Alteration of ecdysone metabolism in Heliothis virescens (F.) (Lepidoptera: Noctuidae) larvae induced by Cardiochiles nigriceps Viereck (Hymenoptera: Braconidae) teratocytes

Abstract

The haemolymph titres of ecdysteroids in last instar Heliothis virescens (F.) larvae parasitized by Cardiochiles nigriceps Viereck, or in larvae injected with teratocytes obtained from this parasitoid, were determined and compared to those of nonparasitized larvae. Ecdysteroids were extracted, purified by high performance liquid chromatography (HPLC), and the titres assessed by radioimmunoassay. The total ecdysteroid titre of parasitized day 5 of 5th instar larvae was consistently lower than in nonparasitized larvae of the same age. The main ecdysteroid inactivation products were both polar and C-26 hydroxylated compounds, which were detected as soon as the 20-hydroxyecdysone appeared to be produced. The subsequent increase of the total ecdysteroid titre registered on day 7 of 5th instar parasitized larvae was almost completely due to polar compounds, whereas 20-hydroxyecdysone remained at low levels. In contrast, the total ecdysteroid titre of nonparasitized H. virescens larvae was higher and inactive metabolites increased in concentration only the day before pupation, after the typical 20-hydroxyecdysone burst. When C. nigriceps teratocytes, obtained from embryos hatched in vitro on a semi-defined medium, were injected into day 1 of nonparasitized host 5th instar larvae (new-slender stage), the total ecdysteroid titres on day 5 and 6 of these larvae were much higher than in nonparasitized controls. However, this high ecdysteroid titre was associated with low levels of 20-hydroxyecdysone and with a high titre of ecdysone and other unidentified ecdysteroids. The 20-hydroxyecdysone was the most abundant ecdysteroid released after enzymatic digestion of the polar ecdysteroids isolated from the haemolymph of parasitized day 7 of 5th instar larvae. In vitro incubation of teratocytes with radiolabelled 20-hydroxyecdysone resulted in the recovery of 34% of the total radioactivity in the HPLC fractions corresponding to polar metabolites. In contrast, only 4.5% of the total radioactivity was recovered in the same polar fractions when ecdysone was co-incubated with teratocytes. These results, along with those presented above, suggest that the inhibition of host pupation observed in parasitized H. virescens larvae is due in part to 20-hydroxyecdysone inactivation through the formation of polar metabolites and C. nigriceps teratocytes seem to play an important role in this biochemical transformation.