Submitted to: Journal Subtropical Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2012
Publication Date: 7/18/2013
Citation: Racelis, A.E., Moran, P.J., Goolsby, J. 2013. Topping of Arundo donax as a pre-treatment to biological control. Journal Subtropical Plant Science. 64:54-60.
Interpretive Summary: Giant reed, an exotic woody grass native to the Mediterranean, has invaded river and canal banks and the shores of lakes and reservoirs throughout the southwestern U.S., negatively impacting the native plants and animals, water conservation, and border security and protection. Various management regimes have been explored, including mechanical cutting or mowing of the shoots, but with little success of control over large areas or over long time frames, except in localized areas at great cost. The recent release and successful establishment of biological control agents in the Lower Rio Grande Basin, which includes south Texas, represents a potential large-scale, long term, and inexpensive option; but, not much is known whether biological control can be successfully integrated with mechanical control. We predicted that well-timed cutting treatments can make the giant reed shoots more prone to attack by a biological control insect, the arundo wasp, which causes tumor-like galls on the shoot tips of new side shoots, which are often abundant on plants that have been cut or mowed. We documented responses of giant reed to cutting at different heights, to explore whether cutting of giant reed can serve as an effective pre-treatment to biological control. Repeated ground level cutting (mowing) was unsuccessful at reducing total weight of main shoots or height, and these re-growing main shoots had few side shoots. Cutting giant reed stems at 1 m or more above ground level (topping) led to the production of many side shoots. The soft, young tissues of these lateral side shoots are good places for the immature larvae of the arundo wasp to feed and develop, as we found many holes in the lateral shoots made by adult wasps chewing their way out of galls. The side shoots would also be good for another biocontrol agent, the arundo armored scale, which we know from other studies likes to settle and feed on plant juices on the side shoots. Other factors, such as shoot tip nutrients or temperature conditions for egg-laying wasp adults might also influence the ability of field shoots subjected to mowing or topping to support arundo wasp populations, but our results do suggest that topping can augment wasp populations as evidenced by relatively higher incidence of successful wasp reproduction (exit holes) in topped plants. We discuss the implications of the plant growth responses to different cutting treatments in the context of a large-scale biological control program.
Technical Abstract: Multiple methods have been employed to control invasive giant reed populations in the southwestern U.S., including modes of mechanical and chemical control, with little success of control at a landscape or temporal scale except in localized areas at great cost. The recent release and successful establishment of biological control agents in the Lower Rio Grande Basin, which includes south Texas, represent a potential large-scale, long-term, and inexpensive option; but little is known regarding the synergistic effects of mechanical control on host plant suitability for biological control. We hypothesized that timed cutting treatments can increase host plant suitability for biological control. We documented phenotypic, plastic responses of giant reed to cutting at different heights to explore whether cutting can serve as an effective control measure and as a pre-treatment to biological control, particularly with the arundo wasp Tetramesa romana, which deposits eggs into shoot tips. Ground level cutting (mowing) is unsuccessful at reducing total biomass or height, whereas cutting giant reed at 1 m or more above ground level (topping) reduces final height. Mowed areas were recolonized by emergent ramets that 9 months later had allocated 60% of total biomass to main shoots and achieved significantly more height gain than did plants cut to 1 m and 2 m. Topping at 1 m or more above ground level induced the production of lateral side shoots at the expense of upward growth. Plants left uncut topped at 2 m or 1 m had 28-fold, 42-fold, and 8-fold more exit holes, respectively, than did plants cut to ground level, which typically had 0 to 2 holes per plant (F3,56 = 21.7, P <0.001, site effect, F2,56 = 0.5, P =0.60, interaction, F6,56 = 2.9, P = 0.01). Other factors, such as shoot tip tissue quality or wasp density might have also influenced the ability of field shoots to support arundo wasp populations. We discuss the implications of plant growth responses to different cutting treatments in the context of a large-scale biological control program.