Abstract
A juvenile hormone analogue (methoprene, JHA) was topically applied to the fourth instar larvae of the silkworm, Bombyx mori, and larval growth, pupation and cocoon weight were examined. Application of JHA (10μg/larva) at 36, 48, and 60hrs after third larval ecdysis delayed somewhat the fourth larval ecdysis and elongated markedly the duration of fifth larval instar. The effecf of elongation of the last larval instar was more prominent when applied in earlier stages of fourth larval instar, and, as a result, cocoon weight as well as body weight of mature larvae were increased. These larvae exhibited several grades of larval-pupal intermediates. Duration of the last instar, larval weight, cocoon weight and abnormal pupation were dependent on dosages of JHA applied. JHA at a dosage of 25μg/larva caused 4-day delay of larval maturation and about 26 percent increase in cocoon weight, while larvae grew almost normally at a dosage of 1μg/larva. The effects of topical application of JHA in fourth instar are almost comparable with those of JHA application at the critical stages of the fifth instar, suggesting a possibility that larval growth and cocoon weight could practically be controlled by applying in the fourth instar.
