Oviposition and tarsal chemoreceptors of the cabbage root fly are stimulated by glucosinolates and host plant extracts

The role of glucosinolates in the oviposition behaviour of the cabbage root fly, Delia radicum (L.) (Diptera, Anthomyiidae) was investigated using egg counts and electrophysiological recordings from tarsal contact chemoreceptors. The glucosinolates present both inside and on the surface of cauliflower leaves were determined. The total amounts obtained with the two methods differed by a factor of 100. The extract of the leaf surface contained about 60 μg per g leaf extracted (gle), the total leaf extract 7.5 mg per gle. The glucosinolate patterns of the two extracts were qualitatively similar, but the ratios of the content of individual glucosinolates showed considerable differences. The D sensilla on segment 3 and 4 of the tarsus of D. radicum females were shown to contain a sensitive receptor cell for glucosinolates. In contrast, the receptor cells of the D sensilla of the other segments did not respond in a dose dependent way to these compounds. The glucosinolate receptors were found to be especially sensitive to glucobrassicin, gluconasturtiin and glucobrassicanapin with thresholds of about 10 ⁻⁸M to 10 ⁻⁹M. Large differences (up to two orders of magnitude) were observed among the different glucosinolates. A significant correlation was found between the behavioural discrimination index and the electrophysiological results. But no obvious correlation existed between the chemical nature of the glucosinolate side chain (e.g. indole, aromatic and aliphatic groups), and their stimulatory activity. However, a significant correlation was found between the overall length of the side chain and the biological activity. Although the flies discriminated clearly between model leaves with and without glucosinolates, a clear dose response curve was only obtained for the indole glucosinolate glucobrassicin. Since the most stimulatory fraction of the surface extract contained no glucosinolates, it was concluded that other compounds, in addition to glucosinolates, do play an important role for the stimulation of oviposition.