Ento-phyto-screening for organic contaminants: The use of insect-induced plant galls as a novel tool for tracking belowground chemical contaminants

Anthropogenic volatile organic compounds (VOCs) are common pollutants produced as byproducts of industrialization which can persist long-term in groundwater. Due to the heterogenous landscape of urban environments, and their volatility, subsurface VOC hotspots can be difficult to delineate. Phytoscreening–using aboveground plant tissue to detect subsurface contaminants–has been employed to detect inorganics pollutants. However, VOCs prove challenging to phytoscreen due to contaminant concentrations often existing below detection capabilities. We hypothesize that insect-induced plant galls, tumor-like outgrowths composed of plant cells that are initiated by insects on many vascular plants that serve as sinks for extraordinary high concentrations of nutrients, accumulate VOCs in higher concentrations than other aboveground plant tissues. To test this hypothesis, we compared the concentrations of semi-VOC 1,4-dioxane, a known carcinogen, in a ~30-km² plume below Ann Arbor, Michigan, USA in galls induced by Viteus vitifoliae (Hemiptera: Phylloxeridae) on grapevine to two other aboveground plant tissues: leaves and shoots. Sampling along a transect within and outside the plume revealed that galls were the only tissue to successfully detect dioxane. Furthermore, galls detected dioxane at sites located outside the plume, expanding its known range. At 40% of sites, dioxane was detected in galls as well as co-located groundwater monitoring wells. Moreover, at an additional 21% of the sites within the plume, galls detected dioxane where the chemical was absent from well water. Overall, our results imply that insect-induced plant galls may serve as a viable and perhaps superior strategy for phytoscreening belowground chemical contaminants, particularly at low concentrations.