To assess whether plant phenology may affect potential for host shifts,S. altissimaandS. giganteawere monitored in the field in Minnesota (both host races), in Pennsylvania (S. altissima, sole host), and in a Pennsylvania common garden of both host plants from both sites. Plant height was measured every other day, and growth rates were calculated through the oviposition period in the field and season-long in the common garden. Greenhouse-grownS. giganteafrom both locations were subjected to oviposition byMinnesota Eurostato compare galling susceptibility. Minnesota hosts had more comparable growth rates and more intraspecific variability than the same species in Pennsylvania.S. giganteagrowth peaked earlier and fell more rapidly than that ofS. altissimafor all populations, suggesting an earlier, narrower window of host suitability. PennsylvaniaS. giganteawill support gall formation, but ramet growth rate was significantly less than that of MinnesotaS. giganteaunder greenhouse conditions, and MinnesotaEurostastrongly preferred Minnesota ramets to those from Pennsylvania. Variations in daily growth rate fluctuations between species may confoundEurosta'sdetection of suitable hosts. These results indicate a higher probability of a host shift occurring in Minnesota than in Pennsylvania and may indicate why colonization ofS. giganteahas not occurred in Pennsylvania.