ALTERATION OF ECDYSONE METABOLISM IN HELIOTHIS-VIRESCENS (F) (LEPIDOPTERA, NOCTUIDAE) LARVAE INDUCED BY CARDIOCHILES NIGRICEPS VIERECK (HYMENOPTERA, BRACONIDAE) TERATOCYTES

The haemolymph titres of ecdysteroids in last instar heliothis viresce ns (F.) larvae parasitized by Chardiochiles nigriceps Viereck, or in l arvae injected with teratocytes obtained from this parasitoid, were de termined and compared to those of nonparasitied larvae. Ecdysteroids w ere extracted, purified by high performance liquid chromatography (HPL C), and the titres assessed by radioimmunoassay. The total ecdysteroid titre of parasitized day 5 of 5th instar larvae was consistently lowe r 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 regis tered on day 7 of 5th instar parasitized larvae was almost completely due to polar compounds, whereas 20-hydroxyecdysone remained at low lev els. In contrast, the total ecdysteroid titre of nonparasitized H. vir escens larvae was higher and inactive metabolites increased in concent ration only the day before pupation, after the typical 20-hydroxyecdys one burst. When C. nigriceps teratocytes, obtained from embryos hatche d in vitro on a semi-defined medium, were injected into day 1 of nonpa rasitized host 5th instar larvae (new-slender stage), the total ecdyst eroid titres on day 5 and 6 of these larvae were much higher than in n onparasitized controls. However, this high ecdysteroid titre was assoc iated with low levels of 20-hydroxyecdysone and with a high titre of e cdysone and other unidentified ecdysteroids. The 20-hydroxyecdysone wa s 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 radio labelled 20-hydroxyecdysone resulted in the recovery of 34% of the tot al radioactivity in the HPLC fractions corresponding to polar metaboli tes. In contrast, only 4.5% of the total radioactivity was recovered i n the same polar fractions when ecdysone was co-incubated with teratoc ytes. These results, along with those presented above, suggest that th e inhibition of host pupation observed of parasitized H. virescens lar vae is due in part to 20-hydroxyecdysone inactivation through the form ation of polar metabolites and C. nigriceps teratocytes seem to play a n important role in this biochemical transformation.