Compensatory response of a legume root‐nodule system to nodule herbivory by Sitona hispidulus

A laboratory study was conducted to examine the hypothesis that herbivory of nitrogen‐fixing root nodules on legumes causes an exact compensatory response in nodule growth. Plants of Medicago sativa (L.) were grown hydroponically in clear plastic growth pouches so that the number and biomass of root nodules could be estimated nondestructively before, and 10 and 18 days after, partial denodulation. For treatments, plants were subjected to 23% denodulation by first‐instar larvae of Sitona hispidulus (F.) (a common herbivore of Medicago and Trifolium) or 50% nodule pruning; additional plants were left untreated. Results indicated that nodule herbivory and nodule pruning caused an overcompensatory response in number of nodules. This was also true for number of nodule units (an indirect measure of nodule biomass) per plant at 10 days after denodulation but had changed to an exact compensatory response by day 18. An inverse relationship between change in number of nodule units and initial number of nodules indicated that compensatory nodulation was regulated by a feedback mechanism. Shoot and root biomasses were not affected by denodulation in this study. 1992 The Netherlands Entomological Society