There is increasing interest in the possibility of using eriophyid mites as biological control agents of invasive alien weeds; however their small size and our lack of knowledge about their general biology present challenges to evaluating their risk to nontarget plants. Aceria salsolae has been proposed as a candidate agent for Russian thistle (Salsola tragus) in the USA. During host specificity testing this mite could sometimes persist on five nontarget species under laboratory no-choice conditions. We conducted a series of no-choice laboratory experiments and a field experiment to try to delineate the physiological and ecological host ranges of this mite and assess its risk to nontarget plants. In the laboratory, A. salsolae increased exponentially on S. tragus, multiplying about 80 fold in 5 weeks. Low levels of reproduction were observed on some plants of Atriplex coronata, Bassia hyssopifolia, B. prostrata, Kochia scoparia and Suaeda calceoliformis in the laboratory during 5 weeks, but mean mite densities remained low (less than 6 fold increase vs. 80 fold on S. tragus). In a field experiment in which plants were inoculated with mites in June and then harvested when they began to produce seed, mites persisted on A. coronata for up to 9 weeks after inoculation, but at extremely low densities, and with no evidence of reproduction. No mites persisted on A. truncata, B. hyssopifolia, or S. calceoliformis. Mite densities were lower on all plants in the field than in the laboratory, probably due to increased mortality and the opportunity to disperse by wind. No signs of damage were observed on any of the nontarget plants in the laboratory or the field experiments. We conclude that this mite is not likely to multiply on any of these plants under field conditions, and that it is not expected to pose a risk to any nontarget plants in the contiguous USA. © 2020

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