I'm not sure why you want to kill each and every ant in your yard. Ants rarely harm the garden; they are more of a nuisance than anything else. The petite crawlers are probably dining on honeydew left behind by problem-causing aphids on your trees. A strong spray with the hose might dislodge aphids, and thus eliminate the ants' food source. Another solution is to wrap or coat the tree trunks with a sticky substance that will inhibit ants from scrambling up. There are a couple of products you can use-a sticky tape or a gluey substance that you spread on the bark. Both products are available at garden centers. Raised anthills are prevalent in areas with a large number of ants. Destroy hills by dousing them with several gallons of boiling water. Ants in the house are another problem altogether. I don't mess around with an indoor ant invasion and commonly use a commercial insecticide such as Raid to eradicate them. Remember that Raid is an indoor product; it's not meant for use in the garden.