Cascading reproductive isolation: plant phenology drives temporal isolation among populations of a host-specific herbivore

All organisms exist within a complex network of interacting species, thus evolutionary change may have reciprocal effects on multiple taxa. Here, we demonstrate “cascading reproductive isolation,” whereby ecological differences that reduce gene flow between populations at one trophic level affect reproductive isolation (RI) among interacting species at the next trophic level. Using a combination of field, laboratory and common-garden studies and long-term herbaria records, we estimate and evaluate the relative contribution of temporal RI to overall prezygotic RI between populations of Belonocnema treatae, a specialist gall-forming wasp adapted to sister species of live oak (Quercus virginiana and Q. geminata). We link strong temporal RI between host-associated insect populations to differences between host plant budbreak phenology. Budbreak initiates flowering and the production of new leaves, which are an ephemeral resource critical to insect reproduction. As flowering time is implicat...