Endocrine interrelationship between the parasitoid Chelonus sp. and its host Trichoplusia ni

The egg-larval parasitoid Chelonus sp. induces the precocious onset of metamorphosis in the 4th (penultimate) stadium of its host Trichoplusia ni, emerges from the prepupa, and then feeds on it. Qualitative and quantitative changes in ecdysteroids and juvenile hormone were measured. Hemolymph of 3rd-to 4th-instar host larvae and the parasitoids they contained, as well as nonparasitized and parasitized eggs, were analyzed. In the host hemolymph a broad peak of ecdysteroids during molting into the 4th stadium and a continuous increase from day 2 (onset of precocious wandering) until day 4 (emergence of parasitoid) were observed; 20-hydroxyecdysone and 20,26-dihydroxyecdysone were predominant. The juvenile hormone titer fluctuated in the 3rd and early 4th stadium and fell to undetectable levels shortly before the precocious onset of wandering. The parasitoid's ecdysteroids started to increase on the molt to the 2nd instar (= early 4th instar of the host) and thereafter fluctuated on a high level, 20-hydroxyecdysone, 20,26-dihydroxy-ecdysone, and ecdysone being predominant. The juvenile hormone titer was high in late 1st-instar parasitoids, decreased to low levels at ecdysis into the 2nd instar, and increased again to high levels in the 2nd-instar larvae at the time when their shape changed from flat to cylindrical. After ecdysis to the 3rd instar the juvenile hormone titer fell. A comparison revealed that both ecdysteroids and juvenile hormone fluctuate independently in parasitoid and host at most stages, suggesting that the parasitoid produces its own hormones. The first data on ecdysteroids and juvenile hormones in the egg stage of a parasitoid/host system are reported. At the stage of eye pigmentation parasitized eggs contained more immunoreactive midpolar ecdysteroids than non-parasitized ones. 20-Hydroxyecdysone and 20,26-dihydroxyecdysone were the predominant ecdysteroids in both nonparasitized and parasitized eggs, but the latter contained several additional ecdysteroids which were not seen in nonparasitized eggs. The titer of juvenile hormone was similar in both. Shortly before hatching the ecdysteroids were low in parasitized and nonparasitized eggs, but the content of juvenile hormone was much higher in the former. At this stage the majority of parasitoids have already eclosed and teratocytes are released. The results of HPLC analysis indicated the presence of juvenile hormone III together with juvenile hormones I and II in parasitized eggs, but only juvenile hormones I and II in nonparasitized eggs.