Total and specific ecdysteroids in the haemolymph of Trichoplusia ni (Lepidoptera: Noctuidae) and its parasite, Euplectrus plathypenae (Hymenoptera: Eulophidae)

Abstract

The effect of parasitism by the wasp ectoparasite, Euplectrus plathypenae, on weight gain and haemolymph total ecdysteroid titre was determined for penultimate (fourth)-instar larvae of the cabbage looper, Trichoplusia ni. The rate of weight gain in parasitized larvae or parasitized larvae with the parasite removed was similar, reaching a maximum of about 80 mg by 88–96 vs 40–48 h for unparasitized and sham-operatec controls. Unparasitized or sham-operated control larvae initiated an increase in haemolymph total ecdysteroid titre at 24 h in preparation for the moulting peak. The timing of the initial increase in haemolymph total ecdysteroids was greatly delayed in parasitized larvae with the parasite removed, with the increase beginning at 208 vs 80 h for larvae with attached parasites. In the latter case, the initial increase was associated with the beginning of parasite feeding. Haemolymph total ecdysteroids reached their highest level at 140 h (7200 pg 20-hydroxyecdysone equivalents/μl) in parasitized larvae, as compared to 2250 pg/μl at 430 h in parasitized larvae with the parasite removed, and 1250 pg/μl at the 28-h moulting peak in unparasitized larvae. By a combination of reverse-phase liquid chromatography and ecdysteroid radioimmunoassay (RPLC/RIA) the major ecdysteroids at the 28-h moulting peak in untreated larvae were shown to co-elute with known standards of 20-hydroxyecdysone and 20,26-dihydroxyecdysone, with similar RIA activity in the highly polar region. At the 140-h peak in parasitized larvae, the major peak co-eluted with 20,26-dihydroxyecdysone and represented 85% of total RIA activity. The RPLC/RIA pattern for haemolymph ecdysteroids of E. plathypenae at 140 h was similar to that of the host. Our data suggest that E. plathypenae alters the hormonal milieu of its host and causes a dramatic increase in haemolymph total ecdysteroid titre, which greatly exceeds that of unparasitized larvae. This demonstration of a dramatic increase in host haemolymph ecdysteroid titre in response to parasite feeding is a novel finding for host-parasite systems.