Fine root consumption patterns of immature insect assemblages in a longleaf pine-wiregrass system

Background/Question/Methods

White grubs (Scarabaeidae), wireworms (Elateridae), weevil larvae (Curculionoidea), and cicada nymphs (Cicadoidea), are widespread fine root herbivores, which potentially influence vegetation dynamics and the belowground allocation and processing of carbon (C) and nitrogen (N). Despite their potential ecological importance and their economic significance as agricultural pests, little is known about the foraging selectivity of root-feeding insects in naturally vegetated ecosystems. We asked whether root-feeding insects in the longleaf pine-wiregrass system foraged for an optimal mixture of fine roots from C₄ wiregrass (higher C:N) and C₃ non-grass species (lower C:N).  Optimal foraging theory suggests a tradeoff between nutritional benefits of selective foraging and additional time spent searching for an ideal diet. Because the mobility of root-feeding insects is limited by soil, we hypothesized that benefits of selective foraging might be outweighed by search time, resulting in random feeding. Samples of fine roots and root-feeding insects were collected from random plots, within which grass dominance had been experimentally manipulated using presence or absence of prescribed fires, leading to higher and lower dominance, respectively. Because C₃ and C₄ plants differ in discrimination against ¹³C, the proportion of C₄ grass in the root samples and in the diets of the herbivores could be estimated.

Results/Conclusions

The majority of insect taxa tracked the δ¹³C values of roots from the same plots in which they were collected, suggesting that they forage non-selectively. There was considerable overlap in the ranges of δ¹³C values among the non-selective insect taxa. However, some taxa demonstrated feeding selectivity, deviating significantly in either a positive or negative direction from the isotopic values of random root samples, with the greatest deviation occurring when root samples were heavily dominated by C₃ or C₄ plant species. This suggests that feeding selectivity within some herbivore taxa may be a dependent on the composition of fine roots available for consumption. We also observed a significantly higher median ¹⁵N enrichment of approximately 4‰ for wireworms, relative to 2‰ for white grubs (p<0.0001), suggesting that the wireworms are omnivorous. We conclude that most herbivores in our study system are nonselective generalists, supporting our hypothesis that selective foraging may be physically constrained by the soil environment. Because some insects appeared to have more specialized diets, herbivore identity may be an important determinate of feedbacks with plant community structure and ecosystem function.