Prescribed fire improves nutritional quality of blue lupine leaves and enhances the reproductive success of the endangered Karner blue butterfly

Background/Question/Methods  Larvae of the federally endangered Karner blue butterfly (KBB, Lycaeides melissa samuelis) feed exclusively on leaves of the blue lupine (Lupinus perennis) and the growth environment of the host plant has been shown to affect KBB reproductive success. We hypothesized that management strategies that increase leaf N content would enhance fecundity and influence population growth rate (λ) of the KBB whose larvae were reared on them. We combined field manipulations of the blue lupine growth environment, captive rearing of KBB, and population modeling to explore how burning, mowing and deadheading affect leaf tissue quality, rates and timing of egg production, and estimates of KBB population growth rate (λ). Using a randomized block design (n=5), six treatments were established in 10 m x 10 m plots in a restoration site containing an herbaceous community with moderate to dense blue lupine cover. Treatments included one control, two burn (April 15 and May 1), two mow (May 1 and May 15) and one deadhead (May 15). Leaves from each plot were fed to larval KBB individuals and tracked through adult stage in a breeding facility. Forty-eight females with confirmed matings (n≥7 per treatment) were then measured for their rate of oviposition (eggs/d) at three d intervals. Leaves were also collected weekly from each plot and analyzed for leaf mass per area (LMA), N and C content.

Results/Conclusions  KBB whose larvae fed on foliage from the earlier burn and mow treatments had elevated oviposition rates during the first 3 d. Differences in the timing and rate of egg production were introduced into an existing KBB population model leading to 13% higher estimates of λ than controls. Leaves from the early burn treatment maintained significantly elevated concentrations of N through June 5 relative to the control. In addition, the early mow treatment had N concentrations intermediate between, but not significantly different from, the early burn treatment and the control. There was no significant difference in LMA among treatments. These results suggest that the early burn and mow treatments, which increase leaf N likely through changes in leaf phenology, enhance the nutritional quality of KBB forage. Consequently prescribed fire or mowing may benefit KBB populations. These benefits may outweigh the direct mortality associated with the treatment, especially when treatments are conducted within a matrix of undisturbed habitat that can provide adequate, untreated refugia.