Determinants of virulence in the entomopathogenic fungus Metarhizium anisopliae (Metsch.) Sorok

Fourteen isolates of Metarhizium anisopliae originally from a variety of hosts were bioassayed against representative species of Lepidoptera, Hemiptera and Coleoptera. Only one isolate was highly virulent (at least 80 percent mortality) to Ostrinia furnacali second instar larvae, with median lethal time (MLT) of 4.0 days. This isolate was likewise highly virulent to older larvae of the insect. Several isolates, originally from lepidopteran, homopteran, and coleopteran species, were highly infective to Dysdercus cindulatus nymphs, indicating their ability to cross-infect species under another insect order. Isolates from soil were as virulent as these isolates. The MLT ranged from 7.9 to 13.6 days. All isolates were avirulent to Spodoptera litura larvae and Epilachna vigintioctopunctata (=philippinensis) grubs. Two isolates caused 100 percent mortality among S. litura larvae when conidia were injected intrahemocoelically, implying that resistance of this insect to M. anisopliae is at the level of the cuticle. Measurements of conidium length, ranging from 5.5 to 8.6 um, and volume, ranging between 16.1 to 34.4 um raised to the third power of the isolates were typical of the short-spored biomass production, lipase and aminopeptidase activities. Although the implication of individual enzyme in virulence is quite evident, the patterns of non-specific cuticle-degrading enzyme production, expressed as percent maximal activity, could not clearly account for the differences in virulence among the isolates. In constrast, variation in mean percent maximal activity, representing the overall enzyme production per isolate, paralleled the differing virulence among isolates, indicating that the whole cuticle-degrading enzyme machinery, rather than the individual enzymes, determine virulence. The overall conclusion is that virulence of M. anisopliae is determined by a range of physiological and enzymatic virulence parameters which come into play at various stages of infection of the host