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

## TL;DR

This study explores how plant chemistry and insect enemies influence the distribution of an invasive scale insect within Phragmites australis stems.

## Contribution

The study reveals how plant traits and natural enemies jointly shape the fine-scale dispersal of an invasive insect.

## Key findings

- Scale insects prefer lower and middle stem sections, where plant defenses are weaker.
- Parasitic wasps are most abundant at the stem base, correlating with higher scale densities.
- Plant nitrogen and phenolic content differ along the stem, influencing insect settlement.

## Abstract

Wetlands along Louisiana’s Mississippi River Delta depend on a tall grass called Phragmites australis to reduce erosion, filter water, and provide wildlife habitat. In recent years, this foundation plant has suffered widespread dieback linked to an invasive insect scale (Nipponaclerda biwakoensis). Although this insect spreads rapidly, scientists have known little about how it moves and settles within individual plants—a key step in understanding and managing its impacts. We studied where scale insects settle along the length of P. australis stems and why. Field surveys showed that adult scales were far more common near the base of stems, where parasitic wasps that attack the scales were also most abundant. In greenhouse and laboratory experiments, we tested whether plant traits influenced where young scales chose to settle. We found that upper stem sections contained more nitrogen and defensive chemicals, while lower sections were less defended. When given a choice, scale crawlers often preferred the middle and lower sections of the stem. Together, our results show that scale distribution within plants reflects a balance between plant chemistry, habitat structure, and natural enemies. Understanding these fine-scale patterns helps explain how this invasive insect spreads and can inform targeted strategies to slow damage to vulnerable wetlands.

Invasive insects increasingly threaten ecosystems worldwide, with wetlands especially vulnerable to unpredictable climate. Phragmites australis 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, Nipponaclerda biwakoensis, 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 N. biwakoensis within host plants remains poorly understood. We examined whether the crawler-stage of N. biwakoensis scales preferentially settled on the bottom or top sections of P. australis stems, and whether plant nutritional and/or defensive traits shaped this preference. In field surveys, scale densities varied along the length of P. australis 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 N. biwakoensis crawler settlement patterns, strengthening our understanding of of insect distribution and guiding predictions of long-term dispersal in N. biwakoensis.