Volatile Organic Compound Emissions in the Invasive Legume Cytisus scoparius: Linking Plant Phenology, Arthropod...

Scotch broom (Cytisus scoparius; Fabaceae) is an invasive nitrogen-fixing shrub widespread in New Zealand, where it impacts forestry, pasturelands, and native ecosystems. Although several biological control agents have been released, Scotch broom continues to expand in regions such as the North Island's Central Plateau. Scotch broom affects the germination and growth of other plants and modifies arthropod communities (including pollinators, herbivores, and predators) within its invaded range. Volatile organic compounds (VOCs) play a key role in mediating plant-plant and plant-arthropod interactions, potentially contributing to this invasive plant's ecological success. However, Scotch broom's VOC emissions in its invaded ranges remain poorly understood. We examined VOC emissions from flowering and non-flowering Scotch broom plants in the Central Plateau and assessed links with biotic and abiotic factors. Our aims were to (1) characterise differences in VOCs between phenological stages; (2) explore shifts in arthropod community composition; and (3) evaluate correlations between VOC emissions, arthropod groups and environmental variables. Flowering plants had higher diversity and abundance of VOCs, with blends dominated by monoterpenes, aromatics, and fatty acid esters, whereas non-flowering plants were characterised by green leaf volatiles (GLVs). Flowering stages supported Hemiptera and Thysanoptera (herbivores), which were positively correlated with fatty acid esters. In contrast, GLVs correlated with Araneae (predators) abundance. Temperature was the strongest predictor of VOC emission patterns, showing significant correlation with most compound classes. These results advance understanding of Scotch broom invasion ecology and highlight the need to further explore individual compounds potentially influencing arthropod composition to inform both native arthropods conservation and future biocontrol strategies.