This week as I pruned some shrubs, I noticed a white powdery substance on the back of some holly leaves. This was a type of scale. As I looked around, I noticed some waxy blobs on my Japanese magnolia. The blobs were wax scale.
There are many different types of scale insects, and they feed on most of the plants in our landscape. Scales can be found on the back of leaves, twigs and branches, and on the trunks of plants.
When the young scale, called crawlers, emerge from the egg, they move a short distance and begin feeding by sucking the sap from the plant. At this point, they become immobile and will spend the rest of their life feeding at this site. Once they start feeding, they produce a waxy coating that protects them. This covering is the reason that scale is so hard to control.
So, how can scale be controlled? I will be using Integrated Pest Management (IPM) principles to decide the best way to control these insects.
There are many principals that make up an IPM approach to controlling pest. The first principle is to use resistant plants. An example of this is planting Crape myrtles that are resistant to aphids. Other principles are proper cultural practices such as proper pruning, fertilization and watering; letting beneficial insects build up to take care of the problem; and physical removal. The last is the use of pesticides.
This week I removed many of the scale by pruning the plant. Because I was doing a renovation prune, I removed the majority of the scale. Next, I will look for new infestations on these plants. As I mentioned earlier, scale insects spread by crawlers that hatch in April. In our area, there is usually one generation per year.
To check for crawlers, put a piece of double-sided tape around the branches in the area where the scales are located. When the crawlers hatch, they will move across the tape and stick to it. The crawlers will be very small and yellow or reddish in color. When the crawlers are present on the tape, it is time to treat.
There are a number of products that can be used to control scale. For organic controls, insecticidal soap or horticultural oils can be used. Malathion will control scale in the crawler stage. With any of these products, spray the interior of the plant or underside of the leaf. The control products have to get on the insect to work.
Imidacloprid is a systemic insecticide that can be used. This product is mixed with water and poured around the roots. It is taken up by the root system and moved throughout the plant and will give control for the season.
There are two other insects that are very common on our shrubs that need to be controlled now: whiteflies and Azalea lace bugs. Whiteflies are a major pest on many shrubs, but I get more questions on controlling whiteflies on gardenias than any other shrub. Gardenias that are infested with whiteflies will have the top side of the leaves turn black. This is called sooty mold. The excrement of the whitefly is called honeydew. Honeydew has high sugar content, and the mold grows on the sugar.
Whiteflies have two generations per year. The first generation will usually hatch in April and the second generation will hatch in August. Controlling the first generation will help reduce the chance of an infestation in August.
There are two methods of control for whiteflies. The first is to start a spray program when adult whiteflies first appear. The products that are used to control whiteflies are insecticidal soaps, pyrethrins, pyrethrums and bifenthrin. These products need to be applied to the bottom side of the leaf because they are contact insecticides.
The other products to use are systemic insecticides such as imidacloprid and disulfoton. They are mixed with water and applied to the root system. These products need to be applied now to give them time to move through the plant.
If azaleas have white or yellow mottling on the top of the leaf and black specks on the bottom of the leaf, they are infected with azalea lace bugs. These insects live on the bottom of the leaf and suck the juices out of the leaf. The lace bugs over-winter as eggs, and will start emerging the end of March through April. There will numerous generations during the year. Again, the key to control is to control the first generation. The same insecticides that control whiteflies can be used on azalea lacebugs.
Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. He can be reached at email@example.com.
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