae/kg). However, infestation rate was not significantly different among treatments. The tendency of infestation load (number of pupae per kilogram of fruit) significantly varied during the rainy season. Thus, plots treated with ICIPE-69 had the highest number of pupae per kilogram (120.20 pupae/kg), and the lowest was on plots treated with Bb337 (44.48 pupae/kg). Marketable yields were 148.75 t/ha (MaD), 145.83 t/ha (K-optimal), 136.67 t/ha (Bb337), and 119.17 t/ha (Bb338). Beauveria bassiana (Bb337) and M. anisopliae (ICIPE-69) applied at 0.012 × 10 8 CFU/mL have a great potential to be used against both Z. cucurbitae and D. vertebratus infesting watermelon and have a little effect on nontarget organisms such as spiders, lady beetles, and ants.