Comparison of the egg‐laying behaviour and electrophysiological responses of Delia radicum and Delia floralis to cabbage leaf compounds

The behaviour and the sensitivity of adult cabbage root fly, Delia radicum and turnip root fly, Delia floralis are compared with host‐plant extracts and isolated crucifer compounds previously identified as oviposition stimulants for D. radicum . The oviposition behaviour of both species is similar; 7–10‐day‐old females are stimulated to lay eggs by the methanol extract of cauliflower leaves that contains thia‐triaza‐fluorenes (CIF) as well as glucosinolates. The glucosinolate fraction is mainly composed of glucobrassicin, which alone stimulate both fly species to lay eggs. The C 5 and D 3,4 sensilla on the prothoracic tarsae of newly‐emerged D. radicum contain neurones sensitive to the glucosinolate fractions tested and to glucobrassicin, whereas the CIF specifically stimulate a neurone in the C 5 sensillum. By contrast, newly‐emerged D. floralis respond less to glucosinolates, especially to glucobrassicin, and have sensitive neurones to CIF in other sensilla than D. radicum . Recordings are also made from the longest sensilla present on the labellum because they are apparently sensitive to glucosinolates. By contrast to earlier investigations, no remarkable phasic‐tonic responses of these neurones are seen. The two species are difficult to discriminate visually, have the same host plants, show identical host‐selection behaviour, apparently respond to the same physical and chemical properties of their host‐plants, but have a clearly different distribution of receptor neurones in the tarsal sensilla.