1. Individual plants are often simultaneously consumed by many different organisms and mediate important indirect interactions between their consumers, even when their consumers are phylogenetically distant and feed on different parts of the plant. 2. We examined the effects on two xylem feeders of sharing a plant host in greenhouse pot trials and outdoor mesocosms. The spittle bug Neophilaenus lineatus, which feeds on grass shoots, and a hemiparasitic angiosperm, Rhinanthus minor, which attaches to the roots, were added separately or together on grass host plants growing in pots or in model grassland swards. 3. We found strong asymmetric competition between the hemiparasite and the insect. In greenhouse experiments, sharing a grass host with R. minor significantly increased mortality of N. lineatus, but R. minor performance was unaffected. 4. Mortality of N. lineatus was also increased on swards containing R. minor in model communities grown in the field. R. minor was more sensitive to sharing a host with N. lineatus in these communities, and experienced a modest (non-significant) reduction in flower production. 5. The hemiparasite was the stronger competitor for xylem resources, possibly because of its ability to extract resources from the plant roots, and because of its significant ability to appropriate the xylem stream via its strong transpirational pull. 6. We believe these are the first experiments to demonstrate competition between insect herbivores and hemiparasitic plants mediated via a shared host plant. The consequences for the invertebrate population were potentially highly significant and we propose that future research should further explore the role of this asymmetric relationship in structuring invertebrate communities
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