Pathogenicity of Bacillus popilliae (Cyclocephala Strain) and Other Milky Disease Bacteria in Grubs of the Southern Masked Chafer (Coleoptera: Scarabaeidae)1

Abstract

Bacillus popilliae Dutky (Cyclocephala strain), causal agent of milky disease in Cyclocephala spp., was evaluated for pathogenicity in grubs of Cyclocephala immaculate (Olivier). Calculated LC50 values for larvae held 21 days in contaminated soil were 2.8 × 105, 2.6 × 105, and 1.3 × 105 sporangia per g of soil for 1st, 2nd, and 3rd instars, respectively. The lower LC50 value for 3rd-instar grubs is probably a consequence of greater intake of food and soil. The calculated ID50 for 3rd instars infected via intrahemocoelic injection was 3.2 × 104 sporangia per grub. These tests indicate that virulence of B. popilliae (Cyclocephala strain) in C. immaculate is comparable to that of B. popilliae type A in the Japanese beetle. Virulence of 16 other species or strains of milky disease (Bacillus spp.) from 15 different hosts was tested via injection in C. immaculata. Strains from three Australian scarabs and a native strain from Phyllophaga fusca (Froelich) were the most infective, producing disease in 36 to 58% of the grubs. Failure of sporangia-talc applications to significantly increase milky disease in field tests is attributed to insufficient time for sporangia to have become incorporated into the soil. There was, however, a consistent trend toward inereased infection in the treated sod.