| Foliage was sampled and crown condition was assessed at 22 forest sites in the Great Lakes region from 1998--2000 to determine the health of American basswood, Tilia americana, and identify associated stressors. Thysanoptera dominated insect abundance, and little evidence of pathogens or abiotic stress was apparent. Thrips calcaratus, the introduced basswood thrips, comprised over 95% of insect abundance, with 131.647 obtained. Its abundance was more variable among sites than trees. The introduced herbivores T. calcaratus and Taeniothrips inconsequens emerged in early spring. In contrast, the native predator, Leptothrips mali, was synchronized with the native basswood thrips, Neohydatothrips tiliae, emerging in late spring and persisting into August. T. calcaratus numbers were associated with basswood dieback, which increased by 10% over three years, while foliage transparency and crown density displayed variable trajectories. Sampling of T. calcaratus by three methods, pole-pruning, shotgun, and tree climbing, was compared. Tree-climber and pole-pruner data were highly predictive of each other. Pole-pruning is limited to lower branches, the shotgun is prohibited near municipalities, and tree-climbing is limited by high monetary cost. Site factors associated with T. calcaratus populations were investigated. Adult T. calcaratus were inversely associated with organic matter, calcium, nitrate, ammonium, soil pH, precipitation, spring temperature, and live crown ratio, and directly associated with low winter temperature and light exposure. Larvae were inversely associated with potassium, precipitation, live crown ratio, and crown position, and directly associated with low temperatures, light exposure, and snowfall. Three experiments were conducted in 2001 to compare host associations of T. calcaratus on European Tilia cordata versus North American T. americana. A laboratory bioassay determined equal preference for T. americana and T. cordata. Likewise, similar numbers of T. calcaratus were obtained from T. americana and T. cordata seedlings planted in the field. T. americana and T. cordata budbreak and leaf opening, and T. calcaratus emergence, occurred at similar dates at ten field sites in Europe and North America. Results suggest that host associations are not responsible for differential damage of T. calcaratus between Europe and North America. Rather, asynchrony with native predators appears primarily responsible. |
