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Thursday, April 15, 2004

WILD WATCH

VEGETABLE WASPS

Fungal alchemists snatch bodies to live


Step back in time a mere 1,000 million years and the three great domains of the Plantae, Animalia and Fungi shared a common ancestor.

What's even more surprising, though, is that fungi are more closely related to animals than they are to plants. That gives pause for thought -- next time you contemplate mushrooms in your miso soup, consider them your distant relatives!

Fungi set some extraordinary records, and not only for being so astonishingly plentiful in Japanese supermarkets. Think of the largest inhabitants of the world, and perhaps the blue whale springs to mind, or the African elephant. Think again: There is a honey fungus in Michigan that covers 600 hectares and is estimated to weigh more than 100 tons!

The heaviest fungal fruiting body alive today is thought to be one at Kew Gardens in London that measures 1.5 meters long and is more than a cubic meter in volume; while the heaviest known was a 45.4 kg bracket fungus from the New Forest of southwest England.

Organisms on this gigantic scale make it into the record books because they are hard to miss. But if you are willing to think small, and do some groveling, there are some fascinating forms of fungi out there that are so small as to be easily overlooked. Even fungal numbers are staggering; mycologists already recognize more than 60,000 species, but estimate there may be as many as 1.5 million species worldwide.

Fungi inhabit most terrestrial environments and live on a wide range of substrates/hosts, ranging from rotting wood to animal matter, including dung. Most, though, live in tropical forests. Along with hordes of insects, protozoans and bacteria, they serve a vital function in ecosystems around the world, for they are great recyclers. They make our recent efforts at resource management appear paltry and pathetic. Fungi are able to convert even cellulose and lignin into chemical forms that plants can reabsorb through their roots.

So fungi consume the detritus and dead fall of the forest and convert it into life-giving nutrition. They are alchemists, turning death into life. Without fungi, the world would be overwhelmed by the accumulation of dead vegetation in both forest and farmland.

However some fungi, or saprobes (they used to be known, mistakenly, as saprophytes -- but "phyte" refers to plants, and plants they are not) are unwilling to wait patiently for death and decay; instead, they hasten the process along.

These are the parasitic forms, and among them it is the "vegetable wasps" or "plant worms" that have fascinated me. These are so named because the fungus spreads throughout its animal host then, in an apparently seamless confluence of animal and "plant," it produces a worm-shaped fruiting body.

20,000 species in Japan

Plant worms infect spiders and various insects including wasps, ants, dragonflies, butterflies, moths, cicadas, bugs, beetles, grasshoppers and flies. A student of mine has been studying Cordyceps nutans, a bug-parasitizing species that ranges from Japan to New Guinea. However, my most recent sighting was of a giant Cordyceps that had parasitized an enormous caterpillar on Ulva Island off southern New Zealand.

Japan, as I have pointed out many times in this column, is -- from a biodiversity perspective -- astonishingly rich, and in terms of fungi alone it probably harbors about 20,000 species. Even more amazing is that of the 400 or so known species of plant worms, some 300 occur here in Japan.

As previously mentioned, vegetable wasps parasitize mainly insects, invading them either by way of the respiratory organs, the organs of digestion, or via the soft joints of the exoskeleton. Having infected a host, they somehow seem to make it become sensitive to light, causing it to burrow into the forest floor among the leaf litter (other species burrow into rotting wood).

There the dying host provides the necessary nutrition for the fungus to develop until it eventually sends up -- for want of a better word -- a stem. Technically known as a stroma, this rises from the host's body, which may be buried a few centimeters deep; it breaks through the leaf litter to rise some 3.5 to 10 cm above the ground and consists of a head part and a pedicel. The head is typically oval or globular and may be reddish or orange. Once mature, this produces the next generation of dustlike, windborne spores.

In Nopporo Forest in Hokkaido, where 17 species of vegetable wasps have been discovered, the commonest hosts were two bugs -- Pentatoma rufipes and Acanthosoma denticaudum -- though why they should be particularly susceptible is still not known. However, there is clearly some link between the species occurring as hosts and their habitat and seasonality. Vegetable wasps reproduce by scattering spores, and as this is a rather random approach to dispersal, they are most likely to infect species that are common in the same environment at the same time of year the fungi is growing. So perhaps for those two bugs in Hokkaido, their only mistake is being in the right place at the right time. This random "selection" of hosts seems further borne out by evidence showing that common hosts differ from region to region.

Searching for vegetable wasps takes a certain perversity. Rule out a gentle afternoon stroll botanizing and birding in the woods, while hoping to stumble over one protruding from the forest floor. They are so well camouflaged down there among the leaf litter, the forest debris and the shoots of other plants, that the only way to find them is by groveling and crawling, covering the ground at an agonizingly slow pace. You'll do best if you search in deciduous forest where sunlight penetration to the forest floor is limited, and where the ground is damp rather than dry. Once found, a little excavation is required to confirm their identity. It's a painstaking task, but one rewarded by a sighting of such a bizarre form of life.



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