Specifi c impacts of two root herbivores and soil nutrients on plant performance and insect – insect interactions

Soil-dwelling insects commonly co-occur and feed simultaneously on belowground plant parts, yet patterns of damage and consequences for plant and insect performance remain poorly characterized. We tested how two species of root-feeding insects aff ect the performance of a perennial plant and the mass and survival of both conspecifi c and heterospecifi c insects. Because root damage is expected to impair roots ’ ability to take up nutrients, we also evaluated how soil fertility alters belowground plant – insect and insect – insect interactions. Specifi cally, we grew common milkweed Asclepias syriaca in low or high nutrient soil and added seven densities of milkweed beetles Tetraopes tetraophthalmus , wireworms (mainly Hypnoides abbreviatus ), or both species. Th e location and severity of root damage was species-specifi c: Tetraopes caused 59% more damage to main roots than wireworms, and wireworms caused almost seven times more damage to fi ne roots than Tetraopes . Tetraopes damage decreased shoot, main root and fi ne root biomass, however substantial damage by wireworms did not decrease any component of plant biomass. With the addition of soil nutrients, main root biomass increased three times more, and fi ne root biomass increased fi ve times more when wireworms were present than when Tetraopes were present. We detected an interactive eff ect of insect identity and nutrient availability on insect mass. Under high nutrients, wireworm mass decreased 19% overall and was unaff ected by the presence of Tetraopes . In contrast, Tetraopes mass increased 114% overall and was signifi cantly higher when wireworms were also present. Survival of wireworms decreased in the presence of Tetraopes , and both species ’ survival was negatively correlated with conspecifi c density. We conclude that insect identity, density and soil nutrients are important in mediating the patterns and consequences of root damage, and suggest that these factors may account for some of the contradictory plant responses to belowground herbivory reported in the literature.