Transmission of Serratia marcescens (Enterobacteriaceae) in adult Heliothis virescens (Lepidoptera: Noctuidae) laboratory colonies

The red strain of Serratia marcescens (Bizio) has been isolated from healthy, diseased, or dead field-collected insects, but it is most frequently reported as a pathogen of insectary-reared insects. In this study, we determined the path of transmission of S. marcescens in adult Heliothis virescens (F.) to the next generation and compared longevity of infected and uninfected adults. Our results show that S. marcescens can be present on or within egg produced by S. marcescens-infected pairs. Highest external contamination and/or internal infection of surface sterilized eggs from moths fed an aqueous solution of 10% honey contaminated with S. marcescens (4 X 10(8) bacterial cells/ml) was 45% and internal infection of eggs from the same group of moths was 19%. The highest percentage of adults producing eggs contaminated and/or infected with S. marcescens was 70% and the highest percentage producing internally infected eggs was 17%. The highest percentage of total adult mortality at day 10, in bacterial treatments, was 90.1% and in the controls was 15%. Living infected moths are usually asymptomatic, but typically become red postmortem. We demonstrated that S. marcescens can be transmitted both vertically (genetic transmission; the passage of pathogen from one host generation to the next in or on egg) and horizontally (transmission of pathogen between insects at any age after birth) in H. virescens, and that the bacterium has detrimental effects on adult longevity. We point out in ths paper that S. marcescens has been frequently reported from poor quality insectary-reared H. virescens (poor vigor and high mortality). This insect which is commonly used for a host of beneficial endoparasitoids, production of viruses and protozoa, and testing various biological control agents, must be pathogen free (such as S. marcescens) to provide quality biological control agents or meaningful data.