Malaise traps as tool for insect monitoring

In-depth knowledge about insects is largely limited to a few conspicuous taxa such as butterflies, bees and dragonflies. Worse still, less than 20% of all insects on Earth have been described, and we have comprehensive distribution data and population trends for an even lower fraction. This means the vast majority of insects are basically unknown, even in well-studied countries like Sweden or Germany. Malaise traps, combined with recent advances in DNA metabarcoding, could be an effective and inexpensive tool for closing these enormous knowledge gaps. Here, we provide examples of how combining these methods has proven useful in addressing a broad variety of key questions, including biomass trends, filling taxonomic gaps, identifying habitat and driver relationships, evaluating the effectiveness of protected areas, and monitoring pollinators. Our examples are based on first results from the German Malaise Trap Program, a nation-wide long-term insect monitoring project launched in 2019. Today, 83 monitoring sites are included covering a wide range of land use types and protection categories. In particular, we relate our recent data to long-term trends in German insect biomass reported by Hallmann et al. (2017, 2025), and discuss strengths and weaknesses of trend estimates, considering potential limitations in monitoring design and data. Additionally, we outline what biomass trends may tell us about changes in overall insect biodiversity, and detail the vast number of new taxa that combining Malaise traps with metabarcoding can identify (including important pollinators), offering new insights into often unmonitored taxa. We also show how we have used Malaise traps and metabarcoding to determine the effect of land use, climate, and protected areas on insect diversity. Finally, we will provide an outlook on how insects will be monitored in the European research infrastructure eLTER in the coming years, and what additional tools are needed given common limitations in this methodology, and the necessity to further deepen our knowledge of insects to develop targeted and more effective mitigation measures.