Plant Chemistry and Enemy Pressure Shape Within-Stem Distribution of the Invasive Scale Nipponaclerda biwakoensis

Invasive insects increasingly threaten ecosystems worldwide, with wetlands especially vulnerable to unpredictable climate. <i>Phragmites australis</i> is a dominant plant species in Louisiana's Mississippi River Delta and a critically important component of the wetland ecosystem. However, the invasive scale insect, <i>Nipponaclerda biwakoensis</i>, has contributed to large-scale dieback of this foundation species, jeopardizing erosion control, water filtration, and wildlife habitat. Despite rapid regional spread, the fine-scale dispersal of <i>N. biwakoensis</i> within host plants remains poorly understood. We examined whether the crawler-stage of <i>N. biwakoensis</i> scales preferentially settled on the bottom or top sections of <i>P. australis</i> stems, and whether plant nutritional and/or defensive traits shaped this preference. In field surveys, scale densities varied along the length of <i>P. australis</i> stems, with gravid females occurring 3.5× more frequently at the stem base than at the top; parasitism rates were similarly elevated, reaching 12× higher at the base. To evaluate potential drivers of this pattern, we quantified carbon, nitrogen, water, and phenolic content in lower and upper stem tissues and conducted complementary laboratory assays to test crawler settlement preferences. Under controlled conditions, crawlers settled most densely on middle stem sections, with lower densities at the base and the fewest near the top. The basal sections also contained 50% less nitrogen and 47% lower phenolic concentrations compared to the upper stem. The divergence in crawler settlement patterns between field and controlled conditions likely reflects the influence of additional environmental factors present in the field-such as habitat structure, microclimate, and natural enemies-that are absent or minimized in laboratory conditions. By applying a trait-based approach to insect dispersal, we link plant functional traits to <i>N. biwakoensis</i> crawler settlement patterns, strengthening our understanding of of insect distribution and guiding predictions of long-term dispersal in <i>N. biwakoensis</i>.