Efficacy of Entomopathogenic Fungi Against Bruchus rufimanus (Coleoptera: Chrysomelidae) in Laboratory and Field Trials Using Dropleg Spraying Technique

Entomopathogenic fungi (EPF)-based biopesticides have attracted growing interest in pest management as alternatives to neurotoxic insecticides. Their potential was evaluated against various pests, including the broad bean weevil (Bruchus rufimanus Boheman 1833), a significant threat to faba bean (Vicia faba L.) crops. This study examined the entomotoxic effects and sublethal impacts (on oviposition) of three fungal strains under laboratory conditions: Beauveria bassiana (GHA), Metarhizium brunneum (USDA 4556), and M. brunneum (V275) on B. rufimanus adults. Subsequently, a large-scale field trial assessed the efficacy of B. bassiana (GHA) against B. rufimanus infestations using conventional anti-drift and dropleg spraying methods. The laboratory LT50 values ranged from four days for B. bassiana to eight days for M. brunneum (V275). The mortality rates recorded after ten days ranged from 86.6% for M. brunneum (V275) to 96.6% for B. bassiana (GHA). The inhibition of oviposition rates ranged from 12% for M. brunneum (USDA 4556) to 36% for B. bassiana (GHA). Field trials showed that the dropleg nozzles targeted faba bean pods, the oviposition sites of B. rufimanus, more effectively than the anti-drift nozzles. However, both fungal and chemical treatments applied via dropleg nozzles offered limited protection, reducing the infestation rates by 7% and 14%, respectively, with only a 3% improvement over anti-drift nozzles. This suggests that the large-scale spraying of chemical or fungal agents, including B. bassiana GHA, is not an optimal IPM strategy for managing B. rufimanus in faba beans. These laboratory and field results highlight the potential of EPF for managing B. rufimanus. However, the limitations of spray-applied plant protection methods underscore the need to redirect research toward more targeted strategies, such as attract-and-infect or endophytic EPF approaches.