Abstract
A laboratory bioassay technique was used to compare the susceptibility to a granulosis virus of 16 field populations of Phthorimaea operculella (Zell.) in Australia. A difference of 11·6× was found between the most and least susceptible population, while a laboratory strain was over 30 × as resistant as some field populations. It is suggested that this variability might reflect the past history of exposure of different populations to the virus, which appears to be endemic. The implications for the use of this virus as a microbial control agent are discussed.