Biocontrol Potential of Co-Applying Entomopathogenic Nematodes and Metarhizium brunneum Against Adult Otiorhynchus sulcatus

The black vine weevil, Otiorhynchus sulcatus (Coleoptera: Curculionidae), is a significant pest causing damage to horticultural crops. Restrictions on the use of synthetic chemical insecticides, due to factors such as resistance and harm to humans and the environment, are increasing the need for sustainable alternatives in integrated pest management. This study evaluated the effectiveness of entomopathogenic nematodes (EPNs) (Heterorhabditis bacteriophora and Steinernema feltiae) and the entomopathogenic fungus (EPF) Metarhizium brunneum, both individually and in combination, against adult O. sulcatus weevils. Laboratory bioassays were conducted using 12-well plates containing sterile sand. The applications included single and combined doses of EPNs (200, 400, and 800 infective juveniles/insect) and EPFs (low: 1×108, high: 1×109 conidia/L equivalent). Mortality rates were assessed daily for 7 days. Single-agent applications showed a dose-dependent response, with EPNs causing faster mortality compared to the delayed pathogenesis of M. brunneum. However, the simultaneous application of EPNs and EPF significantly increased the mortality rate and resulted in higher overall mortality rates. Chi-square (χ2) analysis revealed strong synergistic interactions, particularly when H. bacteriophora was paired with a low fungal dose and S. feltiae with a high fungal dose. In these synergistic pairings, the cumulative mortality rate reached 100% within 4 to 6 days. The findings demonstrate that integrating these biological control agents offers a highly effective and synergistic strategy for O. sulcatus management, providing a rapid immediate effect and potentially long-term sustained protection while allowing for reduced application doses.