none Host-age structure of Zabrotes subfasciatus (Boheman) on larval parasitism by two hymenopterous parasitoids, Heterospilus prosopidis Viereck (Braconidae) and Anisopteromalus calandrae (Howard) (Pteromalidae), were examined in a series of laboratory experiments. A life-history refuge from parasitism by both parasitoids was found, consisting of the egg stage and the small, young larvae. The refuge length differed for both parasitoids. Host individuals were not parasitized efficiently by A. calandrae for the first 14-16 days, whereas they were not parasitized at all by H. prosopidis for the first 12 days. Host-age refuges were important in the stability of these two host-parasitoid systems. Stability increased with increasing length of the refuge period. The shorter refuge period in the H. prosopidis -host system may lead to further instability by creating asynchrony between the host and parasitoid life cycles. The age of the larval host also determined the sex ratio of the parasitoid populations. This factor may further destabilize the H. prosopidis populations, while having a lesser effect on A. calandrae .