Posted by: scrubmuncher | February 10, 2010

Insect agriculture

Agriculture enabled humans to forsake the unforgiving ways of hunter-gathering in favour of a much more salubrious lifestyle. The cultivation of other species, which is what agriculture essentially is, rates as probably the single most important technological breakthrough in human history.

With that said, humans are not the only animals to cultivate other species for their own purposes. Take the industrious leaf-cutters ants of the Americas. These little insects live in huge subterranean communities that survive and thrive by cultivating a species of fungus in underground gardens. When it comes to fungi husbandry, these ants are second to none. They ensure the garden is at just the right temperature and humidity, they collect plant material for the fungi to feed on and they examine their crop for any signs of disease. When a young queen leaf-cutter ant leaves the nest she was born in to set up her own nest she even takes a tiny chunk of the fungus to begin her very own garden.

Leaf-cutter ant

Here's a leaf-cutter ant doing what it does best - cutting up leaves. The bits of leaf are taken back to the nest and this is what the ant's fungus grows on - Manaus, Brazil (Ross Piper)

There’s also a beetle that cultivates fungi for food, not on the same scale as the leaf-cutter ant, but it’s nonetheless impressive for its efforts. It’s sometimes known as the ship-timer beetle, or timber worm as the larvae have a liking for boring into the wood of trees that were once commonly used to make ships. Its scientific name for those of you who like those kinds of things is Hylecoetus dermestoides and here’s a photo of it:

A female ship-timber beetle

A newly emerged female Hylocetus dermestoides. She can lay a lot of eggs, hence the huge abdomen. The males are similar, but much smaller - Shrawley Wood, Worcestershire (Ross Piper)

The story of this fascinating beetle begins with a female depositing her eggs in crevices and cracks in the bark of mature trees, usually no more than 2m above ground level.  She lays her eggs on a variety of diseased and dead trees, but avoids pines, larches and hornbeams. Birch is a favourite and a large, diseased birch with the correct conditions (warm and no less than 30–40% moisture content) is perfect.

Lots of beetles use dead wood as a food source for their young, but this species has some remarkable differences. On the female’s ovipositor there is a small pocket, the mycangium, which is filled with fungal spores and every egg she lays gets coated with spores from this pocket. After a few days the eggs hatch and small, whitish larvae wriggle out in to the world and loiter around their empty egg-shells for a while to pick up some of the fungal spores left by their mother. After a while they begin to tunnel into the wood using their powerful mandibles, carrying some of the fungal spores with them. Initially, the tunnels are very narrow as the larvae are small, but as they grow, the tunnel must also increase in width to accommodate them. The tunnel may run for over 30 cm, snaking into the wood, but it is not the wood the larva is eating. The tunnel is, in fact, a sheltered fungus farm.

The spores provisioned by the female and carried by the larvae infect the wood until the walls of the tunnels are lined with a white layer of fungus. The fungus (Endomyces hylecoeti) is a type of yeast that has struck up a symbiotic relationship with the beetle. The fungus gets access to wood in the safety of tunnels and in return the larvae get something to eat. The larva takes excellent care of its fungus garden doing everything it can to keep the conditions just right for the fungi so that it will have enough food to complete its development. The fungus requires oxygen to thrive, so the larva must rid the tunnel of any debris to maintain a good flow of air. The larva shuffles along the confines of its tunnel pushing any wood dust and waste to the outside, where it falls to the base of the tree.

By the winter, the larva will not be fully grown, so it will have to retreat to the deepest reaches of its tunnel and enter a resting state to survive the cold, harsh conditions. Come the spring, the larva reawakens and continues to feed on whatever fungi survived the winter. Soon, it is ready to pupate and it wriggles to the tunnel entrance and enlarges its width to form a chamber for its imminent metamorphosis.

The way in which this species of beetle has struck up a relationship with a fungus is one of ways in which insects have solved the problem of how to get at the abundant energy locked away in wood. Some insects, such as the termites, have enlisted the help of bacteria. These endosymbionts live in special parts of the termite gut and produce the enzymes necessary for digesting wood. In exchange they get a warm place to grow and divide with easy access to food.

There are also species that haven’t really solved the problem, the larva of which make do by extracting what little nutrients they can from the wood, which is such a paltry amount that it takes them many years, even decades to reach adulthood.

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