AbstractBACKGROUND: The effects of an insecticidal soap on the survival, fitness and behaviour of an aphid parasitoid wasp, Aphidius colemani (Viereck), were studied in the laboratory. The LC50 (soap concentration causing 50% mortality 24 h after treatment) was determined. The survival of parasitoid larvae (% adult emergence), fitness (tibia length of adults) and number of eggs produced per female parasitoid that survived in third‐ and fourth‐instar aphids treated with insecticidal soap LC50 were also assessed. The LC50 for third‐ and fourth‐instar aphids was determined to be 3.25 g L−1. Acceptance by female parasitoids of aphids that survived their LC50 was also tested.RESULTS: The soap concentration causing 100% mortality in adult wasps 24 h after treatment was 17.5 g L−1. The LC50 was 2.75 g L−1. Soap did not have any effect on the survival of parasitoid immatures or on the fitness or number of eggs produced per female parasitoid. Wasps that were in contact with treated aphids did not oviposit as much in them as in untreated aphids, indicating that female parasitoids detected aphids treated with insecticidal soap.CONCLUSION: These data suggest that aphid parasitoids released following treatment with insecticidal soap are likely to accept a lower proportion of the surviving aphids. Biological control programmes could be ameliorated by soap applications if the latter were made 1 day before the release of wasps in the greenhouse. Copyright © 2008 Society of Chemical Industry