Specialized soil types affect host acceptability and performance of weed biocontrol candidates: implications for host specificity assessments

The Eurasian gall-forming weevil Ceutorhynchus cardariae Korotyeav (Coleoptera: Curculionidae) is a biological control candidate for the invasive Eurasian Lepidium draba L. (Brassicaceae) in the western USA. Among 157 nontarget plant species that have been tested, some North American Caulanthus and Streptanthus species, confamilial with Lepidium, were found to be at potential risk of attack by C . cardariae . Many Caulanthus and Streptanthus species grow on serpentine soils, which are characterized by low nutrient content and high concentrations of various combinations of heavy metals. Some of these species accumulate heavy metals, which have been shown to act as deterrents against insect herbivory. Standard pre-release host specificity tests with C. cardariae used plants propagated on horticultural soils, which could have inflated performance by C. cardariae on Caulanthus and Streptanthus species. To examine this possibility, we assessed the performance of C . cardariae on three Caulanthus species, the federally listed threatened and endangered Streptanthus glandulosus ssp. albidus, and Lepidium draba , on plants propagated in horticultural soil or in native serpentine soil. Our study showed that native serpentine soil influenced C . cardariae attack. All plant species, including L . draba , received less feeding damage and gall formation when grown in serpentine soil. In addition, feeding by C . cardariae was much less and fewer galls were formed on the confamilial species than on L . draba, regardless of soil type. Our data show that native confamilial species restricted to specialized soil types may be at less risk of herbivore attack than predicted based on tests conducted in horticultural soil.