Hormonal requirements for the larval-pupal ecdysis induced in the cultured integument of Chilo suppressalis

The switchover from a larval to a pupal epidermal commitment was studied on integument tissue fragments from early last-instar larvae (1–2 days after ecdysis) of Chilo suppressalis cultured in Grace's medium containing 0.01–0.5 μg/ml 20-hydroxyecdysone for 24–72 hr. Fragments were subsequently cultured in medium containing 1 μg/ml 20-hydroxyecdysone for 24 hr and maintained in hormone-free media for 6 additional days. The degree of switchover induction was measured as the ratio of the number of tissue fragments showing pupal characteristics to the total number of fragments used. The degree of switchover increased with the duration of culture, as well as with the concentration of the hormone (up to 0.1 μg/ml), in the first hormonal treatment. Above this concentration, apolysis and new cuticle formation were induced without change in the epidermal commitment. Cultured integument fragments from larvae in the diapause stage, 40–50 days after hatching, and from those in the penultimate stage, showed the switchover under almost the same hormonal conditions as those used with tissue from the early last-instar larvae. After the first hormone treatment, culture in hormone-free medium was unnecessary for cuticle tanning. Juvenile hormone II added to the medium (3 ng/ml) in the first hormonal treatment completely inhibited the switchover induced by 20-hydroxyecdysone. The potential use of the C. suppressalis integument as a bioassay system for juvenoids is discussed.