The gregarious, multivoltine braconid wasp Glyptapanteles liparidis (Hymenoptera: Braconidae) is an important natural enemy of larvae of the gypsy moth, Lymantria dispar (Lepidoptera: Erebidae), a serious forest pest insect. The gypsy moth has only one generation per year and the overwintering stadium is the egg. Thus, to maintain the population, the wasps depend on hosts that overwinter in the larval stage. The aim of the thesis was to investigate whether larvae of the browntail moth, Euproctis chrysorrhoea (Lepidoptera: Erebidae), that hibernate in nests on their food plants, are suitable overwintering hosts of G. liparidis parasitoids. Additionally, the overwintering behaviour of the browntail moth larvae were documented and oxygen consumption and supercooling point were recorded during the winter months. Larvae of browntail moth were collected from of a colony on Sorbus aria trees and kept under long day and short day photoperiods at 20C and under outdoor conditions for overwintering. Irrespective of temperature and photoperiod, the larvae stopped feeding in autumn and entered a state of rest, remaining in the second larval instar. Frost resistance increased continuously from October to February and decreased in spring. A decrease in respiration rates was not observed. This suggests an obligatory diapause in the overwintering larvae. Browntail moth larvae that were parasitized by G. liparidis wasps in autumn (October, November) showed similar behaviour as unparasitized larvae. For overwintering, the parasitoids remained as eggs or first instars inside their host. Parasitoid emergence from the host could only be observed in a small number of outdoor larvae. When larvae were parasitized in spring after diapause was broken, the parasitoids developed continuously within their hosts. It is reasonable to assume that browntail moth larvae are not only alternate hosts to gypsy moth larvae but also suitable overwintering hosts for G. liparids wasps.