The effects of azadirachtin on the feeding behaviour and virus transmission of the green peach aphid, Myzus persicae (Sulzer)
Azadirachtin was isolated from neem seeds, by flash column chromatography, with a purity of > 95%. Yields from Ghanaian seeds were higher than those from Pakistani seeds (0.04% w/w and < 0.0015% w/w respectively). An unsuccessful attempt was made to produce 14C-Labelled azadirachtin for use in systemic movement studies. Qualitative and preliminary quantitative chromatographic analyses of the leaf extracts of tobacco seedlings (Nicotiana clevelandii) whose roots had been immersed in azadirachtin solutions showed systemic movement of the compound from the roots to the aerial parts. The electrical penetration graph (EPG) method was used to analyse the feeding behaviour of apterous, adult Myzus persicae (Homoptera : Aphididae) on N.clevelandii seedlings, treated systemically with azadirachtin. The percentage of the 9h recording period devoted to non-penetration activities and to stylet pathway patterns, the number of probes inititated and the numbers of sieve tube penetrations all increased with increasing azadirachtin concentration. Azadirachtin treatment significantly reduced the percentage of probes that reached sieve elements and increased non-penetration activity before and after the first period of ingestion from the sieve elements. The percentage of the recording period spent in the EPG pattern associated with sieve tube penetration was significantly reduced by an azadirachtin concentration of 300ppm, and the duration of each individual sieve tube penetration was significantly reduced by an azadirachtin concentration of 100ppm. A novel quantitative method, based on honeydew production, was used to measure aphid feeding on artificial liqued diets containing various concentrations of azadirachtin. During a 52h period on diets containing azadirachtin at concentrations of 100 and 300ppm, adult apterous M.Persicae produced 4-5 times less honeydew than those feeding on control diets and aphids on diets containing 500 and 1000ppm azadirachtin produced no more honeydew than aphids which were starved for the same period. During this period the rate of nymph production by aphids on all treated diets fell to less than half that of aphids on control diet. When the aphids were subsequently transferred to untreated diets for 44h, a large proportion of the nymphs produced by aphids which had been on diets treated with 100-500ppm azadirachtin were born dead with undeveloped appendages.