Magnolia Scale (Neolecanium cornuparvum) and Tuliptree Scale (Toumeyella liriodendri) are native “soft scales” (order Hemiptera, family Coccidae). Coccids are called soft scales because the females are hidden beneath a helmet-like soft leathery covering that provides some protection. However, they are easily crushed.
A key feature of soft scales compared to “armored scales” is the production of sugary, sticky “honeydew.” Armored scales do not produce honeydew.
Soft Scales Suck
Soft scale adults and nymphs insert their piercing-sucking mouthparts into phloem vessels. They tap plant sap to acquire both carbohydrates which provide energy and amino acids which are the building blocks for proteins and enzymes.
However, the phloem sap holds only trace amounts of amino acids compared to vast amounts of dissolved carbohydrates. This means the scale must process a large quantity of sap to extract the necessary amino acids. They discharge the excess sugar-rich liquid from their anus in the form of “honeydew" which is just a nice name for scale diarrhea.
Magnolia and tuliptree scales are notorious for producing copious quantities of sticky, drippy honeydew. The sugary liquid may cause heavily infested trees to literally buzz with insect activity as flies, bees, and wasps seek a sweet treat. A high percentage of the flies are often members of the blow fly family, Calliphoridae. Their maggots may have a taste for decaying flesh, but adults like sweets.
The honeydew drips onto the leaves and stems of the host plant as well as understory plants to eventually become colonized by black sooty molds. Although the molds cause no harm to the overall health of infested trees, the blackened leaves can reduce the aesthetic appeal of heavily infested trees.
Hosts
The primary hosts of magnolia and tuliptree scales are members of the magnolia family (Magnoliaceae) which is comprised of only two genera: Magnolia and Liriodendron. Magnolia scale only infests members of the Magnolia genus while tuliptree scale infests members of both genera as well as a few other tree hosts outside of the family.
Magnolia scale has the greatest impact on non-native magnolias and associated hybrids compared to native magnolias. Native magnolias are more resistant perhaps because of natural defenses that developed through a shared evolutionary history with magnolia scale. Non-native magnolias that are most commonly infestationed include star magnolia (Magnolia stellata), lily magnolia (M. liliiflora), and saucer magnolia (Magnolia × soulangeana).
Tuliptree scale is primarily found on its namesake host. However, as noted above, this soft scale may also be found on magnolias including our native southern magnolia (M. grandiflora), as well as the non-native star and kobus magnolias (M. kobus) and the hybrid saucer magnolia. Additionally, various references note that tuliptree scale may occasionally infest lindens (Tilia spp.), buttonbush (Cephalanthus spp.), and walnut (Juglans spp.) although I’ve never found this scale on anything other than Magnoliaceae hosts in Ohio.
Life Cycle
Both scales have one generation per season. Females and males spend the winter as mid-to-late instar dark-colored nymphs attached to the stems of their host plant. Their resemblance to lenticels makes them inconspicuous. The nymphs mature in the spring with the males developing into small gnat-like insects that fly to females and mate.
The females remain immobile, and eggs are produced in late summer to early fall. Like most soft scales, magnolia and tuliptree scale females can produce large numbers of eggs. Numbers ranging from 1,500 to over 3,000 per female are common. This accounts for why populations can build rapidly.
The females are ovoviviparous meaning that eggs are held internally until they hatch creating the illusion that the females are giving birth to the first instar nymphs (= crawlers). Tuliptree scale females become fully mature in late-July to early-August and eggs hatch over a 2 – 4-week period. Magnolia scale produce eggs over an unusually prolonged period of time meaning that eggs hatch from early August throughout September in Ohio.
The first instar crawlers are highly mobile but become immobile once they insert their piercing-sucking mouthparts into stems. This is the overwintering stage.
Description
Magnolia scale is the largest soft scale found in Ohio with mature females measuring as much as 1/2" in diameter. Pictures of mature magnolia scale females are commonly used as the “poster child” for soft scales owing to their size and shape.
However, magnolia scale females change their appearance dramatically as they develop throughout the spring and summer. The females start the season somewhat flattened and dark gray to grayish-brown making them difficult to detect as they blend with the bark. They look nothing like the helmet-like mature scale pictured above and a heavy infestation may be overlooked.
As spring progresses, the begin to “inflate” and become covered in a heavy coating of white, powdery exudate making them look like they’re “powder-coated.” The white coating makes the scales easier to detect; however, it may also cause them to be mistaken for mealybugs.
Eventually, the magnolia scale females shed their powdery coating as they rapidly expand to their final size and acquire their characteristic helmet-like shape. This generally occurs in late summer and signals that eggs are being produced and 1st instar nymphs will soon be on the scene.
Tuliptree scale females don’t significantly change their appearance as they mature and “inflate” through early to mid-summer. They are not “powder-coated” early in their development.
Mature females can range in size from 1/8 – 1/3" in diameter and colors vary from light grayish green to reddish-orange mottled with black. Their covering develops a distinct flange around the edge making the scale look like WWI-era helmets.
Management
Magnolia scale was once common in Ohio but became rare throughout much of the 2000s except for highly localized outbreaks. This has changed in recent years with this native scale once again becoming common on magnolias throughout the state.
Tuliptree scale remains rare throughout Ohio except for a localized outbreak that’s occurring this season in the southeast part of the state. I’ve been getting reports of high populations producing heavy flows of sticky honeydew in Monroe County and some surrounding counties. Infestations are occurring both in forests and landscapes.
