Insecticidal proteins can be excreted in the honeydew when sap-sucking insects feed on insect-resistant transgenic plants. Honeydew can be an important source of carbohydrates, thus potentially exposing a broad range of honeydew-feeding insects to transgene products.Snowdrop lectin (Galanthus nivalis agglutinin; GNA) dissolved in a 2?m sucrose solution had no antifeedant effect on female aphid parasitoids (Aphidius ervi) but had a direct negative effect on their longevity. When feeding on honeydew from Rhopalosiphum padi feeding on a GNA-containing artificial diet, Aphidius ervi suffered a longevity reduction that was more pronounced than was to be expected based on the detected GNA concentration in the honeydew. Analysis of carbohydrate and amino acid composition revealed that a change in honeydew composition caused by a GNA-effect on the aphids could be a possible explanation for the additional reduction in parasitoid longevity.When comparing the effect of honeydew from Sitobion avenae and R.?padi feeding on GNA-expressing or nontransformed wheat plants on A.?ervi longevity, aphid species was found to have a significant effect, whereas the wheat variety had no effect. The latter result was probably due to low GNA expression levels in the plants. Differences in nutritional suitability between honeydew from R.?padi and S.?avenae could be explained by differences in carbohydrate and amino acid composition. This is the first study to demonstrate that GNA ingested by aphids and transported into the honeydew can negatively affect the parasitoids consuming this honeydew. We recommend that honeydew should be considered as a route of exposure to transgene products in future risk assessment studies.