COS 119-2 - Ant tending in Fender's blue butterfly (Plebejus icarioides fenderi): Assessing an understudied mutualistic relationship in an endangered lycaenid

Wednesday, August 9, 2017: 1:50 PM
D139, Oregon Convention Center
Cameron C. Thomas, School of Biological Sciences, Washington State University Vancouver, Vancouver, WA and Cheryl B. Schultz, School of Biological Sciences, Washington State University, Vancouver, WA
Background/Question/Methods

Fender’s blue is a federally endangered butterfly which survives in remnant prairie habitat in Oregon’s Willamette Valley. Less than 1% of historic habitat remains, and much of that has been degraded by invasive vegetation. Research to date has focused on the adult stage, leaving the ecology of larval stages understudied. Fender's blue larvae maintain a facultatively mutualistic relationship with ants, and recent work suggests ant tending may significantly increase larval survivorship, resulting in a higher population growth rate. Our study had three objectives relative to ant tending in Fender’s blue: determine biotic and abiotic factors affecting the ant-larvae mutualism; investigate differences in abundance, distribution, and behavior among tending ant species; and determine factors associated with higher larval survivorship. During the post-diapause larval season in 2016, we collected data on larvae, tending ants, and vegetation community structure, cover, and composition among nine sites grouped into three locations of observation. We also documented ant tending behavior and the response behavior of ants to provocation. We compared larval counts with egg counts from the pre-diapause larval season in 2015 to measure and model survivorship as a response to factors associated with ant tending.

Results/Conclusions

In late February – March 2016, we documented ant tending in 151 post-diapause Fender's blue larvae. We observed ten species in nine genera across three subfamilies of Formicidae tending larvae in West Eugene. Vegetation at the base of the host plant, grass cover, height of the host plant, and observation location were significant predictors of ant recruitment and recruitment time; specifically, thatch left over from annual mowing of tall invasive grasses significantly increases recruitment time. Total tending time, behavior during the tending event, and response to provocation differed significantly among ant species. Habitat variables also differed significantly among ant species, primarily by vegetation structure and soil properties. Survivorship in Fender’s blue larvae varied among sites, and proportion of larvae tended in each plot was the most significant predictor of survivorship in our study. Larval survivorship increases with increased ant tending, and at two of the three locations, survivorship was more than three times higher in heavily tended plots than those tended infrequently. Our results aim to inform restoration efforts relative to the influence of invasive species on vegetation structure during the Fender’s blue larval phase, a stage which is more significant than previously documented.