Technical Abstract: ScathL is a cathepsin L-like cysteine protease from the flesh fly, Sarcophaga peregrina, that digests components of the basement membrane during insect metamorphosis. A recombinant baculovirus that expresses ScathL (AcMLF9.ScathL) kills larvae of the tobacco budworm, Heliothis virescens, significantly faster than the wild-type virus and triggers melanization and tissue fragmentation in infected larvae shortly before death. Since basement membranes are a potential barrier to the spread of baculovirus secondary infection to other tissues in the host, we tested the hypothesis that the rapid death of insects infected with AcMLF9. ScathL was caused by accelerated secondary infection resulting from the degradation of host basement membranes by ScathL. Viruses expressing catalytically active or inactive ScathL were used to examine the effects of ScathL activity on the appearance and timing of budded virus in the hemocoel during infection, the production of polyhedra in infected larvae, and the rate of infection of the gut, trachea, hemocyte, fat body, and malpighian tubules. We conclude that the enhanced insecticidal efficacy of the recombinant baculovirus that expresses ScathL does not result from altered tissue tropism or accelerated systemic infection. Implications for the role of the basement membrane as a barrier to baculovirus dissemination within the host insect are discussed.