Specific impacts of two root herbivores and soil nutrients on plant performance and insect-insect interactions

Background/Question/Methods

A large number of soil-dwelling fauna commonly attack plant roots and storage organs. Although research on belowground, root-based interactions is quickly developing, our knowledge of the patterns and consequences of belowground herbivory within non-agricultural systems remains limited. Here, we tested how two species of root-feeding insects impact the performance of a native, perennial plant and the growth and survival of both conspecific and heterospecific belowground insects. By growing common milkweed (Asclepias syriaca) in low or high nutrient soil and adding seven densities of red milkweed beetles (RMB, Tetraopes tetraophthalmus), wireworms (Hypnoides abbreviatus), or both species, we also evaluated how soil fertility alters belowground plant-insect and insect-insect interactions.

Results/Conclusions

The location and magnitude of root damage depended on the insect species: RMB caused 59% more damage to main roots than wireworms, and wireworms caused 665% more damage to fine roots than RMB. RMB damage decreased shoot, main root, and fine root biomass, however extensive damage to fine roots by wireworms did not result in any impact on plant biomass. The effect of soil fertility on plant biomass depended on the insect species. Under high nutrient conditions, plants were able to increase root biomass three to five times more when exposed to wireworms compared to RMB. The growth of individual insects also depended on the nutrient environment. Wireworm growth decreased by 19% under high nutrient conditions while RMB growth increased by 114%. The survival of individual wireworms decreased in the presence of RMB, and both species’ survival was negatively correlated with conspecific density. Our results indicate that belowground plant-insect and insect-insect interactions in this system are highly context-dependent. We suggest that efforts to develop a general predictive framework for belowground herbivory will benefit from considering both insect-specific and root-specific patterns of damage.