Belowground herbivory in red pine stands initiates a cascade that increases abundance of Lyme disease vectors

• We sampled 31 red pine stands to examine the effects of rhizophagy and gap formation.
• Stands were either healthy or had pockets of dead and dying trees, symptomatic of red pine pocket decline.
• Symptomatic stands had greater ground-level plant biomass and tick population densities.
• Nymphal tick infection rates of Lyme disease and anaplasmosis did not differ between stand types.
• Stand management can affect human disease risk.

There is increasing recognition that infectious disease patterns are often driven by complex underlying ecological processes. In red pine plantations in the Great Lakes region of North America, feeding by rhizophagous insects triggers a cascade that ultimately results in higher densities of blacklegged ticks, Ixodes scapularis, which are the primary vector of the Lyme disease pathogen Borrelia burgdorferi. We sampled 31 plantations in Wisconsin, USA, that were diseased or asymptomatic for a previously described tree mortality syndrome that originates with root infestation. Understory vegetation was greater in diseased stands, as were the proportion of samples containing ticks and the number of ticks per sample. Infection rates with B. burgdorferi were consistent. Tick densities were identical between declining and healthy portions of symptomatic stands, suggesting stand-level factors are responsible, consistent with mammal movement. These results suggest that forest management practices that affect the dynamics of belowground food webs may have implications for human health.