Magnolia scale (Neolecanium cornuparvum) is a type of "soft scale" so named because the females are hidden beneath a helmet-like soft leathery covering that provides limited protection. Soft scales can be squashed!
This native insect is one of the largest soft scales found in Ohio with mature females measuring as much as 1/2" in diameter. The size and overall shape of mature magnolia scale females commonly make it a poster child for soft scales.
However, the current immature magnolia scale females look nothing like their mature form. The immature females are still somewhat flattened and obscured by a heavy coating of white, powdery material; they look like they’re covered in powdered sugar.
The current form of magnolia scale females may cause them to be mistaken for mealybugs. The confusion is understandable because magnolia scale females have not yet acquired their most identifiable helmet-like form.
Adding to the confusion is the commonly associated appearance of a couple of significant predators drawn to the scale buffet. Sigil lady beetles (Hyperaspis spp.) and Australian mealybug destroyers (Cryptolaemus montrouzieri) have white wool-coated larvae that are wolves in sheep's clothing. The lady beetles look almost exactly like mealybugs.
As as the immature magnolia scale females "puff up" to their full most squishable size, the waxy material will eventually peel away to reveal the pinkish-tan colored females beneath. The maturing females drip copious quantities of honeydew. The sticky, sugary liquid may cause heavily infested trees to literally buzz with insect activity as flies, bees, and wasps seek a sweet treat.
This native scale has a strong affinity for non-native magnolias and associated hybrids. Common hosts include star magnolia (Magnolia stellate), lily magnolia (M. liliiflora), and saucer magnolia (Magnolia × soulangeana). Native magnolias are more resistant perhaps because of natural defenses that developed through a shared evolutionary history with the scale.
All in the Family
Tuliptree scale (Toumeyella liriodendra) is another large native soft scale that may be found on magnolias. The occurrence of this scale on magnolia is a reminder that our native tuliptree (a.k.a. tulip poplar, yellow poplar) (Liriodendron tulipifera) belongs to the magnolia family, Magnoliaceae. Tuliptrees are a type of magnolia.
Tuliptree scale may infest star magnolia, saucer magnolia, and southern magnolia, M. grandiflora). Oddly, magnolia scale does not infest tuliptrees.
However, the occasional occurrence of tuliptree scale on magnolia can be confusing given that the two soft scales look nothing alike. Indeed, during “outbreak” years when localized populations are high, tuliptree scale may be found joining the scale party on trees infested with magnolia scale. Thankfully, tuliptree scale has a life cycle that’s similar to magnolia scale so the same management options can be applied to both of these soft scales.
Magnolia Scale Life Cycle
Magnolia scale has one generation per season. Females and males spend the winter as first 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 but rapidly expand in size as they mature through the spring and summer. Eggs are produced in late summer to early fall and are held internally until they hatch creating the illusion that the females are giving birth to the first instar nymphs (= crawlers).
The unusually long period that females develop eggs translates into eggs hatching 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.
As with all soft scales, magnolia scale adults and nymphs insert their piercing-sucking mouthparts into phloem vessels. They tap plant sap to acquire both carbohydrates which provide energy as well as amino acids which are the building blocks for proteins and enzymes.
However, the phloem sap contains only trace amounts of amino acids compared to huge 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 a sticky, sugary "honeydew" which is just a nice name for scale diarrhea.
Magnolia scale is notorious for producing copious quantities of sticky, drippy honeydew. During normal years, 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 seriously reduce the aesthetic appeal of heavily infested trees.
The honeydew also attracts a plethora of freeloading sugar-sippers including stinging insects such as wasps as well as flies. 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.
As with most soft scales, magnolia scale is seldom a direct killer of established landscape 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.
1. Host Range. Host selection is a highly effective first line of defense against magnolia scale. As noted above, the native magnolia scale can wreak havoc on non-native magnolias and associated hybrids which never developed defenses against this sap-sucking pest. So, planting native magnolias can significantly reduce the impact of magnolia scale. Of course, I recognize this management approach limits enjoying the full range of colors and forms offered by all magnolias available for Ohio landscapes, so it’s important to pay close attention to the remaining management tools.
2. Tree Health Management. Maintaining healthy trees is an important component of magnolia scale management, particularly for non-native magnolias. The first and best approach to managing this native scale is to reduce tree stress-inducing conditions such as watering trees during a drought.
Although providing proper soil fertility is an important component of tree health management, fertilizer applications should be used with caution, particularly high nitrogen applications. Numerous studies have shown that high nitrogen benefits sap-sucking insects by increasing the amino acid concentration in the sap. High nitrogen applications are almost a sure-fire recipe for high scale populations.
3. Rely on Allies. Bio-allies such as the lady beetles mentioned above are also an important component of scale management programs. While predators and parasitoids are seldom effective enough to eliminate magnolia scale on non-native magnolias, they certainly make an important contribution to the effort.
I was disheartened to find a developing magnolia scale population on a saucer magnolia in my landscaping. How dare they! However, I was heartened to find that Sigil lady beetles had found my scale infestation. In fact, the lady beetle population was so high, it was difficult to find magnolia scale females out of danger from the wolves in sheep's clothing so I could take pictures for this Alert.
4. Scrub it Away. Physically removing soft scales using a dish scrubber or soft-bristled scrub brush is a direct approach to scale management. Although scrubbing away softs scale is best applied to small trees, it can be highly effective in reducing burgeoning scale populations. However, timing is important with the current maturing females providing a good target. Waiting until females develop into their mature form risks escape from the scrubbing by the small, flattened 1st instar nymphs.
5. The Role of Insecticides. Integrated Pest Management (IPM) involves combining pest management tactics that fall under the general strategy headings of biological, physical, and chemical to keep pest populations below an acceptable threshold. A cornerstone of IPM is to never use one tactic at the expense of another. For example, the first choice for insecticides should be those with a limited impact on beneficial insects.
Topical insecticide applications targeting 1st instar crawlers later in the growing season can be effective. However, they require thorough stem coverage and the extended egg hatch of magnolia scale means multiple applications may be required based on label recommendations. Also, beneficial insects may come in contact with topical insecticides adding an important point to consider in deciding which product(s) to use.
"Horticultural oils" (e.g. summer oils), as well as insecticidal soaps, can be effective on 1st instar crawlers and have a limited impact on beneficial insects. However, oils and soaps require direct contact to kill the scale crawlers and there is no residual activity. Consequently, multiple applications and thorough coverage are required throughout the scale egg hatch. Spring applications can also be effective; however, there is a risk for damaging flower buds.
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 have a limited impact on beneficials. The same is true of insecticidal products based on the active ingredient azadiractin which also behaves as an IGR. Keep in mind that although azadiractin is found in the neem tree (Azadirachta indica), it is only found and extracted from neem seed kernels. It is not found in neem oil. However, neem oil can be used as a horticulture oil against soft scales.
Systemic neonicotinoids offer another effective option that minimizes impacts on beneficial insects. Products based on the active ingredients imidacloprid, dinotefuran, or thiamethoxam are highly effective against magnolia scale if applied late in the season to target 1st instar crawlers.
Of course, as with any pesticide, it’s critical to read and closely follow product label directions. Insecticide failures against magnolia scale (and other soft scales) are commonly traced to “applicator error” such as incorrect mixing or mistakes in making 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.