Curtis Young (OSU Extension, Van Wert County) reported during our BYGL Zoom Inservice last Tuesday that Miss Kim Manchurian lilac (Syringa pubescens) is in full bloom in northwest Ohio. This bloom event has long been a dependable phenological indicator for egg hatch of Oystershell Scale (OSS) (Lepidosaphes ulmi), but NOT for egg hatch of Japanese Maple Scale (JMS) (Lopholeucaspis japonica).
OSS and JMS are so-called “armored scales.” They look similar, belong to the same family (Diaspididae), and can be found on the same hosts; sometimes at the same time. However, they do not have the same life cycles which may account for why we’ve been seeing a dramatic rise in JMS in Ohio over the past several years. Misidentification can lead to miss-targeting insecticide applications.
OSS has one generation per season while JMS may have more than one generation, at least in southern Ohio. Where OSS has two generations per season, the scale spends the winter as eggs while JMS overwinters as females. So, OSS crawlers emerge much earlier in the season compared to JMS. Although both of these armored scales are susceptible to many of the same insecticides, application timing is very different. Mistaking one for the other seriously affects management.
Armor All Basics
Armored 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.
Of course, 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. I experienced this first-hand last year when I came across a heavy JMS infestation on American elms (Ulmus americana). The trees were also infested with European Elm Scale (Gossyparia spuria, Family Eriococcidae (Felt Scales)) which produces copious quantities of honeydew that commonly becomes colonized by black sooty molds.
As you can see in the image below and many of my other images, the black sooty molds colonizing the honeydew from the European elm scale had an impact. On one hand, the accretion obscured some of the armored scale coverings. On the other hand, the charcoal black accentuated some of the white scale coverings. In fact, my attention was drawn to the trees by small white spots highlighted by a black background.
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.
Blurred Lines
Confusing JMS with OSS, and vice versa, is understandable. OSS has long been one the most common armored scales found on woody ornamentals. Indeed, the scale has a worldwide distribution, it has a wide host range (over 150 plant species), and shares many tree and shrub hosts with JMS.
The common name for JMS is also misleading; it doesn’t confine its plant-sucking depredation to Japanese maples. Its host range encompasses over 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.
Although this scale was first discovered in the U.S. in 1914, most of the detections have occurred over the last decade. It’s still considered relatively new to Ohio. Currently, JMS has been detected in Washington DC as well as 16 states including AL, CT, DE, GA, IN, KY, LA, MD, ND, NY, OH, PA, RI, TN, and VA.
OSS spends the winter as eggs under the armored shells of the females. The female shells look exactly like light brown to grayish brown oyster shells with one end pointed and the other broadly flattened. Carefully lifting them and taking a look under high magnification will reveal the pearly white eggs.
OSS has one generation per season. Overwintered eggs hatch at the accumulated Growing Degree Days (GDD) of 497 (base 50F). 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 for OSS crawlers as illustrated in the image below.
JMS overwinters as females 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 base 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 JMA 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.
Integrated Pest Management (IPM)
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.
