AbstractSpectral sensitivity of the wasp Trybliographa rapae (Westwood), a parasitoid of cabbage root fly larvae, was measured by the electroretinogram (ERG) technique and by a complimentary behavioural technique, to 15 selected wavelength bands from 340 to 670 nm. Peaks of electroretinogram sensitivity were found in the ultraviolet, blue and green-yellow regions of the spectrum. This corresponds to known classes of photoreceptor present in the Hymenoptera. Behavioural peaks of phototactic attraction were found in the ultraviolet and green-yellow regions, but not in the blue. No differences were observed between the sexes. We suggest that ultraviolet and green-yellow wavelengths initiated ‘escape’ and ‘foliage’ orientated behaviour respectively. Blue wavelengths appear to have a less specific function but may be important for colour discrimination. Blue wavelengths did, however, become significantly more attractive to female wasps after a pre-experimental conditioning period involving exposure to blue wavelengths in combination with swede infested with cabbage root fly larvae. Thus T. rapae has been demonstrated to show plasticity in its visually motivated behaviour. This may prove advantageous in the development of an artificial egg collecting technique for this species.