Growth-blocking peptide titer during larval development of parasitized and cold-stressed armyworm

Growth-blocking peptide (GBP) has been isolated for the first time from the haemolymph of the host armyworm Pseudaletia separata whose development was halted in the last larval instar stage by parasitization with the parasitoid wasp Cotesia kariyai. Recent studies demonstrated that GBP not only exists in the plasma (haemolymph without cells) of parasitized last instar larvae, but also in the plasma of nonparasitized penultimate (5th) instar larvae. Monoclonal antibodies were prepared to measure the titers of GBP in nonparasitized and parasitized larval plasma. One of three monoclonal antibodies raised against GBP, which is the most specific for GBP, was used to quantify the concentration of plasma GBP. As this antibody recognized two plasma peptides other than GBP in crude plasma fractions, each plasma peptide fraction was separated by a reversed phase HPLC, and then plasma GBP level was measured by ELISA. The highest level of plasma GBP detected on Day 0 of the penultimate instar larvae was gradually decreased throughout the larval growth except for the temporary increase on Day 0 of last larval instar. After parasitization on Day 0 of last larval instar, two peaks of plasma GBP titer were detected during the last larval instar, one day and six days after parasitization. This characteristic increase and decrease in plasma GBP level was also observed by transferring last instar larvae of the armyworm from 25 to 10 degrees C, as a result of which larvae delayed pupation by more than 15 days. From these results, it is reasonable to propose that plasma GBP in lepidopteran larvae might control certain upstream steps in a cascade of events leading to pupation; thus, an elevated level of plasma GBP interferes with normal metamorphosis from larvae to pupae.