As new growth begins to appear, it's time to feed mature fruit trees. Most require the equivalent of one pound of actual nitrogen annually. Divide the amount of fertilizer required into three equal lots and apply them six weeks apart. Check trees for pests and use a forceful spray of water to dislodge aphids, spider mites and whiteflies. If a pesticide is needed, use a chemical that has short residual activities, such as insecticidal soap, horticultural oil or pyrethrin, to protect beneficial insects. To keep ants 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 ant trails. Also place ant stakes or small containers with toxic bait by ant nests and trails.