MSU Study Reveals Key Insights into Invasive Scale Insect Behavior in Mississippi River Delta
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EmailAn MSU study found that an invasive scale insect concentrates on lower stems of wetland grasses, showing how plant chemistry shapes its spread and informing future management efforts.
A recently published study by Michigan State University entomologist Andrea Glassmire and co-authors sheds new light on the behavior and habitat preferences of an invasive scale insect that threatens critical wetland grasses in Louisiana’s Mississippi River Delta (MRD).
The research focuses on Nipponaclerda biwakoensis, an invasive scale insect that damages Phragmites australis, a dominant wetland grass in the MRD. While some P. australis subspecies are invasive in parts of North America, the subspecies prevalent in the MRD plays a critical role in stabilizing shorelines and reducing coastal erosion. Dieback of P. australis was first detected in the region in 2016, nearly coinciding with the first detection of N. biwakoensis that same year. Since then, the insect has rapidly spread to neighboring states including Texas, Mississippi and Alabama.
N. biwakoensis is native to Japan, China and Korea. The insects attach to leaves and stems and feed by sucking sap from the plant. They are very small and covered by a waxy coating, making them difficult to remove and protected from predators and pesticides. Heavy infestations can damage plants and eventually lead to plant death.
Despite the rapid spread and damage caused by this invasive insect, little has been known about how it disperses. In particular, researchers have lacked information about the insect’s crawler stage, a brief period lasting approximately 72 hours during which the insect is able to spread. “The crawler stage is a critical bottleneck for dispersal because many scale insects, including female Nipponaclerdia biwakoensis, become permanently immobile after this brief life stage,” Glassmire explained.
Glassmire and her collaborators conducted controlled experiments to test whether plant traits influenced where young scale insects in the crawler stage chose to settle on P. australis. The experiments revealed that upper portions of the plant stems contained higher levels of nitrogen and defensive chemicals, while lower stem sections were less chemically defended.
“Insects are generally nitrogen-limited, leading us to expect scale settlement on top portions of the stem with softer tissue and higher nitrogen availability,” said Glassmire. “Yet these tissues also contained higher concentrations of phenolic compounds, and scales predominantly settled on the lower stem, suggesting that phenolic defenses outweighed nutritional benefits.”
Field surveys supported these findings. Researchers found that adult scales were far more common near the base of plant stems, finding 20 times more scales on the bottom than the top sections of the stem. Additionally, they found 12 times more parasitized scales on the bottom section of the stem compared to the middle and top sections, but patterns showed that parasitoids may be responding to, rather than driving, low crawler settlement.
The results suggest that the distribution of scale insects within the plant reflects a balance between plant chemistry, habitat structure and natural enemies. Nutritional and defensive traits appear to play a key role in where N. biwakoensis crawlers settle on P. australis. Understanding where the insects concentrate and why they avoid other parts of the plant begins to explain how N. biwakoensis spreads so effectively through wetland systems.
These findings are particularly important for the MRD, the seventh largest river delta in the world. Coastal deltas such as the MRD provide ecosystem services valued at an estimated $12 billion to $47 billion annually, including flood protection, water filtration, fisheries, recreation and cultural and ecological benefits. The region is already facing multiple stressors, including coastal erosion, hurricanes, sea-level rise and human activity.
By identifying the behavioral factors that influence where invasive scale insects settle on plants, the study provides insights that can inform future research and the development of targeted management strategies. Such approaches may help slow damage to vulnerable wetlands and protect the ecosystem services they provide.
