As with any other organic life form, ornamental shrubs and trees can have problems with insect pest's. As human beings, who see these critters attack our plants, the question becomes: To kill, or not to kill the critters. Sometimes, spraying a broad spectrum insecticide can actually do more harm than good when beneficial insects are killed in the process. So when we see a bug on our shrub(s), we should first attempt to identify the type of insect, and whether or not it is listed as a harmful pest or a beneficial insect.
If you see a ladybug on a leaf it's probably busy feasting on and controlling damaging aphid insects naturally. So, as you can see, identification of an insect can be very important.
Though some insects can do a lot of superficial damage to plants or trees they usually don't kill plants. That said, repeated infestations can compromise a plant or tree and leave it susceptible to diseases that can be harmful or deadly.
Many insects are here today and gone tomorrow without doing a bit of harm. The most damaging insects are often the one's you can't see; either they are too small (mites), or they're hiding out on the undersides of leaves (aphids, lacebugs). Others, such as the non-native Japanese beetle can hang around for weeks or months chewing away at foliage and flowers.
If you see an insect on a plant, and there's no apparent damage being done, resist the urge to immediately spray to kill. If you see damage it might be time to take control measures. If it's just a few insects here and there, you can pick or spray them off the plant. For example, at the first site of mealybugs, I usually just spray them off with a blast of water from the garden hose, which usually does the trick. On the other hand, if infestation occurs I'll spray to kill.
Insect Control Products I Use...
When there is infestation, there are several generally safe products I use to control harmful insects on most shrubs and trees. When using these products always follow instructions on the label.
Note: To avoid harming beneficial insects and pollinators, I always spray in the very early morning or late evening hours, when these beneficial insects are not active. Too, I try to avoid spraying flowers if possible.
Neem Oil. I use this one on ornamental shrubs and trees and in my vegetable garden. Neem oil (azadirachtin) acts as a feeding repellent that can prevent attack by many types of pests and diseases. It also can affect the growth and development of some pests to suppress problems. Susceptibility to neem varies greatly among different insect species so check the label carefully to be sure that it covers pests that you are trying to manage.
Pyrethrin. Products containing pyrethrins are very broad spectrum insecticides. They attack the nervous system of almost all insects and other arthropods, including spiders and mites. Pyrethrins are very effective as contact sprays but are deactivated very quickly by sunlight, moisture, and air so no residue is left behind.
Sevin / Carbyrl. When it comes to controlling the Japanese Beetle, nothing works better than liquid Sevin spray. Sevin is also effective on many other types of insects listed on the product label, including fleas, ticks and ants.
Malathion. I use this one to control whiteflies on gardenias and other plants, and lacebugs on azaleas and other plants.
Cyonara Lawn and Garden Insect Control. Cyonara is an excellent professional-grade insect control spray that treats a wide variety of the most common insect pests on shrubs and trees as well as on lawns, yards, vegetable gardens, roses and flowers. Cyonara is fast acting on insects with a quick-knockdown and offers 8 weeks of residual protection.
Fertilome Tree and Shrub Insect Drench. I use this one on crape myrtles, roses and other plants and trees that I know are always bothered by certain very damaging insect pests. Once this systemic insecticidal drench is applied it moves down through the soil, where it is absorbed by the roots of the plant or tree. Once absorbed, it moves up through the tree or shrub providing year-long insect protection. Fertilome Tree & Shrub Insect Drench even moves into new growth after application, thus protecting it, too.
Important Note: Before spraying any shrub or tree in your landscape with an insecticide always read instructions on the label and check to make sure that the plant you want to spray is on the "safe list."