Biology of an adventive population of the armored scale Rhizaspidiotus donacis, a biological control agent of Arundo donax in California

Authors: BRAMAN, CHARLES - University Of California; LAMBERT, ADAM - University Of California; OZSOY, ZEYNEP - Colorado Mesa University; HOLLSTEIN, ELLEN - University Of California; SHEEHY, KIRSRIN - University Of California; MCKINNON, TARA - University Of California; Moran, Patrick; Gaskin, John; Goolsby, John; DUDLEY, TOM - University Of California

Submitted to: Insects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2021
Publication Date: 6/29/2021
Citation: Braman, C.A., Lambert, A.M., Ozsoy, Z.A., Hollstein, E., Sheehy, K., McKinnon, T., Moran, P.J., Gaskin, J.F., Goolsby, J., Dudley, T.L. 2021. Biology of an adventive population of the armored scale Rhizaspidiotus donacis, a biological control agent of Arundo donax in California. Insects. 12(7):588. https://doi.org/10.3390/insects12070588.
DOI: https://doi.org/10.3390/insects12070588

Interpretive Summary: The non-native, invasive giant grass known as arundo or giant reed is a bamboo-like grass that grows up to 30 ft tall along the banks of rivers, creeks and lakes in the hot, dry southwestern regions of the U.S. from Texas to California. Arundo forms dense thickets in these areas that consume and waste scarce water, block access to water, obstruct canals, bridges, levees and flood control channels, fuel wildfires, and displace native plants and animals. Chemical control with herbicides and mowing or digging/removal control are expensive, and do not provide long-lasting, widespread control. Biological control of weeds, using insects and mites imported from the weed's native range, is a safe and effective method to control major, widespread invasive weeds of forests, pastures, rangelands, freshwater shoreline areas and wetlands. Two insects from arundo's native range in Mediterranean Europe, a shoot tip-galling wasp and a root- and shoot-feeding armored scale, were released for biological control in Texas and northern California for biological control of arundo, and they have reduced the live biomass of arundo in the Lower Rio Grande Basin of Texas and Mexico. Evaluations are ongoing in northern California. Researchers at the University of California-Santa Barbara in southern California previously discovered an accidentally-introduced or 'adventive' population of the arundo wasp in Santa Barbara, Ventura, Los Angeles, San Bernardino, Orange and San Diego Counties. In this paper, these researchers report the first finding of an adventive population of the arundo armored scale. The arundo armored scale is a tiny (about 1/16 of an inch as adult), round to oblong insect that is motionless for most of its life cycle, and feeds on vascular tissues and fluids inside the arundo roots and stems. Immobile adult females deposit mobile 'crawlers' which must find a new place on arundo to settle within 1-2 days or die. Crawlers that find a place insert their mouthparts into arundo, lose their legs and antennae and become immobile, secrete a brown, waxy covering for protection, and molt through one additional immature stage before reaching the adult stage. Males take 2 months to develop from crawler to short-lived winged adults, which mate with immobile females. Females take 4 to 6 total months from crawler to complete adult expansion and development of a new generation of crawlers. The UC-Santa Barbara researchers surveyed arundo populations from San Diego to San Luis Obispo and found the adventive arundo armored scale only in the Santa Clara River near Santa Paula and Saticoy, in Ventura County, CA. Genetic DNA fingerprint analyses in collaboration with researchers from the USDA-ARS and Colorado Mesa University found that the adventive southern California arundo armored scale is related to but not identical to known collections of the arundo armored scale from eastern Spain and southwestern France, which were used for laboratory host range evaluations to confirm safety prior to release, and for field releases in Texas and northern California. Collections of the arundo armored scale from Greece and Italy were more distant genetically. The adventive southern California population produces one generation per year in the field, with crawlers appearing between March and June. In greenhouse tests, crawlers were able to settle on common reed, a related plant with both native, invasive, and hybrid forms in the U.S. Adult females developed on common reed, but were 10- to 27-fold less abundant on common reed than on the invasive weed, giant reed. A parasitic wasp intentionally released in California for biological control of California red scale also deposited eggs inside adventive arundo armored scales and killed them in the laboratory, leading to production of adult wasp offspring. The results overall indicate that the adventive arundo armored scale is

Technical Abstract: The invasive grass Arundo donax (giant reed) presents major concerns in Mediterranean, subtropical, and tropical riparian habitats. The armored scale Rhizaspidiotus donacis is approved for biocontrol in North America, but has a recently-discovered adventive California population. A series of studies documented the adventive R. donacis’ Californian distribution, phylogeny, phenology, potential host spillover to Phragmites australis subspp ., and host viability for a common biocontrol parasitoid. The adventive scale population was found within a single watershed in southern California and is genetically nearer, but not identical, to Iberian (eastern Spain and southwestern France) scale populations tested in the USDA biocontrol program and released in Texas and northern California, than to scale accessions from Italy and Greece. Rhizaspidiotus donacis developed on Phragmites haplotypes but at much lower densities than Arundo. The adventive population is univoltine, producing crawlers from March through 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 and adventive populations presents opportunity for exploring benefits of hybrids and/or alternative genotypes where establishment has been difficult. While both likely occur rarely in situ, spillover to vulnerable endemic Phragmites or deleterious parasitoid effects on scale.