Comparative Pathology of Isolates of Spodoptera frugiperda Nuclear Polyhedrosis Virus in S. frugiperda and S. exigua

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Abstract
S. frugiperda nuclear polyhedrosis virus was highly pathogenic of both S. frugiperda and S. exigus. Plaque-purified variants from 2 of the original isolates showed much greater differences in pathogenicity to the 2 insect species than the original isolates. Plaque-purified variants from 1 of the isolates (D) nearly lost pathogenicity for S. exigua while remaining pathogenic for S. frugiperda. Some of the plaque-purified variants produced atypical symptoms, even in S. frugiperda. These variants did not liquify larvae and release polyhedra when the larvae died as is typical for nuclear polyhedrosis infections in Lepidoptera. These variants also produced fewer polyhedra/g larval wt and often had fewer virons/polyhedron. Light and EM studies of S. frugiperda and S. exigua infected with 1 of the original isolates (A) and 2 of the plaque-purified variants (B2 and D7) indicated that only S. frugiperda infected with isolate A had the highly productive infection and viral morphogenesis typical of nuclear polyhedrosis viruses. Exposure of S. frugiperda to isolate B2 or D7 resulted in a slightly delayed infection characterized by increased amounts of abnormal viral morphogenesis and polyhedra of decreased size. S. exigua infected with isolate A or B2 had greatly reduced and delayed infections that were accompanied by highly variable abnormal viral morphogenesis; virtually no normal polyhedra were produced in these instances. Isolate D7 produced neither nucleocapsids nor polyhedra in S. exigua. Large paracrystalline aggregates of nucleocapsids were common in S. frugiperda infected with isolates B2 or D7 and in S. exigua infected by isolate A. Infection of S. exigua by isolate A or B2 was typified by the accumulation of large amounts of excess envelope membrane in the form of strands and vesicles of various sizes. Normal virogenic stromata were characteristics only of S. frugiperda infected with isolate A or B2. Polyhedra produced in S. exigua by even the most pathogenic isolates (A and B2) contained few if any normal varions and were not infective for either S. frugiperda or S. exigua.