Growth pattern of Bacillus popilliae in Japanese beetle larvae

Abstract

Induction of milky disease in 50% of Japanese beetle ( Popillia japonica) larvae by feeding requires about 10 9 spores of Bacillus popilliae per gram of soil. The infectious process occurs in four phases: (1) An initial incubation phase of about 2 days during which there is no evidence of infection in the hemolymph. (2) A vegetative phase of poliferation in the hemolymph which lasts until day 5 when prespores occur and a few spores first are observed. (3) An intermediate phase between day 5 and day 10 characterized by concomitant vegetative growth, prespore formation, and sporulation; maximum vegetative populations of about 10 9 cells per ml hemolymph occur during this phase but the number of spores exceed that of vegetative cells by the end of the phase. (4) Thereafter, a sporulation phase which terminates by day 14 to day 21 with typical milkness and death of larvae; vegetative populations steadily decline and large numbers of spores accumulate during this phase. Milky larvae contain an average of 5 × 10 10 spores per ml hemolymph. Throughout the process microscopic evidence indicates many vegetative cells die without forming spores; dead cells disappear from the hemolymph by some unknown lytic or phagocytic process. Thus the massive spore populations which characterize milky disease result from accumulation of spores during a prolonged period of simultaneous vegetative growth and sporulation rather than from an extended period of exclusively vegetative growth followed by sporulation of most cells.