This information is from the US EPA and OEPA, 4/2012
The best way to dispose of small amounts of excess pesticides is to use them - apply them - according to the directions on the label.
State and local laws regarding pesticide disposal may be stricter than the Federal requirements on the label. Be sure to check with your state or local agencies before disposing of your pesticide containers.
If the container is partly filled, contact your local solid waste agency.
If the container is empty, do not reuse it. Empty pesticide containers should be rinsed at least three times. Use the rinse water as you would the regular strength pesticide. You can put the rinsed container in the trash.
Place it in the trash, unless the label specifies a different procedure.
Pesticides should NEVER be burned, buried, mixed together, poured on the ground, dumped in water, or poured down the drain. This could lead to pollution of drinking water, lakes and streams, or dangerous exposure to pesticides for humans, animals or plants. Do not pour leftover pesticides down the sink, into the toilet, or down a sewer or street drain. Pesticides may interfere with the operation of wastewater treatment systems or pollute waterways. Many municipal systems are not equipped to remove all pesticide residues. If pesticides reach waterways, they may harm fish, plants, and other living things.
EPA's guidance to pesticide registrants about instructions for disposal of household-use pesticides provides more information on this issue.