1. Cultural Management
As with most soft scales, magnolia and tuliptree scales are seldom direct killers of established healthy trees. However, a substantial loss of sap from a heavy scale infestation represents a serious loss of energy resources to the trees. The accumulated stress coupled with other stress-producing conditions may cause leaf yellowing and loss, branch dieback and canopy thinning, and even the death of entire trees.
Thus, the best approach to reducing the impacts of these native scales is to first focus on reducing tree stress. For example, maintaining adequate soil moisture by watering during droughts can reduce the impact of a scale infestation. Some stress-inducing issues are preordained such as planting trees in highly compacted soils or in non-irrigated parking lot tree planters.
Although providing proper soil fertility is a vital component of tree health management, fertilizer applications should be used with caution, particularly high nitrogen applications. Numerous studies have shown that high nitrogen benefits phloem-sucking insects by increasing the amino acid concentration in the sap. High nitrogen applications are almost a sure-fire recipe for high scale populations.
Fertilizer applications should be guided by soil tests with nutrient deficiencies corrected by selecting products that only provide the specific element(s) found to be lacking. Using “general fertilizer” products with nitrogen benefits soft scales and other phloem-sucking tree pests more than trees.
Plant selection is also an effective cultural approach to pest management. As noted above, our native magnolia scale has a strong affinity for non-native magnolias and associated hybrids.
2. Mechanical Management
Soft scales are weakly attached to the bark surface. Thus, it’s easy to physically remove them using a dish scrubber or soft-bristled scrub brush.
A campus project titled, “Scrubbing Scales, Saving Trees, Engaging Students” was published in the American Entomologist in 2019 and describes how students were armed with toilet brushes and directed to scrub the soft scale, calico scale (Eulecanium cerasorum), from trees on the University of Kentucky campus. The approach was highly effective.
Here is a link to the paper: https://entomology.ca.uky.edu/files/rieske_et_al._2019_living_laboratory_scale_scrub_ms_am_ent_65_43-49.pdf
Although scrubbing away soft scales is best suited to small trees, it can be highly effective in reducing burgeoning scale populations. The timing is important with the current maturing females providing a good target. Waiting until females develop into their fully mature form risks the small, flattened 1st instar nymphs escaping the scrubbing.
3. Biological
It’s highly likely that the “now you see them, now you don’t” nature of magnolia and tuliptree scales in Ohio is largely driven by the 3-Ps: Predators, Parasitoids, and Pathogens. I observed the dramatic impact of a scale predator firsthand in 2021 and posted a BYGL Alert titled, “Magnolia Massacre.”
Magnolia scale had established a significant beachhead on a small saucer magnolia in my landscape before I discovered them. However, I watched Signate Lady Beetle (Hyperaspis signata) munch the heavy infestation down to zero in one season.
The images below were taken during the massacre. As you see, the Signate lady beetles have white wool-coated larvae that are wolves in sheep's clothing. The larvae look almost exactly like mealybugs.
The voracious lady beetles didn’t stop with my tree. They also eliminated heavy magnolia scale populations on two small star magnolias in my neighborhood. The three trees have remained scale free since the magnolia massacre.
4. Insecticides
Integrated Pest Management (IPM) involves using pest management tactics that fall under the general headings of biological, cultural, and chemical to keep pest populations below an acceptable threshold. Although IPM strategies include the use of chemicals (e.g., insecticides), the graphic below illustrates that cultural and biological tactics should be used first, with chemical tactics held in reserve.
This approach to using IPM can limit possible negative environmental impacts as well as reduce the selective pressure that produces pesticide resistance. Of course, these two concerns played a significant role in the development of the IPM concept in the first place.
Another cornerstone of IPM is to avoid using one tactic at the expense of another. Thus, it’s desirable to select insecticides that have a limited impact on beneficial insects.
For example, topical applications of pyrethroid insecticides (e.g., bifenthrin) targeting 1st instar magnolia and tuliptree scale crawlers later in the growing season can be effective. However, this chemical class of insecticides is non-selective meaning they can kill bio-allies as well as scale crawlers. Multiple applications may also be required to cover an extended egg hatch which further reduces help from the “biological” corner of the IPM triangle.
Systemic neonicotinoids offer an effective option that minimizes impacts on beneficial insects. Products based on the active ingredients imidacloprid, dinotefuran, or thiamethoxam are effective against these soft scales if applied later in the season to target 1st instar crawlers.
The insect growth regulator (IGR) insecticides, pyriproxyfen (e.g., Distance IGR) and buprofezin (e.g., Talus), are also effective against 1st instar crawlers and will likewise have a limited impact on beneficials. The same is true of insecticidal products based on the active ingredient azadiractin which behaves as an IGR. Keep in mind that although azadiractin is found in the neem tree (Azadirachta indica), it’s only found and extracted from neem seed kernels. It is not found in neem oil.
Of course, as with any pesticide, it’s critical to read and closely follow product label directions. Insecticide failures against soft scales may result from “applicator error” such as incorrect mixing or mistakes in performing calculations leading to an insufficient amount of product being applied. Or, not maintaining recommended pre-treatment and/or post-treatment soil moisture with soil drench applications. The label is not only “the law;” it also provides information that maximizes efficacy while minimizing environmental impacts.