Why would ants be caterpillar-herders?

By Chelsie Vandaveer

December 19, 2001

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Australia is home to twelve species of small butterflies called azures in the genus Ogyris [o gi' ris]. Depending upon the species, the Ogyris butterflies are dependent either on ants in the genus Papyrius [pap ear' ee us] or Camponotus [cam pon' o tus]. Ten of the butterfly species are also dependent on mistletoes (Amyema spp.) [a my' e ma] as hosts for their larvae. The semi-parasitic Amyema mistletoes are dependent on trees, mostly Eucalyptus.

Female azure butterflies most frequently lay their eggs on mistletoes that have been colonized by the appropriate species of ant. The developing larvae are tended by the ants. The Papyrius ants even take the larvae of Ogyris amaryllis into their nest near the base of the tree.

The developing azure butterfly larvae are tended by the ants.
Geneva Azure Butterfly, Pupa Tended by Ant, Australia Photographic Print
Oxford Scientific
Buy Photographic Print at AllPosters.com

The ants guard the growing caterpillars against predation from parasitic wasps and other ant species. In The Ants (Holldobler and Wilson, 1990) it was reported that "ant-tended larvae during the study were 4 to 12 times more likely to survive to pupation than an otherwise similar group of untended larvae."

The caterpillars of the Ogyris genus are night-feeders although it is felt the ants may foster the nocturnal behavior. The caterpillars spend the daylight hours hidden under the bark of the mistletoe's host tree or within holes in the mistletoe made by the mistletoe weevil. The caterpillars emerge at night to feed on the mistletoe, herded by the ants. The Papyrius ants and amaryllis azure caterpillars travel as much as 40 meters from nest to mistletoe and back to the nest.

The ants are not dependent upon the caterpillars, but do receive compensation. A gland on the back of the caterpillar secretes a liquid containing sugars and amino acids (protein building-blocks). Since tending a caterpillar is a considerable amount of work for the ants, the liquid must be a valuable resource.

Don Herbison-Evans and Stella Crossley have photographs of Ogyris genoveva splendida butterflies which are tended by Camponotus ants. To view the photographs, click on the link:

http://linus.socs.uts.edu.au/~don/larvae/lyca/genover.html

The Australian National Botanic Garden has a photograph of the mistletoe, Amyema miquelii, in bloom. To view, click on the link:

http://www.anbg.gov.au/images/photo_cd/732131822186/022.html

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