The global decline of arthropod populations threatens biodiversity, ecosystem services, and food webs, with pesticides as a major driver. Arthropods are crucial for pollination, decomposition, and as prey for vertebrates. This decline, linked to intensive agriculture, affects over 74% of the world’s agricultural land with pesticide contamination. Despite EU regulations, pesticide pollution impacts aquatic and terrestrial ecosystems. Current risk assessments focus on survival, overlooking sub-lethal effects that can disrupt physiology and behavior affecting fitness and population viability. This project investigates the sub-lethal effects of pesticide exposure on the fitness of riparian Tetragnatha spiders, which are key predators in riparian ecosystems. These spiders feed on aquatic insects and thereby transfer pesticides to terrestrial environments. Despite their ecological importance, the impact of pesticides on their reproduction remains poorly understood. I will evaluate how sub-lethal pesticide exposure affects fitness-related traits, including sperm quality, oocyte development, and reproductive behavior, addressing gaps in ecotoxicological assessments. I hypothesize that pesticide exposure will reduce sperm quantity and quality, cause DNA fragmentation, impair oocyte development, and disrupt mating behavior, leading to reduced fitness and population viability. The project combines laboratory and field experiments along the Mura River in Slovenia, an area with documented pesticide contamination. Field data will inform laboratory experiments where spiders are exposed to various pesticide concentrations through contaminated prey. We will assess reproductive parameters and behavioral changes to understand the broader implications of pesticide exposure. This interdisciplinary research will advance evolutionary ecotoxicology, enhance understanding of pesticide impacts on reproduction, and inform conservation strategies, aligning with EU environmental objectives.