The effects of fire on the behavior of an endangered butterfly: A study of Fender's blue in Oregon's Willamette Valley

Background/Question/Methods Fender’s blue Plebejus icarioides fenderi is an endangered butterfly found in Oregon’s Willamette Valley. Its prairie habitat has been largely lost to development, agriculture and invasive species, with remaining prairie fragments threatened by succession in the absence of historic fire. Prescribed burning is a tool used to slow the encroachment of woody plants including poison oak Toxicodendron diversilobum, Himalayan blackberry Rubus armeniacus, and Scotch broom Cytisus scoparius and to stimulate growth of the butterfly’s larval hostplants, Kinkaid’s lupine Lupinus oreganus and spur lupine Lupinus laxiflorus. Previous demographic studies indicate that Fender’s blue females lay more eggs in burned habitat. We explored the behavioral basis for this pattern, to assess if butterflies spent a larger proportion of their time ovipositing in burned habitat relative to unburned habitat and whether their movement behavior responded to burning. In this study, conducted during spring 2011, we measured the effects of fire on Fender’s blue behavior by observing female butterflies in unburned vegetation and in areas that were burned the previous fall. We quantified the time butterflies spent foraging, ovipositing and flying in burned and unburned vegetation. We also recorded flight behavior (step lengths and variation in turning angles) to assess the rate of movement through burned and unburned habitat.

Results/Conclusions We found that female Fender’s blue butterflies spent a larger proportion of their active time flying in unburned habitat while they spent a larger proportion of their time ovipositing in burned habitat. Analysis of flight path data indicate that butterflies take shorter flight steps and stop more frequently in burned habitat. These behavioral observations have important implications for managing prairie habitat using fire. They suggest that behavioral responses moderate demographic effects of fire on population dynamics.  For example, our past work has shown that fire kills overwintering larvae in the year of burning.  Active selection of higher-quality habitat in burned areas will allow populations to recover more quickly from the short-term negative effects of burning.