Yield of Metarhizium anisopliae Conidia on Four Tropical Cereals and in vitro Evaluation of Virulence against Cowpea Weevil, Callosobruchus maculatus (Fabricius) (Coleoptera: Chrisomelidae)

Aims: To evaluate conidia yield of entomopathogenic fungus, Metarhizium anisopliae Net 275.86DC on different tropical cereal substrates and evaluate its virulence against the bruchid weevil, Callosobruchus maculatus in vitro.

Methodology: The fungus was cultured on Sabouraud Dextrose Agar (SDA) and incubated at ambient temperature (mean daily temperature=30°C) for 14 days. Radial extension was measured daily along pre-marked orthogonal axes and used to estimate growth rate. Conidia from 14 days SDA culture were scraped into 10 mL Reverse Osmosis (RO) water containing 0.02 µl of surfactant, Tween 80. Ten gramms sub-samples of cereal grains; maize, rice, sorghum and millet were washed and soaked in RO water overnight at 4°C. Thereafter, they were transferred into 250 mL glass jars with micro-porous lid in triplicates and autoclaved at 121°C and 15 psi for 30 minutes. The jars were left for 2-3 hours to cool and the water activity (aw) of the substrates were determined, followed by inoculation with 1 µl of 1.0 x 104 conidia mL-1 of prepared seed inoculum. Incubation was done in the dark for 14 days in equilibrated environment (ERH>98%). Conidia were harvested by pouring RO water containing 0.02-0.05% Tween 80 unto the substrates in the jars followed by vortexing. Pure conidia were recovered by vacuum filtration and concentrated into 10 mL suspension using a centrifuge. Conidia concentration (conidia mL-1=Yield) was determined using improved Neubauer haemocytometer under x40 objective of light microscope. Three concentrations, 1.0 x 105, 1.0 x106, 3.9 x106 conidia mL-1were tested for virulence against C. maculatus using in vitro bioassay system with adult insects and the median lethal time, LT50 of the fungus was determined.

Results: The growth rate on SDA media was 2 mm day-1 and the yield on millet was significantly the highest, 5.53 x 106 conidia mL-1. Conidia concentrations from maize, sorghum and rice substrates were 4.73 x 105, 2.8 x 105 and 1.3 x 105 conidia mL-1 respectively. The yields from maize and sorghum were not significantly (P>0.05) different. The fungus was virulent against C. maculatus, LT50=1.6 days at 3.9 x106 conidia mL-1.

Conclusion: Some of the tropical cereals could be adopted for mass production of virulent conidia of the entomopathogenic fungi at relatively low cost of production.