Also you may want to overseed in a less sensitive grass variety. Locate your compost pile away from your well. A composting brochure is available. Plant cereal rye or annual ryegrass in the fall and rototill it down in the spring. This adds organic matter to your soil and the grass roots absorb nitrate in late fall and early spring, keeping it from contaminating groundwater.
Insecticides kill good bugs as well as pests.
Wait and see if it gets worse - good insects like lady beetles might keep the problem under control. If you decide to spray, read and follow all label directions exactly. Others, like bleach, can hurt your septic system. Septic systems can't detoxify all of the chemical compounds found in these products and, consequently, some end up in groundwater. Mix equal parts flour and salt into a little vinegar.