Abstract.1The oak treehopperPlatycotis vittata(Fabricius) is bivoltine throughout its range with discrete generations in the early spring and autumn. Females of both generations diapause.2Latitudinal variation in the timing of life history exists among four populations ofP. vittataalong the eastern coastal plain of the United States.3We disrupted synchronization betweenP.vittatalife history and host‐plant phenology in the springs of 1984 and 1985 by placing southern populations on northern trees with a different phenology. This caused some females to oviposit abnormally in late spring, without entering diapause. The offspring of these females had reduced reproductive success.4Winter diapause can be broken by providing females with a newly flushing tree with high levels of amino nitrogen in the sap. Females terminated winter diapause on trees breaking winter dormancy while females on trees still in winter dormancy did not, under identical photoperiod and temperature regimes.5The role of host‐plant mediated life history variation as a process promoting intraspecific geographic differentiation in the arboreal sap‐feeding guild is disc