Histopathology of green muscardine in larvae of four species of elateridae (Coleoptera)

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Larvae of the elaterids Limonius californicus, Hypolithus bicolor, Ctenicera aeripennis, and Ctenicera destructor appeared susceptible to the disease, green muscardine, incited by the fungus Metarrhizium anisopliae, in the order listed. At 20°C the first symptoms of disease were observed in L. californicus and C. aeripennis about 4 to 5 days, and in H. bicolor and C. destructor about 6 to 8 days, after inoculation. The fungus most commonly penetrated the host directly through sclerotized or membranous cuticle. Penetration appeared facilitated by enzyme( s); this was suggested by differential staining reactions in the procuticle. There is evidence also for mechanical pressure involved in penetration. Occasionally, the fungus entered the host through spiracles and pores of sense organs. No penetration via the digestive tract was noted. Prior to the death of the host, the fungus developed sparsely within the body cavity. Some hyphae radiated inward from the point of ingress. Hyphal bodies were concentrated near the point of ingress, but a few were found remote from this site. Only the hypodermis in the vicinity of penetration whowed signs of histolysis. Following death of the host, the fungus developed rapidly. It appeared to invade the internal tissues and organs in the following sequence: fat bodies; digestive system and Malpighian tubules; hypodermis and nervous system; muscles; and tracheae. It appeared unrestricted in its points of emergence from the host, and seemed to rupture the cuticle by mechanical pressure on egress. Sporulation occurred within 24 hrs. after emergence. The various factors postulated as important in the death of insects parasitized by fungi are discussed. Two of these, namely, pathological changes in the hemolymph and toxin production, may be implicated in the death of the elaterid species studied.