Juvenile Hormone Titers in the Hemolymph during Late Larval Development of the Tobacco Hornworm, Manduca sexta (L.)

1. A sensitive and quantitative bioassay for JH is described based on the JH inhibition of cuticle melanization in larvae of the black mutant of Manduca sexta. The assay is sensitive down to about 2 x 10-5 µg (20 pg) C18JH. 2. Using this assay, the JH titers in hemolymph of larvae of M. sexta from ecdysis to the fourth instar through the first third of the fifth instar have been estimated. The JH titer is highest immediately after ecdysis to the fourth instar and reniains relatively high until after initiation of the last larval molt. During the time that molting is initiated, the JH concentration is about 2 x 10-8 M in terms of C18JH equivalents. Following initiation of molting the titer drops about 30-fold. 3. Immediately after ecdysis to the fifth instar, the JH titer is again high (nearly the same as seen at the initiation of the molt), then by day 1 has declined sixfold and remains constant through day 2. In mature fifth instar larvae, the titer has dropped again to essentially undetectable levels. 4. The rate of degradation of endogenous JH in the hemolymph was estimated following neck- or abdomen-ligation of fourth instar larvae. After either type of ligation, JH activity was lost from the hemolymph with a half-life of about 1.5 hours.