Oystershell Scale (OSS) (Lepidosaphes ulmi) has long presented a management challenge given its wide host range coupled with limited insecticide targets. However, I’ve watched populations of this non-native gradually recede in southwest Ohio over the past few decades owing to help from predators and parasitoids along with the arrival of more effective insecticides as well as a better understanding of application timing.
Unfortunately, OSS has largely been supplanted by the Japanese Maple Scale (JMS) (Lopholeucaspis japonica) in southwest Ohio. Although this scale was first discovered in the U.S. in 1914, most of the detections have occurred over the last decade. It’s now found in most eastern states from New York to Georgia and many Midwestern states including Ohio where significant populations are becoming a common occurrence.
I believe the rapid rise of JMS has been partially fueled by a combination of misidentification leading to the miss-targeting of insecticide applications. These armored scales have entirely different life cycles meaning that management for one will not affect the other.
Armored (or “hard”) scales are so named because they spend much of their life cycle protected under a waxy armored cover called a “test.” The only mobile stages in the life cycle are the adult males, which look like tiny winged aphids, and the first instar nymphs that hatch from the eggs. These are called "crawlers" … because they crawl.
Armored scale nymphs and females feed by inserting their long piercing-sucking mouthparts into plant cells causing them to rupture and collapse. This is unlike soft scales, felt scales, and several other insects that insert their piercing-sucking mouthparts into phloem vessels to withdraw large quantities of sugary sap with the excess liquid excreted in the form of honeydew which is a polite name for scale poo. Armored scales do not produce honeydew.
If you observe honeydew coupled with black sooty molds, look for a soft scale. There’s no scale law preventing armored scales from taking up residence on trees that are also infested with a soft scale or felt scale.
JMS tends to congregate on the undersides of stems. They also gather in bark fissures presumably to reduce the drilling depth required for their piercing-sucking mouthparts to reach delectable stem cells.
Confusing JMS with OSS, and vice versa, is understandable. OSS has long been one of the most common armored scales found on woody ornamentals. The scale has a worldwide distribution and it has a wide host range (over 150 plant species). Many of its hosts are shared with JMS.
JMS has been found in the U.S. on plants belonging to 45 plant genera in 27 plant families. Common landscape and nursery hosts include Acer, Amelanchier, Camellia, Carpinus, Cercis, Cladrastis, Cornus, Cotoneaster, Euonymus, Fraxinus, Gleditsia, Ilex, Itea, Ligustrum, Magnolia, Malus, Prunus, Pyracantha, Pyrus, Salix, Syringa, Tilia, Ulmus, and Zelkova.
The common name for JMS is also misleading; it doesn’t confine its plant-sucking depredation to Japanese maples. In fact, I recently found a JMS infestation in a local park on hosts that included red maple (A. rubrum) and crabapple (Malus sp.). However, two Japanese maples (A. palmatum) located within the general area of the infestation were scale-free!
OSS spends the winter as eggs under the armored shells of the females. OSS has one generation per season. Overwintered eggs hatch at the accumulated Growing Degree Days (GDD) of 497 (base 50F). The full bloom of Miss Kim Manchurian Lilac at 498 GDD is a good phenological indicator of egg hatch and the emergence of first instar nymphs (crawlers). Using double-sided sticky tape is an excellent method for monitoring the emergence of OSS crawlers as illustrated in the image below.
JMS overwinters as females that are covered in a reddish-brown shell which in turn is covered by a brittle white covering. Carefully rubbing the white covering will reveal the armored shell and lifting the shell will expose the light purplish colored females.
Much less is known about the seasonal life cycle of JMS compared to OSS. Research has shown that the scale has two generations per season in Maryland and Virginia and one generation in Pennsylvania. I believe JMS has two generations in southern Ohio based on my observations over the past few years. However, the exact number of generations in the northern part of the state has not yet been established.
According to a University of Maryland Extension fact sheet, JMS eggs begin hatching at 816 GDD (base 50) and peak at 1,143 GDD. Full bloom of smokebush (Cotinus coggygrias) and Chinese lilac (Syringa chinensis) can be used as phenological indicators. The first instar nymphs crawl about for around 8 weeks. Second-generation crawlers appear at 2,508 GDD with peak egg hatch at 3,022 GDD. Crawlers are active for around 7 weeks.
The JMS life cycle where the scale has only one generation per season is poorly understood. This may include northern Ohio although I’m not aware of anyone confirming this. According to a Penn State fact sheet where JMS has one generation, the authors note, “This pest overwinters as females. Adult males are on host plants from late April to late May. One female will lay an average of 25 eggs. The crawlers are active from late May through early August.” Thus, managers should consider using double-sided sticky tape to monitor and refine the timing of insecticide applications.
1. Inspection: this is particularly important for new plants. A low-level infestation on a few new plants can quickly explode into multiple high-density infestations across multiple plants. Make sure to part the foliage on plants with dense canopies so you can peer down onto the lower stems. Also, look at the undersides of the stems and examine bark fissures where JMS often congregates.
2. Don't Destroy Beneficials: scale insects have many natural enemies and JMS is no exception. The key is to apply tactics that kill the scale, not the beneficials. For example, if the JMS population is low and the scale is accessible, it can be physically removed through selective pruning or by scrubbing the stems.
Crawlers are highly susceptible to insecticides, but so are the beneficials. Don't use a topical insecticide such as a pyrethroid that kills beneficials. Research has shown that you'll eventually be dealing with an increased scale population on the rebound.
3. Choose Your Weapons Wisely: several insecticides will kill the JMS crawlers and leave the beneficials alone. According to various university fact sheets, dormant and horticultural oils are effective and they preserve beneficials. However, the job may be incomplete if the JMS population is high. Tank mixing with an insect growth regulator (IGR) such as pyriproxyfen (e.g. Distance) or buprofezin (e.g. Talus) provides greater efficacy, although the IGRs alone also work well.
The systemic neonicotinoid insecticides dinotefuran (e.g. Safari, Transtect) and clothianidin (Arena) as well as the diamide insecticide chlorantraniliprole (Acelepryn) are also effective against JMS. The neonicotinoid imidacloprid (e.g. Merit) is not effective against this or other armored scales.
Of course, as with all pesticide applications, it's critical to read and follow label directions to maximize efficacy while minimizing off-target impacts. Also, you should never rely on a single tactic for suppressing JMS or any other armored scale. For example, an effective strategy may be to target the first generation JMS crawlers with a systemic neonicotinoid to account for the extended time that crawlers are present then follow up with an IGR on the second generation crawlers.
4. Don't Walk Away: it's rare for any chemical suppressant to provide 100% efficacy. You should continue to closely monitor affected plants and be prepared to repeat the IPM steps listed above.