Establishment and dispersal of the armored scale Rhizaspidiotus donacis for biological control of Arundo donax in northern California

Authors: Moran, Patrick; ROGERS, DYLAN - University Of California Berkeley; BITUME, ELLYN - Us Forest Service (FS); Portman, Scott; GOOLSBY, JOHN - Former ARS Employee

Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/26/2025
Publication Date: 6/8/2025
Citation: Moran, P.J., Rogers, D.V., Bitume, E.V., Portman, S.L., Goolsby, J.A. 2025. Establishment and dispersal of the armored scale Rhizaspidiotus donacis for biological control of Arundo donax in northern California. Biocontrol Science and Technology. 35(7):755-776. https://doi.org/10.1080/09583157.2025.2514451.
DOI: https://doi.org/10.1080/09583157.2025.2514451

Interpretive Summary: The non-native giant grass known as arundo, giant reed or carrizocane grows along the banks of canals, rivers and lakes in the dry, hot southwestern United States. This grass can grow as tall as 35 ft (10 m). It wastes water that is needed for irrigated agriculture and drinking. This invasive weed also fuels wildfires, obstructs flood control systems, alters river flow patterns, displaces native plant and animal species and conceals illegal activities that harm the environment. USDA-ARS scientists released a tiny insect known as an armored scale to reduce the growth and vigor of arundo at nine sites in the Central Valley of northern California. Adult females of the armored scale, known scientifically as Rhizaspidiotus donacis, are immobile, with no legs or antennae. They produce tiny live crawlers, which move from plant to plant and settle within a day or two, becoming immobile themselves. Immature scales molt through two stages and reach adulthood either as short-lived (1 to 4 days) winged adult males that mate with females, or as long-lived (3 to 5 additional months) immobile adult females, and they feed on the liquid contents of plant cells in the tuber-like rhizomes (roots), base of shoots and leaf bases of arundo. The armored cale produces one new generation per year. The armored scale was released at nine sites in the Sacramento and San Joaquin River watersheds and in the Delta where the rivers meet before emptying into San Francisco Bay by planting arundo 'microplants', small potted arundo plants that had been infested with the armored scale about six months prior, and so had adult females ready to produce a new generation of crawlers. In short-term establishment surveys conducted one year after release in 2015-2019, the armored scale was found at all nine sites in at least one plot per site, as about 50% of the 78 plots sampled across all sites yielded a rhizome with at least one adult female. This result proved that crawlers dispersed from the microplants and colonized the resident arundo in the plots. Four to eight years after release (in 2022-2023), samping of side shoot nodes on main shoots indicated long-term establishment of the amored scale at all nine sites, as 71% of 52 plots sampled had at least one node with at least one adult female, with an average of 47% of the nodes in each plot having females, and all or almost all of the plots at five sites had females on nodes. At sixteen plots across eight of the field sites sampled in early 2024, adult females were found an average of 33ft or 10 m from 13 of the 16 plots, but up to 240 ft or 73 m from one release plot. The crawlers thus dispersed from the release plots, and are continuiong to disperse and build populations. Based on the known impact of R. donacis elsewhere, this armored scale is expected to reduce live biomass and vigor of arundo over time, decreasing its negative effects on water resources.

Technical Abstract: The rhizome-, shoot- and leaf-feeding armored scale Rhizaspidiotus donacis (Hemiptera : Diaspididae) was released for biological control of arundo (Arundo donax), a water-consuming invasive riparian weed in the arid Sacramento and San Joaquin River watersheds in the Central Valley of northern California, USA. Adult female scales reared on potted arundo shoots were planted into field plots at nine field sites. One year after release, at least one adult female was present on resident A. donax rhizome samples from 47% of plots later selected for dispersal monitoring. Short-term establishment was confirmed among other plots at all nine field sites (11 total release sites). Four to eight years after release, 47% of the nodes of side shoots removed from live main shoots in the plots supported at least one adult female of R. donacis, and at five sites most or all of the plots examined had established populations, verifying long-term establishment at all sites. Pre-release mechanical control of A. donax (ground-cutting, mowing or no cutting) did not influence short or long-term establishment. Side shoot nodes were examined at various distances from 16 plots across eight sites. Reproductive adult females dispersed 10 m or less from 13 of these plots (across eight sites), with annual dispersal under 2 m per year, consistent with armored scale crawler active dispersal ability. Several plots showed more substantial dispersal, up to 73 m, caused possibly by flooding or human disturbance. The armored scale R. donacis was confirmed as being well-established, dispersing and building populations in the Central Valley, with potential to negatively impact A. donax.