Using caterpillar-host plant relationships to bolster forest bird resiliency to reduced cover of a threatened keystone tree species

Background/Question/Methods

Metrosideros polymorpha (‘ōhi‘a) forests are critical to most Hawaiian forest birds, but fungal diseases causing Rapid ‘Ōhi‘a Death (ROD) are changing forest structure and threatening food webs of threatened and endangered (“listed”) species. The Hakalau Unit of Hakalau Forest National Wildlife Refuge (Hakalau) harbors the largest and most stable populations of four listed bird species, presumably due to the availability of food resources, absence of ROD, and extent of managed habitat above the range of mosquito-borne avian malaria. Caterpillars are critical arthropod prey of listed bird species, but caterpillar-host plant associations are poorly known and trophic implications of reduced Metrosideros cover due to ROD are uncertain. Because a previous study indicated the overriding importance of five caterpillar species in the diets of listed bird species at Hakalau, we attempted to identify which ones were hosted by Metrosideros. We gently shook vegetation to dislodge caterpillars onto a collecting sheet and transported them to the laboratory for rearing to adult moths, which we then identified to family, genus, or species. During rearing, the mandibles of a subset of caterpillars were dissected and compared morphometrically to mandibles previously identified as important prey of listed birds.

Results/Conclusions

Eleven native plant species hosted at least two taxa identified to genus or species, with the most (seven) from Metrosideros. From mandibles extracted from caterpillars during rearing, we matched 10 of 19 morphotypes previously identified in Hakalau bird diets, potentially representing >95% of caterpillar prey eaten by Hakalau birds. Metrosideros hosted 8 of the 10 morphotypes, with 4 being exclusive to Metrosideros, including 3 of the 5 morphotypes most important to the 4 listed bird species. The earlier diet study revealed that the listed species consumed combinations of only two of the five key caterpillar morphotypes, indicating a high degree of specialization and vulnerability to reductions in Metrosideros cover. Former pasturelands at Hakalau are being reforested primarily by planting stands of Acacia koa (koa), which support most bird species to varying degrees. Our study indicates that increasing the density of six other native trees besides Metrosideros could benefit birds in koa stands by increasing the availability of caterpillar prey and boosting forest resiliency to invasive diseases and other threats. Supplementary studies using DNA barcoding to refine knowledge of caterpillar-host plant relationships can further help managers evaluate options for adapting to reduced Metrosideros cover due to ROD or other factors.