LOCATING A RESISTANCE MECHANISM TO THE CABBAGE APHID IN 2 WILD BRASSICAS

Feeding behaviour of the cabbage aphid, Brevicoryne brassicae, was mon itored electronically on two resistant Brassica species, B. fruticulos a and B. spinescens, and compared with a susceptible control B. olerac ea var. capitata cv. Offenham Compacta. Aphids, monitored for 10 h on the under side of leaves, performed recognizable feeding behaviour on all species. Electrical Penetration Graphs (EPGs) of aphids on resista nt and susceptible plants showed no difference in behaviour for aphids on resistant Brassica species compared to susceptible until stylets p enetrated the phloem sieve elements when a large reduction in the dura tion of passive phloem uptake (E(2) pattern) on B. fruticulosa was ind icated. Although feeding behaviour on 6 week-old plants of B. spinesce ns was similar to the susceptible controls, behaviour on 10 week-old p lants was similar to that recorded for B. fruticulosa. The mechanism o f resistance is thought to be located in the sieve element as the norm al sieve element salivation (E(1)) signal was either quickly terminate d by withdrawal of the stylets from the sieve element or continued as a disrupted E(2) pattern. Analysis of secondary plant compounds in the three Brassica species only identified significant differences in the glucosinolate profile. No reproducible differences were detected in t he concentration of phenolics or anthocyanins. The major glucosinolate component of B. fruticulosa and B. spinescens was gluconapin rather t han glucobrassicin and glucoiberin as found in the susceptible host pl ant. However, both pure glucosinolates and glucosinolate extracts from all three species did not reduce aphid survival on chemically-defined artificial diets. These results suggest that the mechanism of resista nce may be a mechanical blocking of the sieve element or stylets rathe r than a difference in the secondary plant chemistry of glucosinolates and phenolics.