Abstract Populations of Phoracantha recurva , a cerambycid beetle that appeared in California around 1995, have increased rapidly while populations of the congener Phoracantha semipunctata , that arrived in California before 1985, have dramatically declined in the southwestern part of the state where the two species occur together. Both species colonize stressed Eucalyptus trees and fallen branches in the shared habitat. The proportion of P. recurva in the mixed population increased rapidly from 0.1% in 1995, the first year this species was detected, to 4.7% the following year, and 74% in 1997. To determine whether differential parasitization of eggs of the two beetle species by an egg parasitoid may be a contributing factor in this apparent ecological replacement of one species with a congener, we evaluated oviposition preference and host suitability of eggs of the two congeneric beetle species using strains of the egg parasitoid Avetianella longoi reared on eggs of either P. semipunctata (S-strain wasps) or P. recurva (R-strain wasps) for multiple generations. In both choice and no-choice bioassays, female parasitoids of both strains preferred to oviposit in P. semipunctata eggs, and survival rates were much higher in P. semipunctata eggs than in P. recurva eggs, for host eggs of two age classes (0.5 and 2.5 days old). Preference and survival of progeny of R-strain or S-strain females on P. semipunctata eggs were not significantly different, indicating that the preference for P. semipunctata eggs as a host resource was innate and had not been affected by >15 generations of selection. A substantial fraction of P. recurva eggs parasitized by wasps of either strain survived parasitization and produced neonate larvae, whereas no P. semipunctata eggs survived parasitization. Furthermore, a significantly larger percentage of parasitized P. recurva eggs produced neither a parasitoid nor a neonate larva than parasitized P. semipunctata eggs. The size and sex ratio of progeny of either parasitoid strain were minimally affected by host egg species. Although P. recurva eggs were smaller in diameter and weight than P. semipunctata eggs, eggs of both species were large enough to support the development of several parasitoids, so it is unlikely that insufficient nutrition was a contributing factor to survival of parasitoids in eggs of either host species. Cumulatively, these results suggest that eggs of both species contain adequate nutrition for developing parasitoids, but that ovipositing A. longoi females and their developing progeny frequently are not able to terminate P. recurva egg development.