You are definitely right, but I probably should have specified. Irradiating seeds is vastly different than irradiating already living specimen, and hardly transferable to insects I believe.
Irradiating the entire specimen is not so different if you also irradiate the part of them that produces the seed. You're just irradiating an earlier stage of reproduction, the result is unlikely to be different. Seeds have the advantage that they do not move and contain little water that could interfere with the radiation. The insect will produce it's own seed once irradiated, infertile or not.
