Milbemycins are a complex family of macrocyclic lactones containing a highly characteristic spiroketal group, produced by Streptomyces hydroscopicus subsp. aureolarcrimosus. Milbemycin A4 is the dominant member of a group of analogues containing a 25-ethyl substituent. Milbemycin A4 is a highly selective and potent nematocide and insecticide. Like the closely related avermectins, milbemycins are thought to act by opening glutamate sensitive chloride channels.