Phenoloxidase activity in the haemolymph of parasitized and unparasitized Heliothis virescens

Abstract

Hymenopterous parasitoids of Heliothis virescens are able to evade the immune mechanisms of their host, especially the ability of the host to encapsulate foreign objects within a haemocyte capsule. This encapsulation is often accompanied by melanization. Our present findings show that hosts parasitized by Microplitis croceipes, Cardiochiles nigriceps and Campoletis sonorensis show changes both in the amount of melanin formed when their haemolymph is exposed to air and in the total protein as determined by the Folin phenol reagent. Furthermore, these changes are promoted by each of the three parasitoids in a different way. However, none of these parasitoids was able to alter the levels of host phenoloxidase as measured by the ability of the haemolymph to convert 4-methylcatechol to its quinone. The hymenopteran egg must therefore escape encapsulation in Heliothis in a way other than through inhibition of phenoloxidase, since the melanization reactions in the host's haemolymph are apparently unimpaired, particularly during the egg stage, by either the egg or venom of any one of the three parasitoids.