Immigrant inviability can contribute to reproductive isolation (RI) during ecological speciation by reducing the survival of immigrants in non‐native environments. However, studies that assess the fitness consequence of immigrants moving from native to non‐native environments typically fail to explore the potential role of concomitant reductions in immigrant fecundity despite recent evidence suggesting its prominent role during local adaptation. Here, we evaluate the directionality and magnitude of both immigrant viability and fecundity to RI in a host‐specific gall‐forming wasp, Belonocnema treatae. Using reciprocal transplant experiments replicated across sites, we measure immigrant viability and fecundity by comparing differences in the incidence of gall formation (viability) and predicted the number of eggs per female (fecundity) between residents and immigrants in each of two host‐plant environments. Reduced immigrant viability was found in one host environment while reduced immigrant fecundity was found in the other. Such habitat‐dependent barriers resulted in asymmetric RI between populations. By surveying recent literature on local adaptation, we find that asymmetry in immigrant viability and fecundity are widespread across disparate taxa, which highlights the need to combine estimates of both common and overlooked barriers in cases of potential bi‐directional gene flow to create a more comprehensive view of the evolution of RI.

A critical question in understanding the origin of trade-offs and their impact on trait evolution is whether trade-offs between species originate from within-species variation. Despite their importance, studies linking interspecific divergence between closely related species to intraspecific trait variation are still rare. In this study, we describe a trade-off between dispersal and reproductive effort between two sympatric sister species of wasps in the genus Belonocnema (Hymenoptera: Cynipini: Cynipidae) that form galls on live oaks: B. fossoria, which specializes on Quercus geminata, and B. treatae, which specializes on Q. virginiana. Specifically, our results suggest that B. fossoria has evolved reduced flight capability and smaller wings, but a larger abdomen and greater total reproductive effort than B. treatae, which has larger wings and is a stronger flier, but a smaller abdomen and reduced total reproductive effort. Despite these significant morphological and reproductive differences, these traits remain unchanged when transplanting B. fossoria and B. treatae onto the alternative host plant, suggesting that divergence in these traits is likely genetic as opposed to a plastic response to the different rearing environments. However, we did not find evidence of intraspecific trade-offs between wing length and reproductive traits within either B. fossoria or B. treatae, indicating that trade-offs in life history traits between the two species is a result of independent adaptations in response to different environments. Our study informs our understanding of the evolution of trade-offs among life history traits by examining trade-offs at different biological organizations.