EFFECTS OF DIET-AGE AND STREPTOMYCIN ON VIRULENCE OF AUTOGRAPHA-CALIFORNICA M NUCLEOPOLYHEDROVIRUS AGAINST THE TOBACCO BUDWORM

Addition of the antibiotic streptomycin to two artificial diets routin ely used in bioassays of neonate larvae of Heliothis virescens (tobacc o budworm) infected with Autographa californica M nucleopolyhedrovirus (AcMNPV) increased lethal times of the virus. After storage of diets for 3 weeks at 4 degrees C, lethal times of infected larvae were signi ficantly slower compared to those for larvae bioassayed using diets st ored for 2 weeks or less. The effect of diet-age on rate of mortality was not the result of a change in total protein content or pH of the d iet, but was apparently the result of some other alteration in the qua lity of the diet (e.g. microbial spoilage, palatibility, and/or nutrit ional value unrelated to total protein). Although we did not determine why lethal times were slower in response to streptomycin concentratio n or diet-age, we did find that slower lethal times were correlated wi th slower relative growth rates (RGR) of infected larvae. In addition, RGR of infected larvae decreased as a function of increasing streptom ycin concentration, diet-age, and the interaction of the two factors. These results demonstrate that it is difficult to obtain consistent an d comparable bioassay results if antibiotic composition and diet-age a re not controlled. We suggest a standardized diet or highly standardiz ed procedures for a given diet be developed that permits comparison of bioassays among and within laboratories. (C) 1997 Academic Press.