The juvenile hormone titre during the penultimate and ultimate larval stadia of Trichoplusia ni

Abstract

The juvenile hormone titre in penultimate and last-stadium larvae of Trichoplusia ni (Hübner) was determined by a method utilizing gas chromatography-mass spectrometry with selected ion monitoring. Juvenile hormone II was the predominant homologue detected; highest levels were only ⋍400 pg/g. The juvenile hormone titre was high during the fourth stadium and during apolysis of the last stadia then declined to barely detectable levels just before cocoon spinning, and peaked again during the prepupal stage. Prepupal levels of this hormone were similar to those detected in 4th and 5th larval stadia suggesting an important biological role for juvenile hormone in the prepupa. Starvation of day-1, 5th stadium larvae or inhibition of in vivo juvenile hormone esterase activity with O-ethyl- S-phenyl-phosphoroamidothioate (EPPAT) resulted in higher levels of juvenile hormone in mid last-stadium larvae. T. ni larvae undergoing precocious development caused by parasitization with Chelonus sp. showed greatly reduced levels of juvenile hormone II compared to control larvae, but contained a considerable level of juvenile hormone III. Separate analysis of the host carcass and the parasite larva supported the hypothesis that the hymenopterous parasite was the major source of the juvenile hormone III detected.