No abstract

Spectral 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.

The spectral sensitivity of the compound eye of the cabbage root fly, Delia radicum (Linnaeus), was measured using the electroretinogram (ERG) technique, at fifteen selected wavelengths between 340 nm and 670 nm. The form of the ERG was found to be diphasic in nature. A primary peak of spectral sensitivity in the UV (340-350 nm), and a smaller secondary peak in the blue-green region (460-546 nm) were found, together with a shoulder of sensitivity, representing a 'pseudo-peak' as reported for other Diptera, in the red region (630 nm). No significant differences were found between the dorsal and ventral regions of the eye. The peak response in the green region (546 nm) agrees well with existing behavioural data on colour attraction and visual discrimination of host plants by the cabbage root fly.

The effect of allelochemicals from its host, the larva of the cabbage root fly, Delia radicum (L.) (Diptera: Anthomyiidae), and the host‘s food plant on the ovipositor probing response of the parasitoid Trybliographa rapae (Westw.) (Hymenoptera: Cynipidae) were investigated. Trybliographa rapae probed both cabbage root fly infested and uninfested swede (Brassica napus var. napobrassica), although significantly more wasps responded to infested swede. Antennal sensilla are likely to be the mediators of this response. The synomones and kairomones involved are extractable in water, diethyl ether and methanol. No response was observed to washed, starved cabbage root fly larvae. Wasps spent significantly longer searching infested swede than uninfested, although probing frequency remained constant. It is suggested that the initiation of probing in T. rapae is dependent on a threshold concentration of general synomones or host related synomones and kairomones, whereas time spent searching a particular area is dependent on the environment perceived by sensilla on the ovipositor.