Entomology: Animal Husbandry in The Animal Kingdom

Scientists have long admired the industrious ants that keep aphids as cows and milk them for their sweet “honeydew” secretion. Now a graduate student in entomology has added to the brief catalogue of insect husbandry Louisiana State University’s Gary Ross has watched Mexican carpenter ants (Camponotus abominalis) protect caterpillars against their natural enemies and live on the juices that the caterpillars excrete. Though both parties benefit from the odd relationship, nature ensures that it is always brief: by the time they are 83 days old, the caterpillar cattle sprout orange-rimmed wings and fly away.

Ross discovered the colorful symbiosis in the pine-covered Tuxtla Mountains of southern Mexico. The ants, which are less than half an inch long, live in nests tunneled out of wood. The caterpillars, one inch long and much heavier than the ants, are the naked, vulnerable larvae of a rare butterfly.

Sunset Supper. The ants never let the caterpillars out of their care. During the day they keep the caterpillars in individual burrows a few inches long, and plug the entrances with pellets of earth. A few ants always stay inside to guard each precious caterpillar cow. The burrows are always close to the caterpillars’ favorite food plant, a low white-flowered bush, but they may be as far as 50 ft. from the ant colony.

When the sun has set, the guardian ants come out of the burrow and climb into the bush. They inspect every twig and leaf, looking for marauders, especially the fierce, predatory ants that infest the pinewoods and would quickly slaughter the caterpillars. When Ross put spiders or beetles on the bushes, the protecting ants found them at once and quickly dragged them away. About 7:30 p.m., the caterpillar is let out of its burrow. Shepherded by the carpenter ants, it climbs to the topmost leaves of the bush and starts feeding greedily. The ants climb aboard and drink its honeydew on the spot.

Escaping Butterfly. When dangerous dawn approaches, carpenter ants swarm over the caterpillar, herd it down the bush and into the burrow. Then they crawl in beside it and pull mud pellets over the entrance. Even when the caterpillar turns into a chrysalis, they stay on guard until it emerges as a butterfly and flutters freely away.

Ross’s caterpillars have never been found except in pinewoods inhabited by the protective ants, and the observant entomologist believes that they are wholly dependent on their keepers. In a rather heartless experiment, he kidnaped caterpillars and put them on their food bushes with no ants to protect them. Not one survived for a day.