Low concentrations of fertilizer and herbicide alter plant growth and interactions with flower-visiting insects

Abstract

The increasing extent and intensity of agricultural land use has led to an increase in the volume of agrochemicals applied to the landscape, including those used to improve the nutritional quality of soils (fertilizers) as well as those used to control undesirable species in the agroecosystem (pesticides). Chemicals can disperse in the air and surface and ground water, leading to exposure of non-target organisms. Ruderal, disturbance-tolerant plants on which many flower-visiting insects rely are commonly exposed to these chemicals in agroecosystems. Our research questions were: 1) how does non-target exposure to agrochemicals affect plant growth? 2) what are the indirect effects of non-target exposure to agrochemicals on flower-visitation by insects? We designed a two year field experiment imitating field-realistic fertilizer run-off and non-target herbicide exposure scenarios to explore the impact of low concentrations of fertilizer and herbicide alone, and in combination, on communities of seven plant species, including six native perennials and one non-native annual commonly found in agricultural systems in Ireland (Cirsium vulgare, Epilobium hirsutum, Plantago lanceolata, Origanum vulgare, Filipendula ulmaria, Hypochaeris radicata, Phacelia tanacetifolia). We created field-realistic exposure scenarios by applying concentrations of mineral fertilizer similar to those detected in ground water, and glyphosate levels equivalent to 7.6 % of a standard field application, to the foliage and soil of the plots. We found low concentrations of fertilizer and herbicide affected plant growth: fertilised plants were taller when flowering, while plants exposed to herbicide flowered at shorter heights and produced shorter leaves. The size of the floral display had the largest effect on insect visitation, with larger floral displays significantly more likely to receive a visitor in a given sampling event. The size of the floral display also interacted significantly with the fertilizer treatment for both the abundance and species richness of floral visitors. Overall, our results suggest that there are direct and indirect effects of agrochemical exposure on plants in field margins, and that these effects change the interactions between ruderal plants and flower-visiting insects.