Impacts of the introduced basswood thrips ( Thrips calcaratus Uzel) on forest health in the Great Lakes region

Abstract

Insect, disease, tree condition, weather, and soil data were sampled from 22 northern hardwood forest sites containing American basswood, Tilia americana L., in Michigan, Wisconsin, and Minnesota from 1998 to 2000. Basswood dieback increased by 10% during this period. Insect abundance on basswood foliage was dominated by Thysanoptera, of which the introduced basswood thrips, Thrips calcaratus Uzel, accounted for over 95% of total catch. There was little evidence of pathogens or abiotic stress factors having pronounced effects on basswood. Weak associations between basswood dieback and T. calcaratus abundance suggest that it is an important component of basswood decline in the Great Lakes region, but that other factors are also involved. The native predator, Leptothrips mali (Fitch), is highly synchronized with the native basswood thrips, Neohydatothrips tiliae (Hood), with both species emerging in late spring and persisting into August. The two introduced herbivores, T. calcaratus and Taeniothrips inconsequens (Uzel), peak in late spring before becoming almost totally absent by August. Numbers of adult T. calcaratus in early May can be used to predict subsequent numbers of larvae. T. calcaratus abundance was more variable among sites than among trees within a site. This indicates that future investigations will benefit most by increasing the number of study sites rather than more intensively surveying fewer sites. A significant year-by-site interaction was observed for both adult and larval T. calcaratus. Adult populations were inversely associated with organic matter, calcium, nitrate, ammonium, soil pH, summer precipitation, April minimum temperature, April precipitation, and basswood live crown ratio. They were directly associated with low winter temperature and basswood crown light exposure. Larval T. calcaratus were inversely associated with soil potassium, precipitation, basswood live crown ratio, and basswood crown position. They were directly associated with low winter and spring temperatures, basswood light exposure, and winter snowfall. Interpretation of these relationships must be tempered, however, by the presence of autocorrelations among many of these variables. This study shows that basswood is declining throughout the Great Lakes region, and T. calcaratus, along with other contributing factors, are related to this phenomenon.