Genetic Monitoring of Tamarisk Biocontrol Agents (Diorhabda spp.) Along the Gila River Using Mitochondrial CO1 Barcoding 

 Francisca Esquivel1*, Zeynep Ozsoy1, Matthew J. Johnson2, Amanda Stahlke1 

1Department of Biological Sciences, Colorado Mesa University, Grand Junction, CO, USA

2EcoPlateau Research, Bend, OR, USA

As the four tamarisk beetle species (Diorhabda spp.) introduced for biological control of invasive tamarisk (Tamarix spp.) continue to colonize into new areas of North America, it remains unknown whether beetle species identity affects efficacy as biocontrol agents or nontarget impacts on the surrounding ecosystem. In riparian habitat, where the federally endangered Southwestern Willow Flycatcher (SWFL; Empidonax traillii extimus) breeds, multiple beetle species co-exist and could hybridize, biocontrol agent evolution and species interactions could degrade flycatcher breeding habitat and affect this endangered species recovery. Since 2020, we have been using genetic and genomic tools to monitor as D. carinulata has moved east and D. sublineata has moved west along the Gila River. Previous studies found differences in spring emergence and diapause timing between these two species, though recent experiments suggest novel adaptations to southern climates that may eliminate these differences. Furthermore, since these two species cannot form successful hybrids, we could be monitoring competitive exclusion as one species becomes more dominant than the other. There may also be differences in size between species, which could correspond to greater or lesser rates of tamarisk defoliation overall.  To determine which species are present in the Gila River Basin and to help determine how beetle species identity predicts efficacy or nontarget impact, we performed molecular identification (mt-CO1) of 130 beetles collected between May and August of 2023. We found that of the current 130 samples sequenced east of the San Carlos Apache Tribe Reservation, all were identified as D. sublineata, except for one D. carinulata. On the west side of the Reservation, no adults were observed. Together, these results suggest that D. sublineata may become the dominant species along the Gila River, providing evidence for competitive exclusion. We compare these results to our previous work in species contact zone along both the Gila River and the Middle Rio Grande River. Our work complements a decade of genetic monitoring, improves our understanding of how species distributions of tamarisk beetles continue to change and what that change could mean for the future of SWFL.