Heliconius erato (Photo by Marty Condon)
Margie Gibson - NZP Staff Writer
ZOOGOER - November-December 1989
An eye-pleasing experiment by Marty Condon, a Friends of the National Zoo (FONZ) supported scientist, is attracting visitors in the Reptile House like bright flowers bursting with nectar attract butterflies. And butterflies and blossoms are precisely the focus of the experiment. In the Reptile House, Condon has installed an exhibit featuring passion vine butterflies (Heliconius erato), progeny of adults she brought back from a study trip to Central America. In the wild, these black, red, and yellow butterflies live at the edges of tropical forests where their major food sources--passion vine shoots and flower nectar--are abundant. Condon explains that behind the display lies an intricate relationship between the passion vine butterfly and the plants that sustain them throughout their lives.
Looking at the Reptile House display you see a gregarious group of butterflies flitting around the plants. A closer look reveals the insects' life cycle: mating, egg-laying, and feeding behaviors are all readily observable. Females flutter to tender new shoots of the passion vine to deposit eggs. Mature butterflies alight on the bright red and orange flowers to gather pollen and energy-rich nectar.
In the butterflies' tropical forest habitat and even in the deceptively peaceful looking exhibit, a struggle for survival is under way. Look carefully at the plants and you'll begin to see how nature has armed the participants for the battle. Anyone who has gardened knows that butterfly caterpillars can eat vast quantities of leaves very quickly. Some plants, including those in the exhibit, have evolved defense mechanisms to protect themselves against the caterpillars. For instance, some species of passion vines, the sole food of the Heliconius caterpillar, have tiny nectaries on the underside of their leaves. The nectaries produce sugar water, which attracts ants and wasps. The ants and wasps, in their turn, feed upon the Heliconius caterpillars. This helps check the caterpillar population which, if left to its own devices, could devour the passion vine. Other passion vines have an even more active defense in their arsenal: They have evolved hooked hairs that puncture young caterpillars. Caterpillars are holding their own in this evolutionary battle, though: Some species spin a web over these puncturing hairs and continue their passion vine repast unharmed.
Not surprisingly, reproducing a slice of tropical forest in the Reptile House presents a few problems. Condon explains that the number of butterflies in the exhibit depends on the availability of flowers and shoots for the butterflies to feed upon. Heliconius also feed on the pollen of these plants--a feature that distinguishes them from all other butterflies. She has collected some of these food plants in the tropics and brought them into the U.S. with special permits; others she grows from commercially available seeds. But even the sunny work area on the second floor of the Reptile House does not provide the sunlight necessary to produce enough plants to support a large number of butterflies.
Insect pests are another problem. Lantana, with its firework-like bursts of flowers, is a favorite food plant of these butterflies. Unfortunately for Condon, who is trying to grow lantana, it is also a special favorite of aphid-like whiteflies. They devour large amounts of lantana, reducing the quantity available to the butterflies. So Condon has introduced natural parasites of the whitefly--tiny wasps about the size of a dot over "i"--to see if she can reduce the number of whiteflies.
Rearing the insects from egg to butterfly is a project unto itself. With the help of Karen Vein, a FONZ intern, Condon collects the tiny butterfly eggs from the exhibit and places them in small, covered plastic containers. Condon and Vein add tendrils of the passion vine for food and a small tuft of moistened cotton to keep humidity high. Once the eggs hatch, Condon and Klein clean the containers and keep them supplied with passion vine shoots. The feeding and cleaning chores require three to four hours of their time daily.
While Condon continues her study of these colorful insects, the attractive exhibit provides a lesson in ecology for anyone willing to spend a few moments watching the intricate activities. A living example of the interrelationship between plants and animals, the exhibit illustrates the delicate balance that nature maintains in even a small patch of tropical forest.