Oviposition and chemosensory stimulation of the root flies Delia radicumand D-floralis in response to plants and leaf surface extracts from resistant and susceptible Brassica genotypes

In Brassica crops differences in susceptibility to root fly attack can be largely attributed to antixenotic resistance. Plants of four genotypes (two swedes and two kales) with widely differing resistance in field trials, were compared in laboratory choice assays for their susceptibility to oviposition by the root flies Delia radicum (L.) and D. floralis (Fallen) (Diptera, Anthomyiidae). For both species the preference among the genotypes corresponded to the susceptibility of the genotypes in the field. The preference ranking in response to surrogate leaves treated with methanolic surface extracts of the four genotypes was identical to the preference among potted plants, demonstrating that chemical factors on the leaf surface mediate host preference for oviposition in these species. For both species of fly, glucosinolates are major oviposition stimulants and for D. radicnm an additional, nonglucosinolate oviposition stimulant, presently called CIF, is known. We describe a procedure for chromatographic separation of glucosinolates from CIF in leaf surface extracts. In oviposition-choice assays with D. radicum, the CIF-fractions of the two swede genotypes applied to surrogate leaves received a 1.8 and 4.6 times higher proportion of eggs than the respective glucosinolate-fractions, confirming the major importance of CIF as an oviposition stimulant. The genotype of swede that was preferred by both fly species in tests with plants and methanolic leaf surface extracts, also stimulated oviposition more in tests with the glucosinolate-fractions or the CIF-fractions derived from the surface extracts, respectively. Thus, glucosinolates and CIF together account for the observed preference among the genotypes and may also be responsible for their susceptibility under field conditions. In the two kale genotypes the preference for plants or surface extracts differed from the preference among the corresponding glucosinolate- and CIF-fractions, indicating that additional, as yet unknown chemical factors may also be involved. For both groups of stimulants tarsal chemoreceptors allow electrophysiological monitoring of glucosinolate- and CIF-activity in fractionated surface extracts. For D. radicum the chemosensory activity of both glucosinolate- and CIF-fractions corresponded to the respective behavioural activity in the oviposition preference tests, suggesting that preference for oviposition among genotypes can be predicted from the electrophysiological activity of their fractions. The chemosensory response of D. floralis, in particular to the CIF-fractions, was less pronounced than the response of D. radicum, indicating interspecific differences in the perception of the major oviposition stimulants. We discuss the potential application of electrophysiological techniques in support of other screening methods used in breeding for root fly resistance in Brassica crops.