GEOGRAPHIC AND HOST SPECIES EFFECTS IN EUROSTA-SOLIDAGINIS (DIPTERA, TEPHRITIDAE) MORTALITY

Eurosta solidaginis (Fitch) infesting the host plant Solidego gigantea Ait form larger galls than individuals attacking the presumed ancestral host, S. altissima L. Because previous studies found that Eurytoma gigantea Walsh parasitism is restricted to smaller galls, it was hypothesized that this gall-size difference could result in decreased parasitism of E. solidaginis infesting S. gigantea. E. solidaginis galls were collected along a transect from Pennsylvania to Minnesota and from two sites in Iowa to determine E. solidaginis survivorship and causes of mortality. Although S. gigantea galls were generally larger than S. altissima galls, there was no significant decrease in E. gigantea parasitism for E. solidaginis occupying S. gigantea galls. Galls containing the parasitoid E. gigantea were significantly larger on the host plant S. gigantea than on S. altissima. Parasitoid access to E. solidaginis was not limited by the larger size of S. gigantea galls. Eurytoma obtusiventris Gahan parasitism was restricted to galls on S. altissima and this parasitoid occurred only in the more eastern parts of our transect. Differences in gallmaker survivorship and natural enemy occurrence between Solidago hosts at sympatric sites were dependent on site; predation by the beetle Mordellistena unicolor Lee. was the primary determinant of differences in gallmaker survivorship between Solidago hosts and among geographic sites.