Stored Bacillus popilliae spores and their infectivity against Popillia japonica larvae

Bacillus popilliae spores were stored for about 7 years under three separate conditions: frozen in sterile distilled water, smeared on glass microscope slides, and stored in loam soil at room temperature. In separate experiments, each of the 7-year-old preparations was fed to Popilla japonica larvae at concentrations of 10³, 10⁵, 10⁷, and 10⁹ spores/g of soil. A significant decrease in the percentage of larvae infected occurred in all of the aged spore tests. B. popilliae spores stored in soil, for the extended period, produced 3% larval infection only at the 10⁹ spores concentration; similar results were obtained from frozen spores. When P. japonica larvae were fed spores stored dried on slides, about 20% of the larvae developed milky disease. When aged frozen spores were artificially injected into larvae, 12% became infected at concentrations of 1 × 10⁶ spores/larvae; dried spores at the same concentration infected about 38% of the insect larvae. We conclude from these data that aged B. popilliae spores are significantly less infective against P. japonica larvae than young spores.