Regrowth of arundo (Arundo donax) after herbicide treatment supports a higher density of the shoot tip-galling wasp Tetramesa romana released for biological control

Authors: Moran, Patrick

Submitted to: Southwestern Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2025
Publication Date: 6/18/2025
Citation: Moran, P.J. 2025. Regrowth of arundo (Arundo donax) after herbicide treatment supports a higher density of the shoot tip-galling wasp Tetramesa romana released for biological control. Southwestern Entomologist. 50(1):104-116. https://doi.org/10.3958/059.050.0133.
DOI: https://doi.org/10.3958/059.050.0133

Interpretive Summary: The giant invasive grass known as arundo, giant reed or carrizocane in the United States (Arundo donax) thrives in riparian habitats along rivers, creeks, canals and reservoirs in the hot, dry southwestern U.S. This giant grass (up to 9 meters of 40 feet tall) consumes water, decreasing the amount of water available for irrigated agriculturem, human consumption and recreation, and native species protection. It also fuels wildfires, alters stream flow patterns and displaces native plants and animals. Chemical herbicides such as glyphosate and/or imazapyr are often applied to control arundo and can kill many shoots, but in most places new srundo shoots eventually regrow. The shoot-tip galling wasp Tetramesa romana, harmless to humans, was released for biological control of arundo in the Sacramento and San Joaquin River watersheds of northern California as well as in their Delta where they come together and empty into San Francisco Bay. The wasp female lays eggs on the tips of the main shoots and side shoots of arundo, causing a gall or 'tumor' to develop. Larvae feed inside the gall, pupate, and chew an exit hole when emerging as adults. Landowners mixed and applied the two herbicides noted above at the sites around the same time as the the wasps were being released, but biocontrol release plots set up at each site were not sprayed, to allow the wasps to establish populations at each site. Two to five years later, regrowth was noticed in the sprayed areas, and both the original release plots and regrowth patches were sampled by collectng and dissecting side shoots and counting exit holes made by emerged adults as well as larvae and pupae inside the galls. The density of T. romana per side shoot was 12-fold higher in the regrowth patches across all three sites than in the original biocontrol release plots (a statistically significant difference), and up to 38-fold higher at one of the three sites. Wasp density on regrowth was statistically higher compared to the release plots at two of the three sites. Arundo regrowth patches contained 2.6-times more live biomass than did the original release plots, as they were more vigorous. The wasp T. romana, like many insects that make galls, prefers vigorous tissues for galling. The wasp's increased density on post-herbicide regrowth has implications for integrated management, This wasp could disperse across herbicide-treated areas on regrowth to reach untreated areas, and they may be able to limit thew amount of regrowth that occurs after herbicide treatment.

Technical Abstract: The shoot tip-galling wasp Tetramesa romana Walker was released in the Lower Rio Grande Basin of Texas and Mexico and also in the Central Valley of northern California for biological control of arundo (Arundo donax). Between two and five years after herbicide treatment at three release sites in the Sacramento-San Joaquin Delta, regrowth shoots were examined for galls with exit holes made by adults and wasp larvae and pupae inside. The density of the wasp on side shoots was 12-fold higher in patches of arundo regrowth (0.85, about one wasp on each side shoot) than in untreated patches (0.07, less than one wasp among 10 side shoots) across all three sites and was significantly higher on regrowth at two of three sites. Regrowth patches did not show symptoms of herbicide damage as they contained 2.6 times more live biomass than did untreated patches. Regrowth supported more abundant galls of T. romana because of its vigorous growth relative to untreated patches containing older arundo. The results show that biological and chemical control of arundo can be integrated.