Insect growth regulating (IGR) and toxic effects evoked by Ledum palustre extracts on the metamorphosis stages of Tenebrio molitor were studied using differential thermocouple calorimeters. The timing of normal and failed ecdysis as well as the length of interecdysial periods were measured exactly from calorimetric recordings. Respiratory and muscular responses of poisoned insects were also reflected by characteristic patterns on the recordings.