The parasitic nematode Thripinema nicklewoodi is a potential inoculative biological control agent of the western flower thrips, Frankliniella occidentalis. Laboratory studies were undertaken to assess the effect of T. nicklewoodi infection on: 1) host feeding, 2) host fecundity, and 3) viral competency of F. occidentalis coinfected with a tospovirus. Individual thrips infected with nematodes as larvae and maintained on leaf discs in microcentrifuge vials showed a reduced feeding throughout the adult life span. This per capita reduction in feeding by parasitized individuals contributed to a total reduction in feeding of 87% on chrysanthemum petals and 91% on bean foliage relative to uninfected thrips. Parasitism also reduced the longevity of adult thrips by 3–5 d, although the preadult developmental time was unaffected. In a separate study, thrips infected with nematodes as larvae became reproductively sterile and appeared to have a reduced vector competency for tomato spotted wilt virus (TSWV). Thrips larvae were inoculated with both TSWV and nematodes in a factorial design, and adults were subsequently exposed to a petunia leaf disc assay to test for fecundity and virus transmission using enzyme-linked immunosorbent assay (ELISA). No eggs were recovered from thrips infected with nematodes, and dissections revealed that their embryos remained fully degenerate, which was not observed among healthy thrips. Although the proportion of thrips testing positive to a TSWV nonstructural protein (indicating systemic virus acquisition) was statistically similar between treatments, fewer viruliferous F. occidentalis coinfected with T. nicklewoodi became virus transmitters. Moreover, the per capita frequency of virus transmission among nematode-infected thrips was reduced by ≈50% relative to nematode-free thrips. Our results suggest that T. nicklewoodi may help prevent thrips outbreaks and reduce direct feeding damage and secondary virus spread during the prelethal period of infection.