The Biogeography of a Disjunct Plant-Insect Relationship: Thimbleberry and Diastrophis kincaidii (Hymenoptera: Cynipidae) in the Great Lakes Region

The clonal shrub thimbleberry (Rubus parviflorus) ranges throughout western North America and is disjunct to the northern Great Lakes region. Diastrophis kincaidii (Hymenoptera: Cynipidae) is an obligate gall wasp of thimbleberry that has long been known from western North America but has only been reported from the Great Lakes region since the early 1990s. It has been suggested that Diastrophis kincaidii was only recently introduced into the Great Lakes region sometime in the 1980s. We conducted a survey to determine the distribution of D. kincaidii and its parasitoids within the thimbleberry range in the Great Lakes region. We found that D. kincaidii is restricted to the colder and more xeric habitats within the Great Lakes thimbleberry range. Additionally D. kincaidii was found to have colonized isolated micro-habitats in the region were it attained high population densities. The inquiline community inhabiting D. kincaidii in the Great Lakes Region was similar to past reports. This community included species from eastern and western North America as well as two undescribed species. We suggest that D. kincaidii and its inquiline community have long been a part of the insect fauna of the Great Lakes region and is likely a remnant of an original connected pre-glacial continental thimbleberry population.