Fire increases ant-tending and survival of the Fender's blue butterfly larvae

Studies of fire as a management tool often focus on how fire alters microclimate and affects plant quality. While these processes are important, they may act on a target species indirectly through species interactions. Fender's blue [Plebejus (=Icaricia) icarioides fenderi] is an endangered butterfly found in fire dependent upland prairies in Oregon's Willamette Valley, USA. In this study, we measured oviposition preference and overwinter larval survival and how they responded to an experimental burn. To explore the mechanism underlying differences in these vital rates, we studied gross plant chemistry of Fender's blue larval host plants Kinkaid's lupine (Lupinus oreganus = L. sulphureus spp. kincaidii) and spur lupine (L. arbustus) and monitored how temperature and fire history affect the mutualism between Fender's blue larvae and a variety of tending ants. We used open top chambers to expand the range of observable temperatures, overlaid with the experimental burn to determine how both factors affected ant recruitment and duration of tending bouts. Oviposition rates and overwinter larval survival were higher in previously burned areas. We found no corresponding change in plant chemistry. Ant tending was affected by both temperature and fire history, with larvae able to recruit ants more quickly in warmer temperatures and fire history influencing the length of tending bouts. Differences in ant tending may explain enhanced overwinter survival. This case study highlights the importance of indirect (through species interactions) as well as direct effects in determining the impacts of fire on population dynamics of an at-risk butterfly species.