Thrips calcaratusUzel is an introduced pest of basswood trees in North America. Its basic biology and life history are unknown because it is not injurious in its native European range. In the present study, the general life history ofT. calcaratusin Wisconsin was determined using emergence trapping, foliar sampling, and soil extraction. This species is univoltine; overwintered adults emerge from the soil in early spring, feed on buds as they open, and oviposit into the major veins on the undersides of leaves. The larvae feed on leaves, descend to the soil, and pupate. Degree-day models were computed for the free-living stages ofT. calcaratusand its host plant phenology. There is close synchrony betweenT. calcaratusemergence and basswood bud swelling and opening. Populations of three thrips associates—the pear thrips,Taeniothrips inconsequens(Uzel);the native basswood thrips,Neohydalothrips tiliae(Hood); and the predacious black hunter thrips,Leptothrips mali(Fitch)—were also monitored. The pear thrips is an introduced polyphagous pest that has caused severe damage to eastern maples and western fruit orchards but has not been heretofore reported in midwestern states.