The effect of the armored scale, Rhizaspidiotus donacis (Hemiptera: Diaspididae), on shoot growth of the invasive plant Arundo donax (Poaceae: Arundinoideae)

Authors: CORTES, ELENA - University Of Spain; Goolsby, John; Moran, Patrick; MARCOS-GARCIA, MARIANGELES - University Of Spain

Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2011
Publication Date: 3/24/2011
Citation: Cortes, E., Goolsby, J., Moran, P.J., Marcos-Garcia, M.A. 2011. The effect of the armored scale, Rhizaspidiotus donacis (Hemiptera: Diaspididae), on shoot growth of the invasive plant Arundo donax (Poaceae: Arundinoideae). Biocontrol Science and Technology. 21(5):535-545.

Interpretive Summary: This study examined the impact of the arundo armored scale on the growth of side shoots on the stems of this non-native invasive weed. Arundo (also known as giant reed, carrizocane, and ladron del agua, of "water thief") is a giant grass, as tall as 25 ft, that is a native plant in Mediterranean Europe, including southern Spain. Arundo was introduced to Mexico and the U.S. hundreds of years ago by colonists for use in thatch roofing and for fences, but it has spread to become one of the most harmful invasive plants in the southwestern U.S. This harmful weeds grows along rivers, such as the Rio Grande, as well as reservoirs and canals, consuming precious water resources that are needed for agriculturall and domestic use in large low-rainfall areas such as the Lower Rio Grande Basin of Texas and Mexico, and the huge grass shoots reduce visability and access for border enforcement personnel. An important reason why this harmful grass has been able to cause problems along the Rio Grande and elsewhere is the fact that there are no local insects that feed on it and kill it in the U.S. The USDA-ARS is developing a biological control strategy to reduce the damage caused by arundo. Biological control of weeds involves the release of insects as natural enemies of the targeted weed. In the case of arundo, an armored scale insect from southern Spain has been tested rigorously in the laboratory to ensure that it feeds and develops young only on arundo, and not crop plants or native grasses, and permission to release this armored scale in the U.S. is pending. Armored scales are tiny, legless, non-mobile insects that use their needle-like mouthparts to remove juices from plant tissues, reducing the ability of the plant to grow. In this study, the arundo armored scale was examined at five sites in its native range in southern Spain. One half of the shoots at each site were treated with insecticide to kill the arundo scale, while the other half were not treated. The lengths of selected side shoots on each main shoot were then measured monthly. Arundo shoots infested with the arundo armored scale grew over two-fold more slowly than did shoots protected from the arundo scale with insecticide. Protected shoots grew most quickly in the warm, wet spring months, while infested shoots did not benefit from the spring warming of temperatures. This result matches the life cycle of the Arundo armored scale, which produces young in the spring. The results of this study suggest that the Arundo armored scale will be an effective biological control agent against arundo giant grass in the U.S. and Mexico.

Technical Abstract: In this study, the effect of feeding by the armored scale, Rhizaspidiotus donacis (Leonardi, 1920) (Hemiptera: Diaspididae) on the growth of the plant Arundo donax L. (Poaceae) was evaluated under field conditions in its native range. This study was designed to evaluate the impact of R. donacis, a candidate agent for biological control of A. donax which is invasive in arid riparian ecosystems of the southwestern U.S. and Mexico. The study was carried out at five A. donax sites in the Province of Alicante, Spain, differing in altitude and climate. At each site, 30 infested lateral shoots were selected and 15 were randomly treated monthly with imidacloprid insecticide. Shoot lengths were measured monthly over a one-year period in a comparative growth analysis. Shoots infested with R. donacis had an over two-fold reduced growth rate as compared to treated shoots. Growth of shoots varied by site, and the effect of R. donacis on growth was most pronounced in the late spring, when females produced neonate scale crawlers. The impact of R. donacis on A. donax growth under field conditions in the native range, combined with its narrow host specificity, indicate that R. donacis is a promising candidate for biological control of A. donax in North America and other areas invaded by this weed.