Hyposoter didymator (Thunberg) (Hymenoptera: Ichneumonidae) is an important larval parasitoid of several lepidopteran pests. Under laboratory conditions, the topical toxicity of selected insect growth regulator insecticides, azadirachtin (AZA), diflubenzuron (DFB), methoxyfenozide (MET), pyriproxyfen (PYR), and tebufenozide (TEB), and the naturalyte spinosad (SPIN) was tested against last (third)-instar larvae of this parasitoid up to the maximum field recommended concentration (MFRC). At MFRC, no abnormalities or mortality were scored for MET and TEB (IOBC toxicity classes 1). In contrast, AZA was slightly, PYR moderately, and DFB and SPIN harmful to the parasitoid (IOBC toxicity classes 2, 3, and 4, respectively). These ratings are the result of a reduction in rate of: (i) pupae formation, (ii) pupal mortality, (iii) adult longevity, (iv) parasitism, and (v) adult emergence. In addition, PYR and SPIN caused a direct mortality in the treated larvae. At sublethal concentrations, every insecticide but MET and TEB also affected these life history parameters of the parasitoid. The second part of the study focused on pharmacokinetic parameters important for the toxicological properties of these insecticides. We determined the recovery pattern of 14C-labeled DFB, MET, PYR, and TEB from parasitoid body, larval exuvia, and silken pupal cocoon after topical application to third-instar larvae. For MET, TEF, and PYR, >60% was recovered from the parasitoid body after 24 h, whereas this was only 12% for DFB, which was mostly detected in the silken cocoon (>60%). More than 25% PYR was also detected in the larval exuvia after three days. Data suggest that the silken pupal cocoon and the larval exuvia might play a role in eliminating insecticides. In conclusion, the current results indicated that MET and TEB are harmless to third larvae of H. didymator. In contrast, AZA, DFB, PYR, and SPIN are harmful and their use in IPM should be considered with caution until they are evaluated under field conditions. [copyright] 2004 Elsevier Inc. All rights reserved. Animals and man; Control; Biological control; Biochemistry; Life cycle and development; Development; Metamorphosis; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Pollution; Chemical pollution; Abiotic factors; Chemical factors Lepidoptera; Control by; Hyposoter didymator (Hymenoptera); Hymenopteran parasites; Hyposoter didymator; Parasite responses to insecticides, host control implications Hyposoter didymator (Ichneumonidae); Control of; Lepidoptera; Pollutants; Larval development; Pupal development; Longevity; Emergence; Lepidopteran hosts; Fertilizer and pesticide pollution; Parasite responses, lepidopteran host control implications; Fertilizers and pesticides; Insect growth regulator insecticides & spinosad none