Alfalfa Weevil Resistance Mechanism Characterized by Larval Convulsions1

While screening 150 alfalfa ( Medicago sativa L.) introductions for resistance to the alfalfa weevil [ Hypera postica (Gyllenhal)], a new resistance mechanism was discovered in P.I. 247790 from Peru. A portion of the weevil larvae reared on this introduction convulsed, dehydrated, and died before pupation. One clone produced up to 20% convulsing larvae and 40% mortality. Due to the large degree of variability for this trait in P.I. 247790, it should be possible to increase this type of antibiosis through selection. Another resistance mechanism characterized by delayed larval development was also discovered in P.I. 247790. Larvae reared on one clone had a longer development period than those reared on other clones. This mechanism appears to have limited potential in producing resistant cultivars because development is only delayed and insects are not killed.