A Strain of Serratia marcescens (Enterobacteriaceae) with High Virulence Per Os to Larvae of a Laboratory Colony of the Corn Earworm (Lepidoptera: Noctuidae)¹

An unpigmented strain of the bacterium Serratia marcescens Bizio that is highly virulent when fed to larvae from a laboratory colony of the corn earworm, Helicoverpa zea (Boddie), was found as a contaminant in a sample of the nuclear polyhedrosis virus of the celery looper, Anagrapha falcifera (Kirby), that also infects H. zea. High rates of mortality (>70%) were obtained by feeding doses of less than 100 bacterial cells per larva. Previous reports indicated that when insects could be killed with S. marcescens administered orally, doses on the order of 10⁵ to 10⁶ cells per insect were required 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 mortality was obtained was compared with another colony, and the S. marcescens strain was compared with other strains. Results indicate that the highest levels of mortality are associated with a particular combination of insect colony and bacterial strain. While the potential of this organism as a biological control agent has not been evaluated, these apparently unique interactions could make this system an interesting and potentially useful model for the study of host-pathogen virulence factors.