Refuge ecologist enjoys fungus-farming leaf-cutter ants in Costa Rica

Posted: Friday, February 26, 2010

My travels this winter brought my wife Sara and me to Costa Rica for three weeks of winter respite. The down time of traveling provides good opportunities to catch up with some good books, and my choice this year was Edward O. Wilson's fascinating autobiography "Naturalist." Harvard University's E.O. Wilson is one the world's great naturalists and conservationists, and perhaps the world authority on ants.

Photo By Sara Berg
Photo By Sara Berg
Leaf-cutter ants in Costa Rica carry leaf fragments back to their underground nest. The leaf fragments are about -inch long. Leaf-cutters can defoliate a citrus tree in 24 hours.

Wilson describes his boyhood days exploring the swamps of southern Alabama and the Florida Panhandle, usually by himself. A skinny lad at 15, he became a quite bold snake collector. One day he got a hold of a muscular five-foot cottonmouth water moccasin that was more than he could handle and it gave him quite a fright. Fortunately he was able to free himself from the snake's coils without getting bitten, but the incident definitely got his attention.

Wilson is often quoted as saying, "Most children have a bug period. I never outgrew mine." I feel like I am in good company here, because I too have never outgrown mine. My "bug period" always resurfaces every time I visit the tropics, where everywhere there are amazing bugs to be seen, heard or get bitten by.

Ants were the bug of choice for young E.O. Wilson. At the age of 6 in 1935, he was fascinated by the site and peculiar smell of lion ants of the genus Dorymyrmex which ran around like whirligigs on the stove-hot summer sidewalks of Pensacola. He still revisits this site periodically to check on the ants, although Dorymyrmex has been replaced by another species Pheidole dentate, at least for the time being. On a visit 40 years later Wilson was down on his hands and knees on the same sidewalk looking for ants, when an elderly black man passing by stopped to ask if he needed help.

Like Wilson I am always happy to revisit my leaf-cutter ant friends when I visit Central America. I may have been gone for awhile, but the leaf-cutters have been working away, never missing a beat. As soon as I enter the forest I find their well-cleaned paths and see the endless procession of ants carrying clippings of leaves. The ants have sharp cutting mandibles which vibrate a thousand times a second and neatly saw off fragments of leaves about a half-inch in size. Each ant picks up a leaf fragment, which can weigh twenty times its body weight, and carries it back to a large underground nest.

I have seen leaf-cutter ant columns stretching hundreds of feet from a partially defoliated tree to the nest. Nests can be 18 feet deep and have as many as 3,000 chambers. On the surface the nest look like mounds of fresh soil, with several entrance holes and some air ventilation holes. The ants move day and night, both coming and going along their trails, following pheromone scent tracks.

Leaf-cutter ants are farmers. They don't eat the leaves that they work so arduously to carry back to the nest. Rather, they use the leaves to cultivate a white fungus, which they eat when it is ripe.

In their highly developed caste system, leaf preparer ants clean up the leaves, removing any foreign material such as fungus, algae or other insects. Antibiotic bacteria Streptomyces are cultivated to ward off the fungal parasite Escovopsis, which can wipe out the fungus crop if not suppressed.

Leaf degrader ants chew up the leaves, defecate on them, and mould the mixture into pellets. Planter ants carry strands of the white fungus to inoculate the pellets.

When the fungus is ripe, it is harvested by caretaker ants who deliver the fungus to other ants and the queen.

The ants also cultivate antibiotic bacteria Streptomyces to ward off the fungal parasite Escovopsis, which can wipe out the entire fungus crop if not suppressed.

When a young virgin ant queen leaves the colony on her nuptial flight, she carries some of the fungus as well as the antibiotic bacteria with her as an innoculum or starter for the fungus in her new colony. The fungus and the antibiotic bacteria are packed into special pockets in the queen's mouth parts.

These "infrabuccal pockets" are functionally similar to the pockets in the spruce bark beetle exoskeleton that carry blue-stain fungus from one tree to another, in order to clog the tree plumbing with fungal mycelia and disarm the tree's defenses against beetle attack.

The queen will breed with many males on her nuptial flight, store the sperm for years, and produce 8-10 million eggs during her lifetime.

A leaf-cutter colony is typically active 10 to 15 years, the average lifetime of the queen. Since the queen is the only egg-layer, the colony gradually runs down from lack of new ants after the queen dies.

There are 35-50 species of leaf-cutter ants, all of which live in the New World tropics. Last year I went to Borneo, an island in the Old World tropics half way between Vietnam and Australia, and I missed seeing my leaf-cutter friends. I also missed seeing hummingbirds, which are such a colorful part of the Neotropical bird fauna, but again are missing from the Old World tropics.

Leaf-cutter ants are not the only agriculturists in the insect world; we also have termites and ambrosia beetles, both of which also cultivate fungi. Termites setting up a new termite garden acquire their fungus Termitomyces from wind-dispersed spores coming from mushrooms growing on other termite gardens. The gardens are grown on dead plant material, to which the termites continuously add spore-laden fecal pellets to produce a "comb" which is eaten as the fungus ripens.

Ambrosia beetles are common in recently killed spruce trees on the Kenai. Unlike bark beetles, ambrosia beetles bore into the wood, and carry out all the sawdust in a very tidy manner, rather than eating it.

They inoculate the wood with a fungus which digests the wood and produces the "ambrosia" which is what the beetles actually eat.

Geneticists studying insect DNA find evidence of nine separate evolutionary origins of insect agriculture using fungi. In ants fungal cultivation arose only once, probably 45-65 million years ago in the Amazon rainforest. Termites likewise have had a single origin of fungal culture, in the African rainforest 24-34 million years ago. In ambrosia beetles however fungal culture appears to have arisen seven times, each time independently, 20-60 million years ago.

The fact that fungal farming has arisen independently so many times strongly suggests that it must be a pretty good system, at least for the insects. But good or bad, both the insects and the fungi are now locked into it, because in most cases neither can survive and reproduce without the other.

Ed Berg has been the ecologist at the Kenai National Wildlife Refuge since 1993. Ed will be teaching his weekly 1-credit "Cycles of Nature" course at the Kenai Peninsula College in Soldotna and Homer, starting March 30 and April 1, respectively.

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