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Juvenile hormone (JH) is a major insect gonadotropin. However, no endogenous types of JH have been found in beet armyworm, Spodoptera exigua. Here we analyzed the JH effect on vitellogenesis of S. exigua, which was used for determining the type of endogenous JHs in the species. Females were decapitated at different times after adult emergence to remove the source of JH. This treatment prevented vitellogenic development of S. exigua when the decapitation was executed as early as 4h after adult emergence. Both JH analogs, fenoxycarb and pyriproxyfen, induced vitellogenesis of the decapitated females. Similarly, JH I and JH II could induce vitellogenesis of the decapitated females, but not JH III. Follicle cell patency, allowing access of vitellogenin (Vg) in the hemolymph to the growing oocyte, was induced by the application of JH and JH analogs, but not by RH5992, an ecdysteroid agonist. Phenoxyphenyl compounds such as fenoxycarb, KS-175 (1-(4-phenoxyphenoxypropyl) imidazole), and T3 (L-3, 5, 3′-triiodothyronine) also significantly induced the follicle cell patency of S. exigua. Among JHs, JH I and JH II could significantly induce follicle cell patency, but not JH III. Both JHs and ecdysteroids could induce Vg biosynthesis. These results suggest that JH I and JH II can be the endogenous types of S. exigua and play roles in the vitellogenesis in both Vg biosynthesis of the fat body and Vg uptake by the growing oocytes.

Effect of exogenous juvenile hormones (JHs) on pupal development was assayed in the beet armyworm, Spodoptera exigua. Fenoxycarb, a potent JH mimic, was applied topically to different ages of the pupae, and showed significant inhibition of normal adult eclosion even at 0.1 μg dose when it was applied at the early pupal stage (day 0). As the pupal development underwent, the susceptibility of the pupae fenoxycarb decreased. RH5992, a potent ecdysteroid mimic, did not, however, any similar inhibitory effect on the pupae. Natural JH types (JH I, JH II, and JH III) were applied on day 0 pupae to compare their inhibitory effects on adult eclosion. Both JH I and JH II significantly inhibited adult eclosion at 1.0 μg dose, but JH III did not even at 10.0 μg dose. It was noted that fenoxycarb-treated pupae showed little rectum development. Fenoxycarb did not, however, show any negative effect on the development of compound eye and wing imaginal discs, and on the pupal hemolymph protein pattern. These results suggest that there should be a commitment period requiring an absence of JH for a normal adult metamorphosis during early pupal development and that the endogenous type of JH in S. exigua is JH I or JH II or both JHs like other lepidopteran species.