Broomcorn millet grain cultures of the entomophthoralean fungus Zoophthora radicans: sporulation capacity and infectivity to Plutella xylostella

The shelled grains of glutinous broomcorn millet, Panicum miliaceum, were used as solid substrate to prepare granular cultures of Zoophthora radicans, an entomophthoralean biocontrol agent against numerous insect pests. Steamed millet grains were inoculated by mixing 15 g millet grains (D.W.) with mashed pieces of half a 60-mm-dish colony in 3 ml modified Sabouraud dextrose broth and incubated at 15°C and L:D 12:12 for up to 24 d. 20 grains were sampled at 3 d intervals from day six onwards and individually assessed for their sporulation capacity using a self-designed device for spore collection. The millet cultures after 12 d incubation produced 12.0-14.9 × 104 spores grain−1 during a 7 d period. The maximal sporulation capacity associated with the 21 d-old culture was about half of that of Z. radicans-killed Plutella xylostella larvae (28.7 × 104 spores cadaver−1), which individually were at least three times larger than the millet grains. Based on the time-concentration-mortality responses of second-instar P. xylostella larvae to Z. radicans in three independent bioassays, the spores ejected from the cultured millet grains, from the mycelial mats from liquid culture, and from larval cadavers displayed insignificant variations in infectivity to the host species, and yielded similar LC50 and LT50 estimates. Conclusively, the millet-based technology for production of granular cultures of Z. radicans was easy, inexpensive and highly efficient, and it could be superior to previous methods used in mass production of mycelium-based preparations of Entomophthorales since this new approach requires no special additives, drying, freezing and milling. This technology may suit to mass production of culturable but nutritionally fastidious entomopathogens from the Entomophthorales.