Butterfly - Host Plant Relationships

Among the many rewards of studying butterflies is the opportunity to observe the direct ecological relationship between insect and plant. Butterflies, like many other insects, interact closely with plants throughout their life cycle, beginning with the adult female's selection of a host plant on which to place her eggs. This is a crucial choice since the hungry caterpillars, upon hatching, will immediately begin devouring the food plant. This will continue until they are fully-grown and ready to pupate.

Most butterflies have strong larval food plant preferences. Some species feed on just one species of plant (including several rare species listed as endangered or threatened). The larvae of the federally endangered Karner Blue Butterfly, for example, feed exclusively on wild lupine. More often, a species' caterpillars (such as Monarchs) will eat a few closely related plant species and in some cases, large taxonomic groups of butterflies (whites and sulphurs) feed almost exclusively on a single plant family. A small proportion of butterflies will eat a broad range of plants from unrelated families. A dramatic example is the wide-ranging painted lady, for which more than 100 host plants from various families, including the asters, legumes, and mallows, have been recorded.

Food plant specialization offers certain ecological advantages. If the preferred food plant occupies a predictable, stable habitat, the specialist can establish a long-term local colony and need not spend a lot of time seeking and sampling potential host plants. A specialist can become closely attuned to the appearance, micro-habitat, and seasonality of its host plant and can finely adapt to any other animals and parasites that utilize the plant.

In addition, competition with other insect species for use of the plant is likely to be reduced with food plant specialization. To defend against herbivory, plants have evolved different protective mechanisms. For example, some plants produce distasteful and toxic chemical compounds. There are high metabolic costs to the insects who eat these plants. This limits the number of different groups of plants that any one species can utilize.

One of the disadvantages of food plant specialization is that the specialist depends on the fate of a reduced number of, in some cases, just one plant species. Some butterflies are known to have broadened their host plant choices when their preferred plant becomes scarce. Nevertheless, the extensive and rapid habitat disturbance and destruction across eastern North America have favored those butterfly species that are broad ranging, generalist feeders.

As we begin to devote more attention to the protection of Wisconsin's native insect fauna, we can look to a popular and relatively well-understood group like the butterflies and see how closely and obviously tied to many of our native plants and communities these insects are. For conservationists, this plant-insect bond pointedly demonstrates the importance of protecting not just species of plants and animals, but also the ecological relationships among plants and animals through time.