PHENOLOXIDASE ACTIVITY IN HEMOLYMPH OF NAIVE AND BEAUVERIA BASSIANA-INFECTED SPODOPTERA-EXIGUA LARVAE

The majority of Spodoptera exigua hemolymph phenoloxidase (PO) activit y is localized in the hemocytes of naive larvae. Mid sixth instar larv ae possessed 0.2 +/- 0.5 and 3.4 +/- 2.6 units (1 unit = Delta 0.001 A ngstrom/min at 490 nm) of PO activity/mu l of hemolymph equivalent in the plasma and hemocyte lysate (HL) fractions, respectively. The S. ex igua HL PO had a pH optimum of 7.0 when assayed against DL-Dopa. This enzyme is present in hemocytes as a proenzyme: activation of PO could be inhibited with either 10 mM diisopropyl fluorophosphate or soybean trypsin inhibitor (1 mg/ml). The relative activity of HL PO was not in hibited by the addition of EGTA or EDTA. The PO titers in HL and plasm a fractions sampled from sixth instar larvae during the feeding period (0-60 hr) of the sixth larval instar were not significantly different , However, a 10 x increase in PO activity was observed in plasma sampl ed from the wandering and prepupal stages. A series of assays was cond ucted to monitor the PO activity in Beauveria bassiana-infected S. exi gua larvae. At 24 hr postchallenge the PO titers in plasma and HL frac tions from infected larvae were comparable to PO titers in fractions f rom saline controls. Analysis of PO activities revealed that by 48-60 hr postchallenge the PO titers in the HL sampled from infected larvae decreased sevenfold, whereas plasma PO titers increased. The increased titer of PO in the plasma did not inhibit fungal development. (C) 199 6 Academic Press, Inc.