Early detection of pest infestation is a prerequisite for sustainable crop protection. However, many pest species are difficult to detect and thus infestation is diagnosed from damage observed on the respective crop. This diagnosis is often made too late for implementation of crop protection measures, and serious crop losses may result. The swede midge, Contarinia nasturtii Kieffer, is a major pest of Brassica L. (Brassicaceae) vegetables in Europe that has recently invaded North America. With its small size and short adult life-span, and the cryptic lifestyle of the larvae feeding at the growing points of its host plants, it is usually detected only after damage has already occurred. Furthermore, because field-trapped specimens are rarely fully intact, it is extremely difficult to identify. Therefore, we developed a species-specific molecular diagnostic method that enables reliable identification of swede midge from various sources such as alcohol or sticky glue traps. The method enables large-scale screening of field-trapped specimens and is used to evaluate the attractiveness and specificity of pheromone traps that are currently under development.