After biological control of the mango shoot caterpillar,Penicillaria jocosatrixGuenée (Lepidoptera: Noctuidae), on Guam, populations of four other species of Lepidoptera using mango increased. These included three species of Geometridae—the widespreadAnisodes illepidariaGuenée,Thalassodessp., and Chloroclystis sp.—and the noctuidNanaguna breviusculaWalker.Thalassodessp.,Chloroclystissp., andN. breviusculawere most commonly found on mango inflorescences, and probably became more abundant as a result of an increase in mango flowers.A. illepidariacaterpillars, which eat leaves and flowers, increased l0-fold after suppression ofP. jocosatrix. Comparison of its larval biology with the mango shoot caterpillar showed thatA. illepidariapreferred slightly older, developing leaves. Survival ofA. illepidariawas low on very young leaves, which were highly suitable forP. jocosatrix. Introduced biological control agents acted as keystone predators, preventing the mango shoot caterpillar from preempting mango as a resource for the other Lepidoptera species.