ROLE OF HOST-PLANT PHENOLOGY IN HOST USE BY EUROSTA-SOLIDAGINIS (DIPTERA, TEPHRITIDAE) ON SOLIDAGO (COMPOSITAE)

To assess whether plant phenology may affect potential for host shifts, S. altissima and S. gigantea were 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-grown S. gigantea from both locations were subjected to oviposition by Minnesota Eurosta to compare galling susceptibility. Minnesota hosts had more comparable growth rates and more intraspecific variability than the same species in Pennsylvania. S. gigantea growth peaked earlier and fell more rapidly than that of S. altissima for all populations, suggesting an earlier, narrower window of host suitability. Pennsylvania S. gigantea will support gall formation, but ramet growth rate was significantly less than that of Minnesota S. gigantea under greenhouse conditions, and Minnesota Eurosta strongly preferred Minnesota ramets to those from Pennsylvania. Variations in daily growth rate fluctuations between species may confound Eurosta's detection of suitable hosts. These results indicate a higher probability of a host shift occurring in Minnesota than in Pennsylvania and may indicate why colonization of S. gigantea has not occurred in Pennsylvania.