Aetiology of melanized inclusions in the body cavity of Neodiprion sertifer (Geoffroy) (Hym., Diprionidae) adults

The aetiology of large melanized inclusions occurring in the haemocoel of adults of some diprionid sawflies is not fully understood. Their occurrence has been associated with nuclear polyhedrosis virus (NPV), but also with extreme temperatures, parasitism, and prolonged diapause. High frequencies of inclusions are found most often during NPV epizootics. In laboratory experiments with Neodiprion sertifer larvae, ingestion of large doses of a common bacterium, Enterobacter cloacae , was shown to induce inclusions indistinguishable from those associated with NPV infection. It is argued that a bacterial aetiology is not contradicted by the other conditions reported to be associated with inclusions. The bacterial aetiology provides an explanation for the low frequencies of inclusions in N. sertifer populations with no NPV infection. It is proposed that the ability to form inclusions may have evolved as a defence against bacterial infection.