Repeated reversion of resistance to nucleopolyhedrovirus by anticarsia gemmatalis.

A laboratory population of Anticarsia gemmatalis subjected to 80% mortality by the A. gemmatalis nucleopolyhedrovirus (AgNPV) developed a 5x resistance ratio (RR) within four generations. When exposure to AgNPV was discontinued, the resistant insects returned to their original level of susceptibility within three generations. Two additional cycles of resistance selection and reversion were repeated in the same insect population with similar results, except that the insects reached a 5x RR within three generations in the second cycle and two generations in the third. The insects returned to the original level of susceptibility within two generations in both the second and third resistance/reversion cycles. The resistant insects produced 61% as many viable neonates as susceptible insects. Resistant insects also had longer life spans, a lower rate of larval survival in rearing, and lower pupal weights than susceptible insects. These differences in life histories, all significant at P < 0.01, indicate that A. gemmatalis resistance to AgNPV might have associated costs in fitness. The addition of an optical brightener (OB) to the AgNPV suspension increased the susceptibility of resistant and susceptible insects by 24x and 58x, respectively, resulting in an increase of the RR from 4.7 to 11.2x. Thus, the LC50 of AgNPV with an optical brightener against resistant insects was less than the LC50 of AgNPV without brightener against susceptible insects. Copyright 1998 Academic Press.