Control of the lesser grain borer (Rhyzopertha dominica (F.), Coleoptera: Bostrichidae) by treatments with residual formulations of Metarhizium anisopliae (Metschnikoff) Sorokin (Deuteromycotina: Hyphomycetes)

Treatments with conidia of the entomopathogenic fungus Metarhizium anisopliae formulated in invert emulsion (water-in-oil formulation) or in wheat flour were applied to Rhyzopertha dominica adults infesting Cicer arietinum grains. The application rates were 4.1×10⁵ conidia/cm² of treated area using a concentration of 1.8×10⁷ conidia/ml of the invert emulsion and 8.2×10⁶ conidia/cm² of treated area using a concentration of 6.5×10⁸ conidia/g of the wheat flour formulation. Results have indicated significant mortality (P<0.05) when newly emerged R. dominica adults were introduced and then treated with the invert emulsion and wheat flour formulations (86.7-93.3%, control treatment 10.0-26.7% mortality). When treated grains were left until the emergence of F₁ adults, there was also significant mortality (P<0.05) in both treatments (28.3-60.0%, control treatment 1.7-8.3% mortality). This indicates a residual effectiveness of the treatment with the two formulations against R. dominica adults which extended to >2 months (equivalent to the duration of the insect life-cycle at 23±2°C and 75±5% r.h.). Infestation rate of C. arietinum grains by R. dominica at 23±2°C and 75±5% r.h. was significantly reduced in the treatments with the fungal conidia formulated in invert emulsion (0.7%) or in wheat flour (1.0%) when compared with the control treatment (19.0-23.3%). The preventive treatments significantly retarded R. dominica development (P<0.05) by 8-12 days compared with the control treatment. The infection with the fungus thus delayed adult emergence of R. dominica by 8-12 days. Overall results give promise for control of R. dominica with M. anisopliae mixed with wheat flour or introduced into invert emulsion.