Development of Resistance byAnticarsia gemmatalisfrom Brazil and the United States to a Nuclear Polyhedrosis Virus under Laboratory Selection Pressure

Colonies of velvetbean caterpillar,Anticarsia gemmatalis,collected in Brazil (Dourados, State of Mato Grosso do Sul; and Sertanopolis, State of Paraná) and in the United States (Louisiana) were established in the laboratory, in each respective country, with the objective of evaluating the potential of this insect for developing resistance to theA. gemmatalisnuclear polyhedrosis virus (AgMNPV). Larvae of the three colonies were exposed to a selection pressure of ca. 80% mortality by AgMNPV in each generation, while a portion of the population was left unchallenged (control). Larvae of selected and control populations were bioassayed with AgMNPV isolate LD-79. Log-dose–probit regression lines and associated parameters were used to compare the selected and control insects in each generation. All three insect populations developed a significant level of resistance within 3–4 generations of exposure to AgMNPV, based on comparison of LC50s between AgMNPV-exposed and control insects and non-overlap of 95% CL. The two populations from Brazil developed resistance ratios >1000× within 13–15 generations of exposure to virus, whereas the resistance ratio in the United States leveled at approximately 5× after 4 generations. Thus,A. gemmatalishas a high potential of developing resistance in Brazil, where the virus occurs naturally and is extensively employed as a microbial pesticide. In the United States, the potential for resistance interfering with a microbial insecticide based on AgMNPV is lower.