Time-concentration-mortality response of potato tuber moth pupae to the biocontrol agent Cordyceps tenuipes

The potato tuber moth (PTM) Phthorimaea operculella is a critical potato pest. Larvae infest both foliage and tubers and mature larvae pupate in the soil or other safe places. Cordyceps tenuipes, an entomopathogenic fungus, infect lepidopteran pupae. To determine the effectiveness of this fungus as a biocontrol agent for PTM, we evaluated the time-concentration-mortality (TCM) response of PTM pupae to C. tenuipes using the following bioassays: (1) direct immersion in conidial suspensions, (2) incubation in sterilised or (3) unsterilised soilpremixed with conidia, and (4) incubation in unsterilised soil drenched with conidial suspensions to simulate field conditions. Fungal infection caused 100%, 83.3%, 73.3%, and 85.0% mortality of PTM pupae in assays 1-4, respectively. At 10(8) conidia/mL or conidia/g concentration, assays 1 and 4 had short lethal times (LT50) of 2.2 and 2.6 days compared with 3.7 and 4.8 days for assays 2 and 3, respectively. On day 7 after inoculation, assays 1 and 4 also had low lethal concentrations (LC50) of 1.69 x 10(3) conidia/mL and 1.10 x 10(5) conidia/g compared with those of assays 2 and 3, which showed low virulence, with LC50 of 3.50 x 10(5) and 3.60 x 10(6) conidia/mL, respectively. Our results demonstrate that C. tenuipes is a promising candidate for PTM biocontrol at the pupal stage. Drenching the soil surface with conidial suspensions may be the most effective method of field application.