Action of ecdysone and juvenile hormone on larval-pupal transformation of the epidermis of the cabbage armyworm, Mamestra brassicae in vitro

Epidermis was explanted from a last instar larva of the cabbage armyworm, Mamestra brassicae, and implanted into a penultimate instar larva. While the host was molting, the epidermis responded to its hormonal milieu and formed new cuticle. Larval cuticle formation by the epidermis was highest in larvae younger than one day and decreased rapidly on days 2 and 3 after molt. After then the epidermis no longer formed larval cuticle. This reuslt suggests the switchover from larval to pupal commitment occurs on day 2. When the epidermis from a day 0 larva was preincubated in Grace's medium for varying lengths of time and implanted into a host, larval cuticle formation linearly decreased as the duration of preincubation lengthened; from 80% without preincubation to 0% at 72hr preincubation. When juvenile hormone I (JH-I) was present in the medium, larval cuticle formation continued even 72hr after preincubation. By contrast, β-ecdysone remarkably promoted pupal commitment, and no larval cuticle was formed in the first 24hr of preincubation. Once the cells were committed to produce pupal cuticle, subsequent exposure to JH-I in vitro was unable to reverse the change of commitment. It was thus concluded that the change of commitment in Mamestra was caused by not only the elimination of JH from the epidermal cells but also the exposure to β-ecdysone. JH-I prevented pupal commitment at 10-8M. Methoprene was about 1/70 as active as JH-I, and JH-III was almost inactive. EMD, ETB and KK-42, known as anti-JHs, did not inhibit JH activity with this system and caused larval cuticle synthesis after preincubation in Grace's medium containing both JH-I and these compounds.