Take arms against armored scale

Figure 2. Tiny orange euonymus scale crawlers covering a Japanese euonymus leaf. S.D. Frank, NCSU.

Armored scale insects may be the most difficult pests to manage on nursery crops. Most trees and shrubs in your nursery are hosts for multiple armored scale species. This article will discuss the key components of armored scale IPM using euonymus scale, a common armored scale, as an example.

Biology
Some of the most common armored scale species in nurseries are euonymus scale, tea scale, Japanese maple scale, false oleander scale, and juniper scale but the list will vary based on your region and crop plants. Every armored scale species has a different life cycle. However, all armored scales are immobile beneath a waxy cover, called a test, for most of their life. The test helps to protect them from predators, parasitoids, environmental conditions, and many insecticides. Most species spend the winter as adults or nymphs beneath their test. Crawlers hatch from eggs laid beneath the adult test. Crawlers are the only mobile stage of armored scales and the only stage without a test. They foray out to find a spot on the plant to feed. This is often just a few inches. They insert their mouthparts into plant tissue and begin to feed and to form their protective test.

Euonymus scale is primarily a pest of euonymus but they can also feed on pachysandra, holly, and other plants. Females are hard, gray, and oyster shell shaped. Males are white and not armored (Figure 1). Euonymus scales overwinter as adult females. In late spring, they lay eggs under their test then die. Crawlers emerge from under the test and settle on plant stems and leaves to feed (Figure 2). Within a site, crawler emergence is fairly synchronized and lasts two to three weeks. Euonymus scale can have one to four generations per year depending on climate.
 

Figure 1. Adult female (gray) and male (white) euonymus scales on Japanese euonymus. A.G. Dale, NCSU.

Scouting, monitoring
You will never know all the scale species present on woody plants. They are tiny, camouflaged, and difficult to identify. However, it is essential to distinguish armored scales (family Diaspididae) from scales in other families such as soft scales (family Coccidae). This is because armored scales are less vulnerable to contact insecticides than soft scales. More importantly the systemic insecticide imidacloprid does not kill armored scales but does kill soft scales. Luckily, there are two easy ways to determine if the scales frustrating you are armored.

Use the tip of your pocketknife to flip the test off the scale. Armored scales will remain on the branch after you have removed their tests (Figure 3). Soft scales cannot be separated from their test so the whole insect will flip over with the cover. Not trained in insect surgery? Don’t worry. Another difference is that armored scales do not produce honeydew but soft scales do. Thus, if the leaves below your scale infestation are covered in sticky honeydew or sooty mold you do not have armored scales.

Euonymus scale infestations can be recognized by yellow spots on the top of leaves where scales are feeding underneath (Figure 4). As infestations get worse you will see scale covers and the flaky white males on plants. Stem feeding scales are harder to detect because they often feed in branch collars and other concealed places. Scales remain on plants all year, so you can scout in the fall and winter when you have more time and plants have fewer leaves.
 

Figure 4. Yellow leaf spots that result from euonymus scales feeding under leaves. A.G. Dale, NCSU.

Decision making
Deciding if intervention is necessary begins with determining if the scales you found are alive. Scale tests can remain on plants for a year or more after the insect dies. So before you apply insecticides, do a quick test. You can flip covers and look for the live insect below. Or, you can squash some of the scales with your thumbnail. If the scales are alive, juice will come out and smear on your finger. This crude technique allows you to check many scales quickly. Unfortunately, there are no established thresholds to help growers decide if insecticide treatment is necessary. However, scales can spread very quickly so even a few scales are too many.

Intervention
Armored scales are relatively waterproof; they live under a waxy cover. Therefore, contact insecticides, such as pyrethroids, do not provide adequate control unless applied to the crawler stage. In addition, pyrethroids, organophosphates, and other insecticides that leave a toxic residue on plant surfaces kill natural enemies and can make scale infestations worse. So what is the best approach? Our work with euonymus scale demonstrates that there are insecticide options that kill scales without leaving a highly toxic residue.

Insect growth regulators disrupt insect development preventing eggs and nymphs from developing into reproductive adults. Two insect growth regulators used for armored scale control are buprofezin and pyriproxyfen. These are classified as reduced risk insecticides due to their relatively low vertebrate toxicity. They can kill by contact and ingestion. Some, such as pyriproxyfen, have translaminar activity so they move into leaf tissue but are not systemic.

Neonicotinoids are another class of chemicals that include some compounds for armored scale control. Imidacloprid is the most commonly used chemical in this class but it is not effective against armored scales. It does control soft scales and other related organisms. However, research has shown that in many cases imidacloprid applications can result in higher armored scale abundance than if nothing is applied. This is why it is essential to differentiate armored scales from other families while scouting.

Figure 3. Adult female euonymus scale with cover removed. A.G. Dale, NCSU.

Three neonicotinoids available for armored scale control are acetamiprid, dinotefuran, and thiamethoxam. Neonicotinoids have systemic activity so the chemicals move into plants making them toxic for pests to consume. This means that feeding stages of scales can be killed with systemic insecticides even if you cannot contact the insect directly. Thus, even though applications during crawler emergence are optimal, we have found that these neonicotinoids can kill adult euonymus scales feeding on treated plants. Neonicotinoids can also remain effective for weeks to months depending on variables such as how they were applied and how fast plants are growing. We treated euonymus plants with acetamiprid, dinotefuran, or thiamethoxam when first generation euonymus scale crawlers were emerging. They provided excellent control of the first generation and 6 weeks later plants were still protected from the second generation of crawlers.

Horticultural oils can also be effective for armored scale management. Horticultural oils are refined petroleum products that kill insects by blocking their spiracles (breathing pores) and also by disrupting fatty acids and cell membranes. They work on contact with insects so they are best applied when crawlers are active. They do not leave a toxic residue so multiple applications may be required. However, horticultural oils are inexpensive and kill many other common pests like mites and aphids so they are definitely a valuable tool.

You can’t treat scales you don’t know you have. The key is finding them before they get out of control. Then consider treating them with one of these products. I think your scale management will get just a little easier.

Steve Frank is Assistant Professor and Extension Specialist in the Entomology Department at North Carolina State University. You can reach him at sdfrank@ncsu.edu.

Adam Dale is a graduate student investigating the biology and management of nursery and landscape pests. For more information on nursery pest management visit http://ecoipm.com.