<p>Solitary bees are vital for the pollination of many crops but face decline due to global change drivers such as land use change, global warming, and pathogens. While much research has focused on social bees and adult solitary bee exposure, limited studies address the impact of pesticide-contaminated pollen on developing bee larvae. Understanding these effects is crucial for protecting pollinators and ensuring global food security and ecosystem stability. Our study reports on the effects of realistic doses of chlorpyrifos, a commonly used insecticide, on mason bee biology. Specifically, we investigated how exposure to three different concentrations of chlorpyrifos influences larval survival, developmental timing, and pupal body size in mason bees (<i>Osmia cornifrons</i>) using manipulated larval diets. A significant reduction in survival was observed among mason bees consuming pollen containing the highest experimental chlorpyrifos dose (830 parts per billion; ppb) but not among treatments at lower doses. Chlorpyrifos exposure also significantly impacted pupal size, although some of these effects were not dose-dependent. Our findings indicate that certain insecticides (such as chlorpyrifos) can affect the development of <i>O. cornifrons</i> at field-relevant doses. This work highlights a need for more comprehensive assessments of pesticide impacts on diverse bee species to inform future pollinator conservation efforts.</p>