Comparison of the sensitivity of four Delia species to host and non‐host plant compounds

Abstract

Abstract. The behavioural facilitation hypothesis, tested in the present study, suggests that evolution of host‐plant shifts by phytophagous insects is based on the preadaptation of insects to the chemistry of potentially novel plant hosts. Thus, closely‐related insects should have similar sensitivities to compounds that are shared by different host plants. The chemoreception is investigated for four phytophagous flies, Delia radicum, Delia floralis, Delia antiqua and Delia platura (Diptera, Calyptratae: Anthomyiidae), belonging to the same genus but developing mainly on different plant families, with particular secondary plant compound profiles. In addition, the carrot fly, Psila rosae, an acalyptrate Diptera, is included as an unrelated species that is associated with completely different host plants. For the comparison, the known oviposition stimulants of the cabbage root fly (glucobrassicin, sinalbin, sinigrin and a thia‐triaza‐fluorene compound; CIF‐1) present on the cabbage leaf surface were chosen. Responses from prothoracic tarsal sensilla are recorded to contact stimulation in a dose‐dependent manner. Among the different flies tested, only D. radicum responds to all the compounds. By contrast, D. floralis is only sensitive to CIF‐1, and not specifically on the C5 sensillum, a finding that is in conflict with previously published results. This discrepancy is possibly an indication of the variability among flies originating from different cultures or habitats. With the exception of sinigrin at high concentration, the various compounds tested do not stimulate D. antiqua or D. platura. However, the carrot fly appears to be completely insensitive to sinigrin even at the highest tested concentration of 10−1 M. The responses of the contact‐chemoreceptor neurones to the selected compounds therefore provide little evidence of common sensitivities that would explain host shift in Delia species and specialization at the physiological level. The wide divergence within closely‐related species and rearing cultures appears to indicate that the sensitivity and distribution of sensory receptor neurones is very variable on an evolutionary scale.