Plant chemistry and enemy pressure shape within-stem distribution of the invasive scale Nipponaclerda biwakoensis

Authors: GLASSMIRE, ANDREA - Michigan State University; CRONIN, JAMES - Louisiana State University; DIAZ, RODRIGO - Louisiana State University; DESOTO, ALEXIS - Louisiana State University; SHAPIRO, EMILY - Louisiana State University; Gaffke, Alexander; SNOOK, JOSHUA - Louisiana State University; STOUT, MICHAEL - Louisiana State University

Submitted to: Insects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/16/2025
Publication Date: 12/19/2025
Citation: Glassmire, A.E., Cronin, J., Diaz, R., Desoto, A., Shapiro, E., Gaffke, A.M., Snook, J.S., Stout, M. 2025. Plant chemistry and enemy pressure shape within-stem distribution of the invasive scale Nipponaclerda biwakoensis. Insects. https://doi.org/10.3390/insects17010009.
DOI: https://doi.org/10.3390/insects17010009

Interpretive Summary: The Mississippi River Delta is a vitally important waterway for major industries that drive local and national economies. In addition to its economic importance, the Mississippi River Delta provides important wildlife habitat and has high biodiversity. Currently dieback events of a keystone species, Phragmites australis, is jeopardizing the benefits from the Delta. One of the causes of the dieback is the introduction of an invasive scale, Nipponaclerda biwakoensis, that feeds on P. australis. In order to develop effective control methods, the behavior and preferences of N. biwakoensis needs to be characterized. A scientist for the USDA-ARS Insect Behavior and Biocontrol Research Unit in Tallahassee, FL in collaboration with researchers from Michigan State University, and Louisiana State University investigated the settling and feeding preferences of the invasive scale. It was determined that the scales prefer to settle and feed close to the stem base and that they avoid plant tissues that typically have high levels of defensive compounds. While the scales prefer to feed in sheltered areas, of low-quality tissue at the base of the stem, parasitoids preferentially attacked the scales in these locations. This information provides evidence to support the ongoing research to develop control strategies utilizing parasitoids for biological control.

Technical Abstract: Invasive insects increasingly threaten ecosystems worldwide, with wetlands especially vulnerable under a changing 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 filtra-tion, and wildlife habitat. Despite rapid regional spread, the fine-scale dispersal of N. bi-wakoensis 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 pref-erence. In field surveys, we quantified scale density along the length of individual P. aus-tralis stems and found that gravid female scales clustered 3.5 times more often at the stem base than at higher sections. In a greenhouse study, we assayed lower and upper stems of P. australis for carbon, nitrogen, water, and phenolic content, and conducted correspond-ing laboratory trials to assess scale crawler preferences for stem height. Crawlers aggre-gated 20× more densely on bottom than top stem tissues, which contained 50% less nitro-gen and 47% lower phenolic concentrations. Parasitism rates were also 12× higher at the stem base, suggesting settlement patterns reflect a trade-off between structural refuges, enemy pressure, and plant chemistry. Our results show that crawlers preferentially avoid high-defense tissues while tolerating low-nutrient conditions, highlighting how plant nu-trition and natural enemies jointly shape fine-scale settlement patterns. This mechanistic understanding informs predictions of insect distribution, competition, and fitness, and guides strategies to mitigate N. biwakoensis impacts.