Density-dependent competition between egg parasitoids of fall armyworm

The fall armyworm ( Spodoptera frugiperda ) is attacked by multiple egg parasitoid species that may co-exploit the same host resources, leading to interspecific competition that can influence biological control outcomes. Among the most widely used egg parasitoids are Telenomus remus and Trichogramma pretiosum , yet their relative performance under different host egg densities remains poorly understood. We investigated density-dependent competitive interactions between these two parasitoids on fall armyworm eggs under both simultaneous and isolated release conditions. Host egg densities of 15, 30, and 60 eggs were used to assess parasitism rate, sex ratio, developmental time, and intrinsic competition. The relative parasitism success of the two parasitoids varied with host density. Telenomus remus parasitised more hosts at medium and high densities, whereas Tr. pretiosum performed better at low host density. Multiparasitism was detected, but only one parasitoid species completed development within an individual host egg. Embryological observations indicated that intrinsic competition occurred during early embryogenesis, with T. remus embryos frequently undergoing developmental arrest in multiparasitised host eggs. T. remus produced a more female-biased sex ratio, while Tr. pretiosum completed development more rapidly across host densities. These results show that host egg density can mediate competitive outcomes between egg parasitoids and should be considered when evaluating parasitoid species for fall armyworm biological control.