Establishment and dispersal of the armoured scale <i>Rhizaspidiotus donacis</i> for biological control of <i>Arundo donax</i> in northern California

The rhizome-, shoot- and leaf-feeding armoured scale <i>Rhizaspidiotus donacis</i> (Hemiptera: Diaspididae) was released for biological control of invasive arundo (<i>Arundo donax</i>) in the Sacramento and San Joaquin River watersheds of the Central Valley of northern California. One year after release, short-term establishment of <i>R. donacis</i> was confirmed at all nine field sites (11 release sites). Four to eight years after release, 47% of the nodes of side shoots removed from live main shoots in the release plots at the nine field sites supported at least one adult female of <i>R. donacis</i>, verifying long-term establishment, and at five sites \100% of the plots examined had established populations. Pre-release mechanical control of <i>A. donax</i> (ground-cutting, mowing or no cutting) did not influence short or long-term establishment. To determine dispersal of <i>R. donacis</i>, side shoot nodes were examined at various distances from 16 plots across eight sites. Scale prevalence and abundance were negatively related to distance from release plot. Reproductive adult females dispersed 10 m or less from 13 of these plots, with annual dispersal under 2 m per year, consistent with general expectations of armoured scale crawler active dispersal ability, and maximum dispersal distance was not dependent on scale density in the release plot. Dispersal between 40–73 m in two plots was possibly caused by disturbance. The armoured scale <i>R. donacis</i> was confirmed as being well-established, dispersing and building populations, with potential to negatively impact <i>A. donax</i>.