Influence of increasing herbivore pressure on modification of glucosinolate content of swedes (Brassica napus spp. rapifera)

The effect of increasing herbivore pressure, in the form of larval feeding damage by the turnip root fly, Delia floralis, on the glucosinolate content of swede roots (Brassica napus ssp. rapifera) was investigated. Only one of the 14 root glucosinolates detected, 3-indolyl methyl glucosinolate, rose significantly with increasing levels of insect attack. Although other root glucosinolate concentrations altered following damage, the induced changes were no greater from inoculation with 20 eggs/root than with 5 eggs/root. Swedes roots that had been damaged by D. floralis contained approximately three times the concentration of total indolyl glucosinolates of control roots. This change was strongly influenced by a fourfold increase in the concentration of 1-methoxy-3-indolyl methyl glucosinolate. The total glucosinolate concentration found in swede roots remained unchanged over-all as a result of a fall in the concentration of five of the aliphatic glucosinolates, which balanced the rise in aromatic glucosinolates. The relevance of these results to studies of crucifer-insect interactions are discussed.