Changes in haemolymph juvenile hormone (JH) concentrations of larvae of the southwestern corn borer, Diatraea grandiosella, were used to estimate the activity of the corpora allata. The haemolymph of penultimate nondiapause instars contained a maximum JH titre of 3000 Galleria units (GU)/ml, whereas the titre in the final instar had dropped to 140 GU/ml within 6 h of the ecdysis and continued to remain low. Corpora allata of last-stage non-diapause larvae therefore remain inactive. Some signal other than a rapidly declining JH titre in mid-instar triggers the release of ecdysiotropin because the pupal moulting cyclic did not become independent of the brain and ecdvsial glands until 4–25 and 4–75 days after the final larval ecdysis, respectively. Newly diapaused immaculate larvae had a haemolymph JH titre of ca. 1500 GU/ml. Three JH mimics applied to early last stage non-dispause larvae induced the formation of the immaculate dispause morph. The mimics did not activate the recipient's corpora allata, but elevated the function JH concentration, each having a different intrinsic hormonal activity. Threshold JH concentrations which control larval ecdyses, dispause maintenance, and metamorphosis are presented.
- © 1976 by Company of Biologists