Selection of a nucleopolyhedrovirus for control of Spodoptera frugiperda (Lepidoptera : Noctuidae): Structural, genetic, and biological comparison offour isolates from the Americas

Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) is the princip al pest of maize in tropical and subtropical regions of the Americas. Larva e of this species are susceptible to a nucleopolyhedrovirus (NPV) which has attracted interest as a potential biocontrol agent. Four strains of NPV is olated from infected S. frugiperda larvae in the United States, Nicaragua, and Argentina were subjected to a structural, genetic, and biological compa rison to select a candidate isolate for use in biocontrol experiments in Me xico and Honduras. All isolates had an occlusion body polyhedrin protein of 32 kDa, but the virions of each isolate differed subtly in the pattern and abundance of certain structural polypeptides revealed by SDS-PAGE analysis . Restriction endonuclease analysis of viral DNA confirmed that these isola tes were strains of a single virus species but showed that they were not ge netically homogeneous; each isolate could be differentiated from the others using common restriction enzymes. Droplet feeding bioassays indicated that an isolate from Nicaragua (Sf-NIC) and an isolate from the United States ( Sf-US) had the highest infectivity when tested against 2nd instars originat ing from a Honduran S. frugiperda colony. Na significant differences were d etected in the speed of kill of Sf-NIC (102.7 h), Sf-US (102.3 h), and Sf-A R (103.4 h), whereas that of Sf-2 (97.3 h) was significantly shorter. Addit ional bioassays of the Sf-NIC isolate against 2nd to 6th instars demonstrat ed that LC50 values increased with larval stage from 2.03 x 10(5) OBs/ml fo r 2nd instars to 1.84 x 10(8) OBs/ml for 5th instars. The concentration req uired to elicit a lethal infection of 6th instars was so high that a reliab le estimate of LC50 could not be obtained. The mean time to death for each stage challenged with the Sf-NIC isolate increased with instar from an aver age of 102.7 h in 2nd instars to 136.9 h in 5th instars.