(Z)-OXACYCLOTRIDEC-10-EN-2-ONE DOES NOT APPEAR TO BE RESPONSIBLE FOR RESISTANCE TO ADULT ALFALFA WEEVIL FEEDING EXHIBITED BY MEDICAGO-RUGOSA

Steam distillate prepared from leaves of Medicago rugosa Desr. could inhibit feeding by adult alfalfa weevils (Hypera postica (Gyll.) on membrane filters, whereas an equivalent amount of steam distillate prepared using M. sativa L. had no effect on weevil feeding. Earlier work established that a 12-carbon lactone, (Z)-oxacyclotridec-10-en-2-one, was responsible for the feeding deterrent properties of the steam distillate from M. rugosa. The concentration of this volatile varied with plant age and leaf position. The maximum concentration in leaves of M. rugosa (area basis) was estimated, on the basis of tests using an inert substrate, to be too low to cause a detectable inhibition of feeding. No correlation between lactone content and weevil feeding was detected when leaves from similar positions on plants of different ages were used in a multiple choice bioassay. Although small amounts (< 12.5-mu-g) of (Z)-oxacyclotridec-10-en-2-one can inhibit adult alfalfa weevil feeding on 13-mm-diameter membrane filters, because leaf concentrations (area basis) of this compound are lower than required to cause a detectable inhibition of feeding on the inert substrate, and becuase concentrations do correlate with weevil feeding, it is unlikely that this lactone is responsible for resistance to adult alfalfa weevil feeding exhibited by M. rugosa.