Influence of Developmental Stage of \u3ci\u3eMelilotus infesta \u3c/i\u3eLeaves on Resistance to Feeding by the Sweetclover Weevil

Leaves of 6- to 8•week old Melilotus infesta plants contained progressively higher levels of nitrate as the leaves developed from the tightly pinched to the fully expanded stage. In weevil feeding tests on water extracts of these leaves a close relationship was found between actual feeding and the feeding predicted on the basis of nitrate content. At a given level of nitrate, extracts of young M. infesta leaves were less attractive to the weevil than were extracts of young M. officinalis leaves. Leaves of mature M. infesta plants contained very little nitrate, but water extracts of such leaves remained unattractive to the weevil in feeding tests. These extracts were lower in Stimulant A activity and higher in Deterrent A activity than extracts of mature M. officinalis leaves. Sweetclover weevil resistance is influenced by the balance between Stimulant A, Deterrent A, and Deterrent B (nitrate), and the relative contributions of the different factors vary with stage of development of the leaves and the plant. Nitrate appears to be the predominant water-soluble factor in young leaves, and Deterrent A and Stimulant A assume increased importance in leaves from mature plants