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The susceptibility of third instar larvae of Anticarsia gemmatalis (Lepidoptera: Noctuidae) and Diatraea saccharalis (Lepidoptera: Pyralidae) to ten distinct plaque purified genotypic variants of a selected isolate of the Anticarsia gemmatalis multiple‐embedded nuclear polyhedrosis virus (AgMNPV), was compared. Despite the fact that this isolate, AgMNPV‐Ds20, represents a wild strain of the AgMNPV selected for higher virulence to D. saccharalis, an alternate host, most of the variants are much more virulent to the original host Anticarsia than to Diatraea. Bioassays have shown an over one hundred‐fold variation in LD50 values ranging from 1700 polyhedron inclusion bodies (PIBs) to more than 200 000 PIBs/larva. The PIB production in infected larvae increased with the pathogenicity of the variant to the host, showing an average ten‐fold reduction in Diatraea when compared to Anticarsia for the same variant. The virus particle yield ranged from 6×107 to more than 109 PIBs/g of infected larvae in Diatraea and from 8×108 to more than 1010 PIBs/g of infected Anticarsia larvae. The data show a clear difference of the pathogenicity of the genotypic variants of AgMNPV in vivo both between the original and alternate host and between the individual variants for the same host. These differences found in vivo indicate that monitoring of shifts in variant frequency of wild and laboratory‐propagated viral isolates in these highly heterogeneous populations would help ensure the efficacy of biological control programs.

The multiple-embedded velvetbean caterpillar nucleopolyhedrosis virus (VBC-NPV) of Anticarsia gernrnatalis HiJBNER was shown to be infectious to a variety of noctuid hosts. The mortality data demonstrated the importance of defining the dosage used in host range analysis. Serial passage of the VBC-NPV through the soybean looper, Pseudoplusia includens, was done to select for host range variants of this NPV. After several passages of the VBC-NPV through this insect the virulefice of the progeny virus remained unchanged indicating heterogeneity in the host and not the virus population. However, between the 3rd and 5th serial passage through Pseudoplusia a latent NPV identical to a single-embedded NPV previously associated from A. gemmatalis was activated. The biologica[ and biochemical characteristics of this isolate demonstrated it to be distinct from the original VBC-NPV. A single passage of this activated virus through A. gemmatalis resulted in the production of viral progeny having characteristics of the original VBC-NPV. Serial passage of this virus preparation through P. includens resulted in virus progeny having biological properties associated with both the original VBC-NPV and the activated NPV isolate.

Development of infection of 4 strains of nuclear polyhedrosis virus (NPV) in third instar Diatraea saccharalis larvae at 10 different temperatures was compared. Two wild strains, the Anticarsia gemmatalis MNPV (AgMNPV) and the Trichoplusia ni MNPV (TnMNPV), and two selected strains, AgMNPV‐D10 and TnMNPV‐D11 were inoculated in 11‐day‐old D. saccharalis larvae and kept in temperatures of 17, 22, 24, 26, 28, 30, 32, 35, 37 and 39°C. The results showed that larval mortality increased with temperature, and the increase of temperature caused also the reduction of lethal time 50% (LT50) values. The maximal polyhedral production was obtained at 28°C for the 4 viral strains and a drastic reduction in the number of polyhedra was observed even for a difference of 2°C above or below this ideal temperature. The selected strains showed a 10‐fold increase in polyhedra yield when compared to the wild type strains.

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