Deladenus siricidicola is a principal biological control agent used to suppress populations of the globally invasive pine pest, the woodwasp Sirex noctilio. Previous studies have reported low genetic diversity in D. siricidicola populations in biological control programs in the Southern Hemisphere and identified two additional, distinct lineages in North America and Spain. In this study, we tested the ability of these three lineages to interbreed and produce viable offspring. We used microsatellite markers to confirm admixture in offspring. Secondly, we compared growth rates among parental and admixed replicates on four isolates of the Amylostereum areolatum fungus on which nematodes typically feed in their asexual, non-parasitic phase. We show that all the lineages were capable of interbreeding and that admixture was asymmetric (i.e., skewed towards one of the parents). The offspring from one of the crosses showed significant variation in growth rate on different isolates of A. areolatum, compared to the parental isolates, but specifically on the slowest growing fungal isolate. Our results pave the way for the strategic introduction of genetic diversity into biological control programs and also inform expectations of accidental introductions of D. siricidicola into new regions.