The effect of two insect growth regulators and a neonicotinoid insecticide were tested on immature stages and adults of the parasitoid Aphtyis melinus DeBach (Hymenoptera: Aphelinidae), a key natural enemy of California red scale, Aonidiella aurantii (Maskell) (Hemiptera: Diaspididae), in California. No significant effect on survival or development through to the adult stage due to the insect growth regulators were found when the parasitoid was treated at the egg, larval, or pupal stages. The broad-spectrum neonicotinoid acetamiprid also showed no significant effect on the development of A. melinus to the adult stage. However, 48 h exposure of emerged adults to acetamiprid residues resulted in high levels of mortality. A. melinus adults treated with either of the two insect growth regulators as larvae survived 48 h exposure and showed levels of fecundity similar to control insects. We conclude that the two insect growth regulators are compatible with augmentative releases of A. melinus but that treatments of acetamiprid should be avoided in situations where biological control by this parasitoid is important.