Yes, insecticides do cause bees to die
The type of insecticide in question, called neonicotinoids, is often used to coat crops. In a study published today in the journal Science, researchers grew rapeseed in 33 sites across Germany, the United Kingdom, and Hungary. They coated the crop with neonicotinoids, introduced various bee species to the area, and monitored the health of the colonies compared to control groups. (These chemicals were made by the companies Bayer CropScience and Syngenta, which helped fund the study.) The bees who lived near the plants treated with neonicotinoids had more trouble surviving the winter.