Impact of the arundo wasp, Tetramesa romana (Hymenoptera: Eurytomidae) on biomass of the invasive weed, Arundo donax (Poaceae: Arundinoideae) and on revegetation of riparian habitat along the Rio Grande in Texas

Authors: Moran, Patrick; Vacek, Ann; RACELIS, ALEXIS - University Of Texas Rio Grande Valley; Pratt, Paul; Goolsby, John

Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2016
Publication Date: 1/18/2017
Publication URL: https://handle.nal.usda.gov/10113/5616010
Citation: Moran, P.J., Vacek, A.T., Racelis, A.E., Pratt, P.D., Goolsby, J. 2017. Impact of the arundo wasp, Tetramesa romana (Hymenoptera: Eurytomidae) on biomass of the invasive weed, Arundo donax (Poaceae: Arundinoideae) and on revegetation of riparian habitat along the Rio Grande in Texas. Biocontrol Science and Technology. 27:96-114.

Interpretive Summary: Non-native, invasive weeds often invade the banks of rivers and lakes. These habitats are important for protection of the diversity of native plants and animals, some of whom can survive only in these shoreline areas, known as "riparian" habitats. Riparian areas are especially important in dry regions in which rainfall and hence moisture for plant growth is otherwise scarce. Rivers and lakes in the southwestern U.S. flow through such dry areas, including the Lower Rio Grande River that forms the border of Texas and Mexico. Arundo (Arundo donax), also known as giant reed and carrizocane, is a giant grass that has taken over thousands of acres of habitat along the Rio Grande. This grass, native to Mediterranean Europe, consumes valuable water, making it unavailable for agriculture and human use. The grass shoots, which can grow over 30 ft tall, block access to water and obstruct visibility for border enforcement along the Rio Grande. The shoots also burn easily, creating fire hazards. Dense thickets of arundo prevent all other plants from growing. In 2009, a wasp (Tetramesa romana) colected originally from Mediterranean Europe, that makes 'tumors', known as galls, on the shoot tips of arundo was released into the field to initiate biological control of arundo along the Rio Grande. Adult wasps lay eggs in shoots, eggs hatch, and larvae feed and develop on the swollen shoot tips, which can die from the gall. In a 2014 study, we demonsatrated that the wasp has reduced the weight or 'biomass' of live arundo shoots by 22%, five years since the wasp was released. In the present study, we took the next step-to see if biocontrol damage by the arundo wasp fosters the appearance of other plant species along the banks of the river. We demonstrated the presence of 44 plant species in areas invaded by arundo, across seven field sites between Brownsville and Del Rio, Texas where the arundo wasp has been released. Each field site hosted between six and 12 plant species, and each of the three field plots at a typical site hosted five species. We showed that the arundo wasp is continuing to reduce arundo live estimated weight-by 32% since our last study in 2014. The amount of live arundo weight in our field plots shows a negative association with the diversity of plant species in the plots. In other words, the less live arundo is in a field plot, the greater the number of other plant species can grow in the plot. The number of dead shoots in the plot, and the amount of damage being done by the arundo wasp (galls and 'exit holes' made in galls by adults as they emerge) is positively associated with plant diversity-the more shoots that are being killed by the wasp, and the more damage the wasp is doing, the greater the diversity of other plants that can sprout and grow in the plots. Biological control of arundo with the arundo wasp is thus increasing native plant diversity along the Rio Grande.

Technical Abstract: An invasive grass, Arundo donax, occupies thousands of hectares of arid riparian habitat along the Rio Grande and was the first perennial grass to be targeted with biological control, due to the great negative impacts of this weed on water resources and riparian ecosystems. The shoot-tip galling wasp Tetramesa romana was released in 2009 and has dispersed over 900 km along the river channel. Plots were surveyed for shoot counts of arundo and all other plant species in 2016 at seven sites in regions in which prior studies had documented a decline in arundo biomass since the first wasp releases. Biomass was estimated from live shoot height, and side shoots and exit holes made by emerging adult wasps counted. A total of 44 plant species were encountered in plots, 86% native. Plots averaged five species, and arundo was most abundant in only 9 of 21 plots. Estimated arundo biomass declined by 32% since 2014. Arundo live biomass and shoot density were negatively associated with plant diversity, indicating that live arundo interfered with other plant species. Increasing density of dead main shoots, of shoots bearing wasp galls, and of exit holes on live main shoots were associated positively with plant diversity in a combined model. Pairwise regressions indicated that the effect of the wasp damage measures were mediated through their effects on shoot mortality. By reducing live arundo biomass, the arundo wasp is contributing to recovery of a diverse native plant community at riparian sites along the Rio Grande.