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Natural Selections: Jeff Mitton: Reakirt’s blue caterpillars act in ancient mutualism with ants


Reakirt’s blue butterflies were moving among the flowerheads of white clover in my lawn. I presumed that they were nectaring, but I watched closely and discovered that they were ovipositing. They would fly to a flowerhead, which is composed of more than a dozen flowers, search for an appropriate spot and touch the tip of their abdomens to the outside of one of the flowers. They deposited one egg per per flower and only one or two eggs per flowerhead. In addition to white clover, they use many other species in the pea family for oviposition and larval growth.

Jeff Mitton. Natural Selections

Reakirt’s blue, Echinargus isola, is one of 10 species of blue butterflies in the Front Range. Males are blue on the upper sides of their wings while females are mostly brown with some blue close to the body. These are small butterflies, with wing spans of 7/8 to 1 inch. An identifying characteristic is the large dark spot with a few tiny brilliant scales on the underside of the rear wing.

While the other species of blues overwinter in Colorado as a caterpillar or chrysalis, no portion of the life cycle of Reakirt’s can live through Colorado’s winters. So the first Reakirt’s that appear in spring have migrated north from New Mexico or Arizona. Reakirt’s blues live yearround from Southern California, Arizona and New Mexico to Costa Rica, and in this portion of the range they produce three broods between March and November. Fewer summer broods may occur for butterflies migrating to more northerly places such as Colorado, Ohio and Wisconsin and these populations die out in fall without returning south.

Reakirt’s blue is in the family Lycaenidae, which includes blues, hairstreaks, coppers and harvesters, for a total of about 6,000 species. Biologists estimate that 2,000 species of Lycaenids have evolved mutualistic interactions with ants. A mutualism is an interaction between two species that is mutually beneficial. Reakirt’s blues have evolved morphological and physiological adaptations to support mutualistic relationships with several species of ants.

A study of Reakirt’s blues in the Santa Catalina Mountains in California documented that the caterpillars are parasitized by both a braconid wasp, Cotesia cyaniridis, and a tachinid fly, Aplomya theclarum. In 1999 and 2000, about 40% of the caterpillars were parasitized, which always resulted in caterpillar death — only about 60% of the caterpillars survived to metamorphose.

Parasitism is the selective force maintaining mutualism through the ages and today caterpillars engage several species of ants to reduce mortality from predation.  Reakirt’s blue caterpillars have a nectary organ on their seventh segment that secretes a sweet solution to feed ants, and they also have glands releasing pheromones that attract and appease ants. In return, ants attend caterpillars that feed them and defend them against parasitic wasps and flies. It sounds plausible, but how well does it work?

Dr. Jennifer Weeks, now at the University of Florida, established an experimental garden of caterpillars on whiteflower prairie clover, Dalea albiflora. Some plants were growing naturally while others were wrapped with tanglefoot, which exclude ants. About 40% of the caterpillars were parasitized on plants with ants while twice as many, 80%, were parasitized if ants were excluded. Ants do not provide perfect protection, but they reduce lethal parasitism by half.

Dr. Naomi Pierce, working at Rocky Mountain Biological Laboratory and now at Harvard University, had previously studied a mutualism between ants and silvery blue butterflies, Glaucopsyche lygdamus. She tested the hypothesis that ants protected caterpillars from parasitic braconid wasps and tachinid flies. The experimental design employed two species of lupine used for oviposition and feeding, one in an environment dominated by sagebrush and aspen, the other in a lush alpine meadow. Virtually all caterpillars were attended by ants and rates of parasitism were similar to those in Reakirt’s blue. Caterpillars protected by ants had only half the level of parasitism as the caterpillars on plants wrapped in tanglefoot, which excluded ants.

In summary, both Reakirt’s blue and silvery blue butterflies in natural environments lose about 40% of caterpillars to parasitic wasps and flies, even though caterpillars are attended and protected by ants. However, caterpillars not protected by ants suffer twice the mortality of attended caterpillars. The similarity of results in Reakirt’s and silvery blues takes some of the mystery out of mutualism and helps us understand the high frequency of mutualism in butterflies with caterpillars susceptible to parasitism.

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