Biology of an Adventive Population of the Armored Scale Rhizaspidiotus donacis, a Biological Control Agent of Arundo donax in California

SIMPLE SUMMARY: The invasive giant reed, Arundo donax, impacts river ecosystems world-wide. The plant-feeding scale insect Rhizapidiotus donacis is approved for biocontrol use in North America but a wild (adventive) population was found in southern California. We studied the adventive scale to document its distribution, life history, relatedness to European samples, risk to a native reed, and ability of a biocontrol wasp to develop within it. The adventive was found within a single watershed and is genetically closest to the Iberian scale population. Rhizaspidiotus donacis developed on some Phragmites reed types, but at lower densities than Arundo. It produces one generation each year with mobile juveniles from March through June. Aphytis melinus wasps showed similar interest in adventive R. donacis as their usual host with deposited eggs developing into a second generation. Rhizaspidiotus donacis appears limited in distribution by its life history, precluding broad biocontrol implementation through natural dispersal but allowing for targeted application. The genetic differentiation between imported biocontrol samples and adventive populations presents an opportunity for exploring benefits of hybrids and/or alternative genotypes where establishment has been difficult. While likely rare in the wild, spillover to vulnerable endemic Phragmites or deleterious parasitoid effects on scale populations warrants consideration when planning use of this agent. ABSTRACT: Arundo donax (giant reed) is invasive in Mediterranean, sub-, and tropical riparian systems worldwide. The armored scale Rhizaspidiotus donacis is approved for biocontrol in North America, but an adventive population was recently discovered in southern California. We documented this population’s distribution, phylogeny, phenology, potential host spillover to Phragmites spp., and potential for parasitism by a common biocontrol parasitoid of citrus scale. The adventive scale was found within a single watershed and is genetically closest to Iberian scale genotypes. Rhizaspidiotus donacis developed on Phragmites haplotypes but at much lower densities than Arundo. The adventive population is univoltine, producing crawlers from March-June. Aphytis melinus parasitoids exhibited sustained interest in R. donacis during choice and no-choice trials and oviposition resulted in a small second generation. Rhizaspidiotus donacis appears limited in distribution by its univoltinism and sessile adult females. This presents challenges for broad biocontrol implementation but allows for targeted application. The genetic differentiation between imported biocontrol samples and adventive populations presents an opportunity for exploring benefits of hybrids and/or alternative genotypes where establishment has been difficult. While unlikely to occur in situ, spillover to vulnerable endemic Phragmites or deleterious parasitoid effects on scale biocontrol agents warrants consideration when planning use of R. donacis.