Most mature fruit trees need the equivalent of one pound of actual nitrogen annually. To calculate how much fertilizer you need, divide the percent nitrogen that the product contains into 100 (e.g. 100 divided by 20 percent nitrogen equals 5 pounds of fertilizer). Divide the amount of fertilizer into two or three equal lots and apply them six weeks apart once new growth begins. Because new growth is attractive to pests, check trees for aphids, spider mites and whiteflies. Wash trees periodically with a forceful spray of water to dislodge these pests. If a pesticide is needed, use a chemical that has a short residual activity, such as insecticidal soap, horticultural oil or pyrethrin, to protect beneficial insects. Because ants "farm" some insects, take steps to keep them off trees. Wrap a band of heavy paper or duct tape around the trunk and coat it with a sticky material like Tanglefoot. Trim branches that touch other objects to stop ants from going around the barrier. Also place ant stakes or containers with toxic bait by ant nests and trails.