Efficacy of entomopathogen Cordyceps tenuipes (Peck) Kepler, B. Shrestha et Spatafora against spider mite Tetranychus piercei McGregor (Acari: Tetranychidae)

ABSTRACT Tetranychus piercei (Acari: Tetranychidae) is a spider mite that infests a wide range of plants. It is difficult to control due to its ability to quickly develop resistance to common pesticides. The study aimed to find the effect of relative humidity (55, 75, and 95 ± 2% RH) on the efficacy of the entomopathogen Cordyceps tenuipes (1 × 106, 1 × 107 and 1 × 108 conidia/ml) against the egg and adult female stages of T. piercei. The results showed that the mortality of T. piercei eggs was concentration-dependent and that RH has a synergistic effect on efficacy. The egg mortality increased with the increasing the concentration from 1 × 106 to 1 × 108 conidia/ml at the same RH condition, and it was significantly higher at 1 × 108 conidia/ml at 95% RH than at 75 and 55% RHs. The median lethal time (LT50) for adult females was 41.1 hours at 95% RH and 49.8 hours at 75% RH when treated with a concentration of 1 × 108 conidia/ml. This was significantly lower than the LT50 values at other relative humidity levels and concentrations. The results of the study on potted bean plants showed that double spray of C. tenuipes was effective in controlling T. piercei infestations at a concentration of 1 × 108 conidia/ml. With a single spray of C. tenuipes, the mite population tends to be reduced initially and then increased subsequently. Thus, considering RH and incorporating other strategies is important when using biological control methods to effectively control this pest.