Abstract

Serratia marcescens (Bizio) is a common bacterium associated with insects reared under laboratory conditions, and it is capable of severely compromising the quality of insects produced in insectaries. This entomopathogenic bacterium is a member of the Enterobacteriaceae (facultative anaerobic, gram-negative rods that are cytochrome oxidase negative and catalase positive), and they usually produce a characteristic red or pink pigment, although white to rose-red strains frequently occur. To better understand the impact of this bacterium on laboratory-reared insects, with the goal of reducing its impact, we undertook a series of experiments using Heliothis virescens (F.) as a model host. Although we observed considerable variation in mortality among replications, regardless of the age of H. virescens larvae exposed to S. marcescens and the kind of diet used, relatively few infected larvae survived to the adult stage; the prevalence of adult infection was consistently ≤20%. S. marcescens was not eliminated from larvae reared on an artificial diet containing antimicrobial agents or from larvae maintained on an acidified diet. However, the impact of the bacterium on insects on the artificial diet tended to be lessened. A substantially higher prevalence of insects inoculated with S. marcescens on cotton leaf pieces and subsequently maintained on a standard artificial diet became infected and died relative to the other two treatments. In all instances, the longevity of infected moths was decreased, and living infected moths usually were asymptomatic but typically turned red postmortem. Evidence indicated that the bacterium was transmitted both on the surface and within eggs (i.e., transovum transmission) laid by infected females. Therefore, surface-sterilization of eggs alone is not adequate to eliminate Serratia from the insect population. However, because the frequency of egg infection was low, treatment of eggs in sodium hypochlorite greatly reduced the prevalence of larval infection. Sterile tap water is used commonly to prepare both liquid and solid diets fed to larvae and moths in insectary cultures, and we observed that S. marcescens survived for at least 1 yr in sterile tap water. Sugar and honey solutions (10%) are used commonly to maintain adults, and both solutions supported the growth of the bacterium. Thus, any aqueous solution used in insect culture should be made with freshly sterilized liquids immediately prior to use, and adult feeding-solutions should be changed daily. S. marcescens can cause substantial losses to an insect colony, and the primary goal in the management of this bacterium is prevention. However, S. marcescens can persist in a colony at low frequencies that can easily escape detection until factors (e.g., physiological stressors) incite an epizootic. Efficacious surface-sterilization of eggs is essential, but other sanitation (e.g., sanitation of adult diets) and good husbandry (e.g., reduce/eliminate stress) strategies also are important in reducing the impacts of S. marcescens in insectaries.