(1) Growth environment of insect-borne fungi

 

In general, since fruiting bodies produced by Cordyceps species are relatively large fleshy fungi, it is assumed that the environment required for their development would be similar to mushrooms and other fleshy fungi not associated with insects. The most obvious difference between entomopathogenic and other fungi in terms of the ease with which they may be collected is in the extreme localization of sporulation by entomopathogenic fungi to the cadavers of their infected hosts. For example, the basidiocarps of a basidiomycete very closely associated with its mycorrhizal host tree will emerge over the relatively wide area occupied by that tree's root system, and because of the size of that root system will be much more numerous and almost certainly larger than the sporocarps that can be formed by an entomopathogenic fungus on a single infected insect cadaver.

In many cases, the stromata of Cordyceps species are found in undisturbed habitats where there is clean air, high humidity, and adequate shading by overhanging trees to help retain soil moisture levels. This is especially true in China where Ophiocordyceps sinensis is used as an herbal medicine and is found in only a very restricted environment. Ophiocordyceps sinensis fruits in April on high mountains (3,000-4,000 m) in a limited area and is collected before the snow melts. The favorable environment for Ophiocordyceps sinensis is, obviously, the same as for its bat moth hosts, Hepialus armoricanus (Lepidoptera: Hepialidae). There is more Ophiocordyceps sinesis found in stands of broadleaf trees than of conifers. Among broadleaf trees, it is found mostly under trees that are 15 years or older and within only a few meters of an adjacent stream, in leaf-covered soils with a high relative humidity and a thick underlying humus layer. In Korea, Cordyceps species grow in a variety of different habitats although each may be quite specific for an individual fungal taxon. Species with relatively large and soft (or obviously fleshy rather than hard, brittle or dry) fruiting bodies such as Cordyceps militaris, C. kyushuensis, and C. scarabaeicola are strongly dependent on high levels of available moisture. They are found in very humid valleys and the numbers of stromata increase after the rainy season starts in June. Taxa with tough and hard stromata such as C. nutans and C. sphecocephala are less influenced by environment, so they are easily found in a forest. In other words, the formation of Cordyceps stromata seem to be more sensitive to the environment than for those conidial entomopathogens (Hyphomycetes and Zygomycetes) which only form conidia on the surface of the host insects. Some entomopathogenic fungi are found in the field to be infested by another fungus growing above them. A beginner may think of them as new or different fungal entomopathogens, but this sort of situation is more likely to represent either a saprobic fungus grow.