Use a magnifying lens if necessary. It is not always easy to tell what is causing a plant problem, but some symptoms are evidence that harmful insects, mites, or slugs are present. Slugs and insects with chewing mouthparts can be responsible for plant tissue disappearance, causing skeletonizing of leaves or holes in leaves and flowers. Other insects and mites harm plants by feeding on sap or plant fluids, which causes curling and discoloration of leaves. Insects that bore into plants make holes in trunks, twigs, and stems.
Mites often make a very fine webbing, while slugs leave slime trails.
Many pests leave specific droppings that can be used to detect their presence. Check for descriptions of pests in garden reference books and on Web sites. Many of the best books focus on the host plant and give a list of the pests associated with each. These references often have drawings or photographs and detailed descriptions of each pest. Try to match the pest with the written description, but keep in mind that many insects have immature life stages that look and act very different from the adults.