Pesticides include insecticides (designed to kill insects), herbicides (designed to kill weeds), and fungicides (designed to kill molds and fungi). Insecticides have varying toxicity to bees depending on their mode of action and concentration. Herbicides can kill off the flowering "weedy" plants many bees depend on as food. Current research is looking into the effects of some fungicides on bee health. It is very important to apply pesticides only as needed, and to prevent them from persisting as residue in the environment. When using insecticides, it is critical to apply them so they control the pest insect but will avoid affecting foraging bees and other beneficial insects.
Here are two very good resources with clear information on protecting bees from pesticides:
The Xerces Society for Invertebrate Conservation: www.xerces.org/pesticides/
The Center for Urban Ecology and Sustainability, Department of Entomology, University of Minnesota (Dr. Vera Krischik): "Pollinator Conservation" (.pdf)
Here is a sample paragraph from Dr. Vera Krischik's "Pollinator Conservation" bulletin:
How You Can Help: Understanding Proper Insecticide Use
Dr. Vera Krischik and Emily Tenczar
The conservation of beneficial insects, that includes bees, insect predators, parasitic wasps, and butterflies, is an essential part of Integrated Pest management (IPM) programs. IPM promotes multiple tactics to manage pests and to suppress the population size below levels that will damage the plant. IPM tactics include cultural control, sanitation, biological control, using insecticides friendly to beneficial insects, and finally the use of conventional insecticides. IPM recognizes that the few remaining pest insects will support beneficial predators and parasitic wasps. When scouting plants for pest insects, check for populations of both pest and beneficial insects, such as lady beetles and bees. If beneficial insects are present, wait to spray insecticides to see if the beneficial insects control the pest insects or use specific insecticides that only target the pest insect. Do not apply insecticides while plants are in full bloom. If possible avoid beneficial insects by spraying leaves in the evening when bees and lady beetles are not foraging.