Laboratory studies were conducted to determine the relative susceptibility between 2nd and 5th instar larvae of Malacosoma americanum to foliage of both wild cherry Prunus serotina Ehrh., a preferred host, and sugar maple, Acer saccharum Marsh., an occasional host treated with Bacillus thuringiensis Berliner subsp. Kurstaki, Dipel ES. The effect of these hosts on the efficacy of Bt also was determined for 3rd instar ,M. americanum larvae. Second instar M. americanum larvae had lower LC50 values and were more susceptible to leaves treated with Bt compared to that obtained for 5th instar larvae on both cherry and maple hosts. In the host-plant effect study, 3rd instar larvae of M. americanum, had higher LC 50 values when fed Bt treated wild cherry compared to that obtained from Bt treated maple foliage. The information helps to define a planning strategy for Bt resistance management of M. americanum especially in circumstances when larger larvae on preferred hosts may be encountered.