One of the first and most important things you do when you're trying to decide whether to spray your garden is to properly identify the pests. Many insects look familiar, especially to the untrained eye. Some insect pests can be easily confused with harmless insects, or even beneficial insects. For example, more than a few times, people have brought me samples of ladybird beetle larvae, thinking that because they were on the plant, they must be a pest. No doubt the ladybird larvae were on the plant keeping the aphids under control. Often, gardeners blame plant damage on insects that just happen to be on the plant at the same time that they notice the damage.
The real pest may have left the plant, or the problem may have resulted from a hard-to-detect problem such as a root-rot disease. These mistakes in identification often lead to unnecessary spray treatments.
Good pest identification may be difficult, because similar-looking symptoms may have very different causes. Distorted or curled leaves, for example, may be caused by certain diseases, as well as certain insects.
Gardeners commonly mistake apple and plum tree leaf curling, caused by aphids, with peach leaf curl, a fungus disease (peach leaf curl does not affect apple or plum trees, only peach and nectarine trees). Yellow spots on tomato fruits, caused by stinkbug feeding, are often mistakenly identified as diseases.
Symptoms caused by factors other than pests, things like overwatering, underwatering, nutrient deficiencies or pesticide toxicity, can be incorrectly blamed on insects, mites or diseases.
Even more confusing, plants also are frequently subject to more than one stress or problem at a time. For example, ash trees are sometimes attacked by anthracnose, a fungus disease, and the ash plant bug, a sucking insect pest. Both problems occur in spring, and at first glance, the damage caused by these different organisms is very similar. However, they are controlled in totally different ways. For another example, water-stressed trees and shrubs often are attacked by boring insects such as bark beetles or flatheaded borers. Spraying to control these insects would not solve the original problem, which is lack of water.
Proper identification is important for choosing the correct controls. In order to do a good job of identifying pests and diseases, you need to be a keen observer and have some good reference materials to help you. The first step is to positively identify the plant species that is experiencing the problem. There are several reference books useful for that purpose, including a newly revised one titled "Ornamental Plants — Their Care, Use, Propagation and Identification." The book was written be D. Dwight Wait, emeritus horticulture instructor, Modesto Junior College. A supplemental DVD to accompany the book also is available.
The second step is to learn the cultural and environmental conditions required for each plant you're growing, and make sure these are being adequately provided. Learn how to identify common insect pests and, just as important, be able to recognize beneficial insects. There are reference books available at libraries, bookstores, retail nurseries and garden stores, and at University of California Cooperative Extension offices. Some pest problems can be reliably diagnosed only by experienced professionals. A certified nursery person, certified arborist, Cooperative Extension adviser or other professional should be able to help you make such identifications.
Ed Perry, a farm adviser with the UC Cooperative Extension, can be contacted at 525-6800 or ejperry@ucdavis.edu.
