Is the outbreak status of Thrips calcaratus Uzel in North America due to altered host relationships?

The introduced basswood thrips, Thrips calcaratus, undergoes outbreaks on and damages American basswood, Tilia americana, in North America, but is less common on and does not cause significant damage to little-leaf linden, Tilia cordata, in its native European range. A possible explanation is that altered host relationships in North America allow increased exploitation of T. americana. Three experiments were conducted to compare host associations of T. calcaratus between T. americana and T. cordata. In a laboratory choice bioassay, T. calcaratus occupied foliage of both tree species with equal frequency. A field assay conducted in North America revealed an identical trend on planted T. americana and T. cordata seedlings planted in a paired design. In a separate field experiment bud break phenology, peak T. calcaratus emergence, and foliar development showed similar temporal patterns in 10 T. cordata and T. americana sites distributed across the insect's native range of Europe and introduced range of North America. These results suggest that physiological and phenological differences between native and newly acquired Tilia hosts are not likely responsible for the increased impact of T. calcaratus in its introduced range. Rather, temporal escape from natural enemies seems to be the more likely explanation. This is evidenced by the stronger synchrony of the North American predator, Leptothrips mali, with the native basswood thrips, Neohydatothrips tiliae, than with T. calcaratus. Understanding the basis for increased success of T. calcaratus in its introduced range may provide insight into managing this insect in basswood forests in the Great Lakes region.