A fluorometric assay to estimate pea leaf weevil (Coleoptera: Curculionidae) larval feeding damage using leghaemoglobin in root nodules of faba bean (Fabaceae)

The pea leaf weevil, Sitona lineatus Linnaeus (Coleoptera: Curculionidae), is an invasive pest of field pea, Pisum sativum Linnaeus, and faba bean, Vicia faba Linnaeus (Fabaceae). Larvae feed on leguminous root nodules and associated Rhizobium leguminosarum Frank (Rhizobiacea) nitrogen-fixing bacteria. Larval feeding causes economic damage, but the current method to assess nodule damage is laborious. Leghaemoglobin, the oxygen-carrying molecule in root nodules, is degraded as larvae feed. Measurement of leghaemoglobin could be an alternative method to estimate larval damage. Here, we developed a fluorometric assay to measure the variation in haem fluorescence, which relates to leghaemoglobin content, from nodulated roots. Roots were collected from caged faba bean plants with or without weevil infestation. Faba bean yield and haem fluorescence were inversely correlated with the percent damaged nodules. A plant growth score was positively correlated with haem fluorescence. This method can be used to assess nodule damage in pulse crops from pea leaf weevil and potentially from other biotic stresses, and it may have wider application to studies of nitrogen fixation.