Release, establishment and realised geographic distribution of Diorhabda carinulata and D-elongata (Coleoptera: Chrysomelidae) in California, USA

This report summarises efforts to establish Diorhabda carinulata (Desbrochers) and D. elongata (Brulle) in California for the control of invasive saltcedars (Tamarix spp.), which degrade riparian ecosystems in the western United States. Over 14,000 D. carinulata individuals were released in California among four locations between 1999 and 2002 but beetles only established at the Tinemaha Reservoir site, the most eastern release location. More than 236,000 D. elongata individuals were released between 13 sites from 2003-2009 and establishment was limited to two sites, along the Cache and Pope creeks in northwestern California. The D. carinulata population did not disperse beyond the release area despite the presence of nearby (ca. 20 km) patches of the host plant. In contrast, D. elongata spread along Cache Creek and branches of related tributaries within the same watershed at ca. 14 km per year. A survey of 122 Tamarix stands across 15 California counties revealed that neither introduced beetle colonised other host patches, including those in neighbouring watersheds. Despite exclusive use of T. parviflora for ca. 36 generations, field collected D. elongata adults demonstrated strong preferences for T. ramosissima over T. parviflora when selecting both resting and ovipositional sites in caged choice tests. The proportion of D. elongata ovipositing on T. parviflora varied over time but with no clear trend of shifting host preference despite strong selection pressure. Explanations for the limited establishment and spread of Diorhabda spp. as well as impact to the target weeds are discussed.