Butterflies, Plants, and Ants—A Vital Connection


IN July, delicate blue butterflies in Western Europe know that it is time to produce the next generation. To accomplish that, though, the butterflies need more than a mate. They also need the services of blooming blue marsh gentians and hungry red ants. Why? What role do plants and ants play in the life cycle of these butterflies?

One place to observe this intriguing three-way relationship is at Dwingelderveld National Park in the northern part of the Netherlands. This park is home to a large concentration of these blue butterflies. In spring and summer, the Dwingelderveld heaths are transformed into a multicolored carpet made up of many flowering plants, including blue marsh gentians, pink bog heather, and yellow bog asphodels. The blue butterflies are especially attracted to the dainty flowers of bog heather and the fringed flowers  of blue marsh gentians—but for two different reasons. The flowering bog heather is a popular food stop serving nectar, and the marsh gentian is viewed as a potential storage place. But what will the butterflies store there?

A Survival Plan

After mating has taken place, the female butterfly looks for a marsh gentian that is taller than the surrounding vegetation. The butterfly settles down on the flower and deposits a few white-colored eggs. Four to ten days later, the eggs hatch, and between two and six tiny caterpillars begin their new life by burrowing into their food supply. After two to three weeks of nonstop munching, the caterpillars lower themselves to the ground.

Interestingly, the caterpillar usually waits until evening to descend. This is significant, for in the evening two species of red ants, also living in the national park, leave their nests in search of food. The caterpillar lands right in the path of these foraging ants. Although the caterpillar’s move may seem suicidal, it is actually part of a survival plan. So, what happens next?

Before long, some red ants bump into the caterpillar roadblock. Quickly they tow the caterpillar to their nest. Once inside, the caterpillar is treated as a guest of honor and lives safely and comfortably through autumn, winter, and spring in an all-you-can-eat environment. Granted, the caterpillar has limited menu choice—some ant grubs and their main staple, food regurgitated by worker ants. But the ants get their end of the bargain too. They regularly milk the caterpillar for the desirable honeydew that it produces. Even when the caterpillar enters the pupal stage, it continues to provide the ants with some honeydew as well as other secretions that the ants fancy eating. But by then, the end of this coexistence is rapidly approaching.

 From Guest to Intruder

During the pupal stage, the caterpillar begins to turn into a butterfly. When the change is complete, the pupa splits open and a butterfly emerges. It is noteworthy that this event usually takes place early in the morning. Why? Because in the morning the ants are not very active, and unlike the time when the caterpillar lowered itself from the plant to the ground, this time is best for it to avoid attracting its hosts.

When the ants eventually come to milk the pupa, they are shocked to find a foreign winged creature in their nest—and they immediately attack the intruder. Quickly the caterpillar-turned-butterfly dashes for the exit to save its limbs and its life. Once outside the nest, the butterfly climbs up a twig and the ants call off the chase.

At a safe height, the butterfly now stretches its wings and lets them dry. Then, nearly a year after it began its life, the big moment arrives and the butterfly flaps its wings for the first time. There it goes—fluttering above the heath! Within days it will mate, and soon it will begin its search for a tall blue marsh gentian. After all, it’s time to start preparing for the next generation.

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Butterfly Endangered

The blue butterfly’s habitat is the heath. Heaths were formed many centuries ago in areas of Western Europe where man had chopped down the primeval forests. In the past, purple-flowering heaths stretched as far as the eye could see, covering large parts of Belgium, Germany, and the Netherlands, but today only scattered pockets remain. As a result, the blue butterfly is rapidly losing ground. In the past ten years, it has disappeared from 57 of its 136 known natural habitats in the Netherlands. In fact, its survival is so threatened that its name has been added to the European List of Endangered Butterflies, a document compiled by the Council of Europe that lists the names of endangered butterfly species.

To ensure that Dwingelderveld National Park will remain a safe haven for the blue butterfly, the park’s caretakers now try to maintain the heathland by applying the same farming methods that were used by farmers centuries ago. As in the past, shepherds with flocks of sheep roam the heaths, and cattle graze on fields covered with tougher grasses. The grazing sheep and cattle clear spots where ling, bog heather, and other plants can germinate. (Presently, some 580 species of plants grow in the park.) In response, the blue butterflies in Dwingelderveld also do their share—their numbers are growing. In fact, this largest and most important heathland park in Europe is such a hospitable home for butterflies in general that 60 percent of all butterfly species living in the Netherlands can be seen there.

[Pictures on page 16]

A butterfly visits a blue marsh gentian and deposits her eggs

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Red ants care for pupae

[Credit Line]

Ants on pages 16 and 17: Pictures by David Nash; www.zi.ku.dk/personal/drnash/atta/

[Picture on page 17]

Pink bog heather

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Yellow bog asphodel

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Sheep and cattle are helping to restore the butterfly’s habitat