Host hemolymph proteins and protein digestion in larval Habrobracon hebetor (Hymenoptera : Braconidae)

Host plasma proteins and protein digestion in larval parasitoids were studi ed during trophic interactions of the ectoparasitoid Habrobracon hebetor Sa y (Hymenoptera: Braconidae), with a host, larvae of the Indianmeal moth, Pl odia interpunctella Hubner (Lepidoptera: Pyralidae). We could detect no app arent differences in host hemolymph protein patterns up to 72 h after paral ysation and/or parasitization by H. hebetor. A 190 kDa putative apolipophor in I present in host hemolymph could not be detected in the midguts of feed ing H. hebetor larvae indicating that it is rapidly digested. The major 60 kDa storage proteins (putative hexamerins) in host hemolymph were detected in the parasitoid midgut and were completely digested 24 h after cessation of feeding and the beginning of cocoon formation. Host hemolymph had a pH o f about 6.4. The pH optima of the midgut proteinases in the larval parasito id were in the alkaline region, but midgut fluid in feeding parasitoid larv ae was about pH 6.8. Based on enzyme activity against selected artificial p roteinase substrates including azocasein, N-alpha-benzoyl-L-Arg p-nitroanil ide (BApNA), succinyl-Ala-Pro-Phe p-nitroanilide (SAAPFpNA), succinyl-Ala-A la-Pro-Leu p-nitroanilide (SAAPLpNA), and inhibition by selected proteinase inhibitors, serine proteinases appear to be the predominant class of enzym es involved in protein digestion in the midguts of H. hebetor. There is als o an active aminopeptidase (LpNA) associated with the microsomal fraction o f midgut preparations. There was no evidence for preoral digestion or inges tion of proteinases from host hemolymph by the parasitoid larva. There was a very active BApNAase in the soluble fraction of midgut extracts. This act ivity increased on a per midgut basis up to 24 h after the beginning of coc oon formation but decreased rapidly by 48 h. Two major (P1 and P3) and seve ral minor proteinases were detected in midgut extracts of H. hebetor analys ed with gelatin zymograms. The apparent molecular mass of P1 varied from 95 to 49 kDa depending on protein loading. P3 had an apparent molecular mass of 39 kDa that was independent of protein loading. In summary, electrophore tic evidence indicates that host hemolymph protein patterns do not change s ignificantly for at least 72 h after paralysation by H. hebetor. The role, if any, of envenomization in preventing breakdown of hemolymph proteins dur ing this time remains to be determined. Because the predominant host hemoly mph proteins, a putative apolipophorin I and the putative hexamerins, are r eadily digested by the serine proteinases present in the midguts of this pa rasitoid larva, these or similar proteins would provide an easily digested source of dietary amino acids that could be used for development of artific ial diets for this beneficial insect. Published by Elsevier Science Ltd.