Effects of Rhizobium symbiosis and nitrogen augmentation of faba bean on host acceptance and feeding preference of the pea leaf weevil (Coleoptera: Curculionidae)

Legumes depend on symbiotic rhizobia to fulfill nitrogen requirements for plant growth and reproduction. This symbiosis not only influences plant nutrition but also mediates plant defense against herbivores. The pea leaf weevil, Sitona lineatus Linnaeus, is an oligophagous herbivore of legumes. Adults feed aboveground on the foliage of a variety of legume species. Larvae feed below ground on root nodules of field pea (Lathyrus oleraceus, Lamb [syn. Pisum sativum, Linnaeus]) and faba bean (Vicia faba, Linnaeus) and the associated Rhizobium leguminosarum Frank (Rhizobiales: Rhizobiacea) bacteria. We tested the effect of rhizobia symbiosis and soil nitrogen augmentation on weevil host acceptance, feeding preference, larval development and the plant performance of faba bean in field and laboratory studies. While adult pea leaf weevils showed no preference in laboratory assays, they preferred rhizobia-inoculated faba bean plants in the field. Rhizobia inoculation increased faba bean yield despite having no effect on nodule production. Nitrogen amendment increased male feeding and female oviposition in laboratory assays. Nitrogen amendment did not protect faba bean yield, even though it decreased larval development and root nodulation. More adults were collected from rhizobia-inoculated plants compared to nitrogen-treated plants. These findings suggest that nutrition-defense trade-offs in faba bean may drive host selection behaviours and pest dynamics of pea leaf weevil. We discuss the role of rhizobia in enhancing plant defense and maintaining yield stability. We also discuss potential negative impact of rhizobia on plant fitness, as rhizobia may produce nutritionally rich plants that are more susceptible to pea leaf weevil damage.