COS 34-3 - Assessing the impacts of an invasive ant species on first-generation monarch butterfly reproductive success

Abstract: One hypothesis for the significant population decline of monarch butterflies (Danaus plexippus) over the past two decades is reduced reproductive success. Within the southeastern U.S., the primary production region of first-generation monarchs and a potential population bottleneck, this decline has occurred concurrently with the spread of the invasive red imported fire ant (Solenopsis invicta, RIFA). This relationship and evidence of RIFA’s negative impact on native arthropod communities have sparked multiple research efforts investigating the potential effects of RIFA infestation on monarch reproductive success; however, results so far have been inconclusive. Research challenges include accurately estimating monarch survival probabilities and effectively reducing RIFA activity in sampling areas given their extensive foraging range and ubiquity in highly infested areas. The objective of this study was to investigate the potential effect of a large-scale RIFA suppression treatment (2,070-ha) on monarch oviposition selection and survival from egg to pupation in a Texas coastal prairie with high RIFA infestation. Data collection was conducted from April-May 2022 in 1-ha sampling quadrats containing butterflyweed milkweed (Asclepias tuberosa) patches located within the RIFA-suppressed and untreated areas. Oviposition preferences and survival probabilities were calculated using weekly stage-based field counts of eggs and larvae analyzed with a Bayesian state-space model. Additionally, vegetative and arthropod community data were collected at each quadrat. Our results indicate that RIFA suppression did not impact monarch oviposition preferences. The best predictors of oviposition selection were milkweed patch density and time since last prescribed burn. We found a 0.93 probability that mean survival to pupation was higher in RIFA-suppressed sampling quadrats than in untreated quadrats. Conversely, there was no difference between treatments in estimated larval survival to the third instar indicating that impacts of RIFA infestation disproportionately affect later instar caterpillars. Predatory ant abundance was a better predictor of larval survival to pupation than RIFA alone, although most high abundance ant quadrats were associated with RIFA. The results of our studies have relevant implications for land managers interested in promoting monarch habitat use and recruitment success. Future conservation efforts should consider not only impacts of invasive predators on larval survival, but also their effects on foraging behaviors of native predators as well as other habitat management practices to maximize monarch reproductive success.