The egg parasitoid Avetianella longoi Siscaro attacks two species of eucalyptus longhorned borers (Phoracantha semipunctata F. and P. recurva Newman) in southern California. During the past decade, P. recurva has replaced P. semipunctata as the dominant borer species, apparently due to differential rates of parasitism. The present study reveals that this replacement is due in part to a physiological defensive response mounted by one species (P. recurva) against parasitoid eggs and larvae, similar to the encapsulation/melanization immune response observed in larval host/parasitoid systems. Fluorescence microscopy and vital dyes confirmed that the defensive response was cellular in nature. Both Phoracantha species exhibited a cellular wound-healing response around the wasp egg pedicel, but the encapsulation of the wasp eggs/larvae was elicited only in eggs of P. recurva. This is the first conclusive evidence that hosts in the egg stage can mount a cellular immune response against a metazoan parasitoid, and may provide insight into the mechanisms underlying the host specificity of many egg parasitoids.