A strain of Serratia marcescens (Enterobacteriaceae) with high virulence per os to larvae of a laboratory colony of the corn earworm (Lepidoptera : Noctuidae)

An unpigmented strain of the bacterium Serratia marcescens Bizio that is hi ghly virulent when fed to larvae from a laboratory colony of the corn earwo rm, Helicoverpa zea (Boddie), was found as a contaminant in a sample of the nuclear polyhedrosis virus of the celery looper, Anagrapha falcifera (Kirb y), that also infects H. zea. High rates of mortality (> 70%) were obtained by feeding doses of less than 100 bacterial cells per larva. Previous repo rts indicated that when insects could be killed with S. marcescens administ ered orally, doses on the order of 10(5) to 10(6) cells per insect were req uired to obtain comparable levels of mortality. Its virulence may have been increased through unintentional selection during the in vivo production of the virus in corn earworm larvae. The insect colony with which high mortal ity was obtained was compared with another colony, and the S. marcescens st rain was compared with other strains. Results indicate that the highest lev els of mortality are associated with a particular combination of insect col ony and bacterial strain. While the potential of this organism as a biologi cal control agent has not been evaluated, these apparently unique interacti ons could make this system an interesting and potentially useful model for the study of host-pathogen virulence factors.