The DNA restriction map for the enzymes BamHI, BglII, PstI, and XbaI of SeMNPV-US (Se-US), the best-studied Nucleopolyhedrovirus strain isolated from Spodoptera exigua in California, was used as a reference to construct that of SeMNPV-SP2 (Se-SP2), a closely related Spanish strain of the same virus. After coinfection of S. exigua with both the Se-US and Se-SP2 strains per os, a recombinant virus (SeMNPV-SUR1 [Se-SUR1]) was detected after one passage which quickly replaced the parental viruses. A physical map of Se-SUR1 DNA was constructed for BamHI, BglII, PstI, and XbaI and compared to that of the parental viruses, Se-US and Se-SP2. Se-SUR1 is the result of at least four crossover events between Se-US and Se-SP2 and not of selection of a minor variant in one of the parental viruses. Bioassays of the recombinant and parental strains against L2 beet armyworm larvae demonstrated that their 50% lethal dose values were not significantly different. The differences in median lethal time values are too small to explain the replacement of the parental viruses by the recombinant virus upon successive passage in vivo, although it cannot be ruled out as an explanation for the selective advantage of the recombinant strain, Se-SUR1. The consequences of the release of nonindigenous or recombinant baculovirus strains in agro-ecosystems are discussed
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