The avermectins (AVM) are macrocyclic lactone derivatives produced by Streptomyces avermitilis, which have potent anthelmintic as well as insecticidal activities (Egerton et al., 1979; Ostlind et aL, 1979; Putter et al., 1981). These compounds consist of four major components (A~a, A:a, Bl~, B2a) and four minor components (A~b, AEb, Bmb, BEb) but it is presumed that all avermectins share a comm...