Managing for Lepidoptera

The extensive prairie fires that occurred prior to the 1830's killed billions of insects each and every time they occurred. However, these fires rarely burned the entire landscape. Patches of habitat were often left unburned and patterns of burn intensity varied enough to reduce the impacts on insects. Following these fires, the incredible fecundity (ability to reproduce) of most insects would permit rapid recolonization of their habitat.

In today's landscape, though, prairies are fragmented into small remnants, which often are separated by miles of totally unacceptable habitat for certain Lepidoptera (butterflies, skippers, and moths). Could our prescribed burning or other management on natural areas eliminate a rare butterfly population?

Lepidoptera management is a subject area with lots of questions and few answers. Endangered Resources staff have talked to butterfly experts and attended workshops on Lepidoptera management and have learned that prescribed burning must be done with care to avoid inadvertently eliminating a species. Some Lepidoptera species are clearly sensitive to fire, and some of these require recolonization to recover. For others, recolonization accelerates population recovery. Since it is difficult to say with certainty which species can recover without recolonization, it is important that we take precautions to protect populations of these species when conducting burns. One way to preserve prairie insects is to divide the area to be burned into units and burn some, but not all, of those units in any one year. We incorporate this technique into the management plans for all of our State Natural Areas. First, we determine management goals for the entire natural area. In the case of prairie communities, a major goal usually includes improving the quality and size of the prairie through burning. Then we determine what species of concern are found on a particular area and which of these will be affected by our management. The larger the prairie area the better the chances that the affected species can be retained. The area is divided into burn units, and we devise a burning schedule that will allow those rare species to seek shelter in the unburned areas or build up a population large enough to recolonize the burned area, in case such backup is needed.

Invertebrate refugium (the unburned portion) at Blue River Bluffs State Natural Area.

For example, at Dewey Heights Prairie along the Mississippi River experts have identified six butterfly and one moth species of special concern. Although we are not doing any specific management for them, some prairie is left unburned when we conduct prescribed burns.

At Muralt Bluff Prairie in Green County there is a large and healthy population of the regal fritillary (Speyeria idalia). The larval food plants of this colorful butterfly are bird's-foot violet and other prairie violets. Therefore, we have delineated the burn units so that there are populations of violets in more than one unit and we make sure that at least one of these areas with violets is left unburned.

The inconspicuous and state-endangered Poweshiek skipper (Oarisma powesheik) is found at Scuppernong Prairie in southeastern Wisconsin's Kettle Moraine region. Poweshiek skippers overwinter as larvae in prairie grasses and their populations are highly localized. When burning at Scuppernong we leave about one of the five acres of skipper habitat unburned.

At Young Prairie in Walworth County there are two Lepidoptera of concern, the state-endangered silphium borer moth (Papaipema silphii) and the black dash (Euphyes conspicua). The larvae of the borer moth live in the roots and stems of prairie dock (Silphium terebinthinaceum). However, this species overwinters from September to early May. Because the eggs may be vulnerable to fire, prescribed burning after May 1 could minimize the impact on this species, but this has the potential to harm other species. More research is needed to help make this determination. The larvae of black dash feed on sedges. If sedge growth is delayed by late spring burning, the black dash larvae might starve. Therefore, we burn only a portion of this prairie at any one time so that the development of at least some of the sedges is timed correctly for the butterfly.

Although this type of management requires additional time and expense, it is a necessary precaution to ensure that the entire insect component of our natural communities is preserved while maintaining the natural fire regime under which the community has evolved. Prairie plants and insects depend on each other; protecting these interrelationships is well worth the extra effort.