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Impatiens capensis (Balsaminaceae) Meerb. is a Necessary Nurse Host for the Parasitic Plant Cuscuta gronovii (Cuscutaceae) Willd. ex J.A. Schultes in Southeastern Michigan Wetlands

Abstract Generalist parasites have been shown to have the potential to substantially affect the structure of the communities they inhabit. In order to predict the potential effects of these parasites, understanding the relationships they have with their host species is critical. In this study, the host range of Cuscuta gronovii was determined at two different times during the growing season, which corresponded with two life stages of parasitic individuals, seedling and adult. Field observations suggest that most of the successful infections by seedlings occurred upon only Impatiens capensis, one of the many species that it infects as an adult. This paper reports three tests of the hypothesis that I. capensis is a necessary nurse host for the parasite C. gronovii. In early summer survey plots, I. capensis was found to be infected by C. gronovii seedlings significantly more than predicted by the null hypothesis that all species in the plot were equally likely to be infected. In a late winter survey, in which all plants in a wetland infected with C. gronovii were identified and their positions mapped, I. capensis hosts were significantly closer to non-I. capensis hosts than predicted by the null hypothesis that infected I. capensis were distributed independently of non-I. capensis hosts. Finally, in a field experiment, newly germinated C. gronovii seedlings that were tied onto I. capensis and two other common hosts succeeded in infecting only on I. capensis. These results suggest that I. capensis acts as a necessary nurse host for C. gronovii in spring flooded southeastern Michigan wetlands.