Peanut early flowering stage is beneficial to Metarhizium anisopliae survival and control of white grub larvae

The study aims to determine the timing of application for high efficacy of Metarhizium anisopliae as a biocontrol agent. A field experiment was undertaken with M. anisopliae applied to the soil at five intervals during the peanut crop lifecycle, at seed germination (day 0) through to pod filling period [75 days after sowing (DAS)], and assessed the change of M. anisopliae density by sampling rhizospheric soil, subsequently at regular intervals and testing counts (CFU/g dry soil) through to harvest. The crop was sown into soil with an established white grub population, with larval density determined at harvest when the trial was concluded. Applications at 0, 15 and 30 days in the crop growth cycle, saw M. anisopliae mean propagule counts drop significantly after 15 days before increasing over the following 15–45 days. We observed an elevated mean increase in counts 30–45 days after application at the early flowering stage (30 DAS). Irrespective of application timing, in general, M. anisopliae densities declined to less than the initial 10% in the late stages of peanut development. At harvest, larval densities in all M. anisopliae treatments were significantly less compared to the control, with the highest mortality (72%) in M. anisopliae treatment applied at early flowering (30 DAS). Relationship analysis showed that white grub density was significantly related to peanut yield. A regression of yield on number of damaged pods also supported that treatment at the early flowering caused the highest impact in terms of reducing damage to pods and improving yield. These results suggest that applying M. anisopliae at the early flowering stage optimizes survival of M. anisopliae in the soil profile, meaning greater probability of larvae contacting the pathogen, leading to greater mortality.