Growth and Survivorship Differences in Eurosta (Diptera: Tephritidae) Galling Sympatric Host Plants JOHN P. LICHTER,l ARTHUH. E. WEIS,. ANDCURT R. DIMMICK3 Department of Biological Sciences, Northern Illinois University, DeKalb, Illinois 60115 Environ. Entomol. 19(4): 972-977 (1990) ABSTRACT Life history characteristics of the goldenrod gallmaker, Eu.rosta solidagini8 Fitch (Diptera: Tephritidae), were compared between individuals using separate host species. Infestation rate, gall size, gall maker larval weight, gall maker mortality, and parasitoid/ predator attack frequencies were examined. Galls from Solidago alti8sima L. (Compositae) averaged 10% less in diameter than those of S. gigantea Ail. Discordantly, the larval weights of the S. altissima larvae were 26% greater. Gallmaker mortality was approximately equal, although the underlying causes differed. Some of these differences can be explained by difference in gall size. The S. alti8sima galls, with their smaller mean size, were more vulnerable to attack by the parasitoid wasp, Eurytoma gigantea Walsh (Hymenoptera: Eurytomidae). On the other hand, S. gigantea galls, with their greater mean size, were more vulnerable to attack by birds. Early larval death of the gallmaker was greater on S. gigantea. Although survivorship did not differ on the two host plants, the larger size of larval from S. alti8sima would suggest that females emerging from this host would have greater fecundity. Although S. altissima appears to be the better host, S. gigantea was more heavily infested. The seemingly maladaptive lack of correlation between infestation and performance could be explained if the insects from the two plant species represent two host races. KEY WORDS Insecta, Eu.rosta, Solidago spp., parasitoids, gallmaker THE TEPHRITIDEurosta solidaginis Fitch induces a spheroid gall on the stem of goldenrod. Although Solidago altissima L. (also known as Solidago can- adensis var. scabra) is the most commonly used host (Uhler 1951, Miller 1959), it has been recorded from a variety of host plants throughout its geo- graphic range (Wasbauer 1972, Abrahamson et al. 1989a). Across the upper midwestern United States, Eurosta galls are frequently seen on Solidago gi- gantea Ait (Abrahamson et al. 1989a; A.E.W., un- published data). Isozyme analysis by G. L. Waring & W. G. Abrahamson (personal communication) suggests that the insects on these two hosts consti- tute genetically differentiated races. Sympatric populations of S. altissima and S. gigantea often support large infestations of the insect. Melville & Morton (1982) suggested that these two distinct host plant species are part of polyploid complex stemming from the diploid S. canadensis var. canadensis (2N = 18) (but d. Semple 1984). S. altissima is a hexaploid throughout its range (2N = 54), whereas S. gigantea collections from Illinois are tetraploid (2N = 36) (Semple 1984). Although the host range of Eurosta is well doc- umented and a deep knowledge of its natural his- I Current address: Department of Ecologyand Behavioral Bi- ology, University of Minnesota,Minneapolis,Minn. 55455. 'Current address: Department of Ecology and Evolutionary Biology,University of California, Irvine, Calif. 92717. • Current address:Department of BiologicalSciences.Northern Arizona University, Flagstaff, Ariz. 86001. tory on S. altissima has accumulated (Uhler 1951; Miller 1959; Weis & Abrahamson 1986; Abraham- son et al. 1989a,b), little information on the com- parative ecology between host plants is available. In this study we examined the influence of host plant on the size and survivorship of larvae. A key goal was to determine whether host plants influence the rate of natural enemy attack. Previous obser- vation had indicated that Eurosta gall size varies with host plant species. Gall size on S. altissima is known to influence the vulnerability of the gall- maker to natural enemies (Cane & Kurczewski 1976, Weis & Abrahamson 1986). Whereas the parasitoid Eurytoma gigantea cannot penetrate the walls of large galls to oviposit (Weis et al. 1985), the fre- quency of attack by the downy woodpecker is greater on larger galls than smaller ones (Weis & Abrahamson 1986). If gall size differs between host plants, t~en differential attack by natural enemies would be expected. We tested this prediction in the field. Materials and Methods The study site was located at Castle Rock State Park, Ogle County, Ill., where both host plants grow abundantly on the floodplain of the Rock River. Over-wintered galls were collected during the spring of 1987. Comparisons were made between gallmakers from the two hosts in gall size and larval weight. If the gall is the food source for the gallmaker, the 0046-225Xj90j0972-0977$02.oojO © 1990 EntomologicalSocietyof America Downloaded from https://academic.oup.com/ee/article/19/4/972/2480462 by guest on 17 November 2021