Magnolia scale (Neolecanium cornuparvum) is a type of "soft scale" so named because of the helmet-like soft leathery covering that protects the females. This is one of the largest soft scales found in Ohio with mature females measuring as much as 1/2" in diameter.
I've gotten numerous e-mails about magnolia scale in southern Ohio over the past several days. A few of the senders correctly identified the scale and asked for management advice; however, others were a bit off the mark. Their questions ranged from wanting to know about the "white stuff" adorning magnolia branches to seeking the identity of the "mealybug" infesting their tree to asking why bees, wasps, and flies are swarming a magnolia.
The confusion is understandable because magnolia scale females have not yet acquired their most identifiable form. The females are still somewhat flattened and may be obscured by a heavy coating of white, waxy material. As the females "puff-up" to their full squishable size, the waxy material will eventually peel away to reveal the pinkish-tan colored females beneath.
As they puff-up, the 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.
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 first instar crawlers are highly mobile but become immobile once they insert their piercing-sucking mouthparts into stems. This is the overwintering stage.
Magnolia scale adults and nymphs insert their piercing-sucking mouthparts into phloem vessels to tap plant sap. 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.
Magnolia scale sucks sap to acquire carbohydrates which provide energy. They also extract amino acids which are building blocks for proteins and enzymes. However, the sap only contains 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 a prolific honeydew producer. During normal years, the sticky 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 do 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 flies. In fact, 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.
Although significant magnolia scale infestations are rare on 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. However, fertilizers 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.
Magnolia scale infestations attract a wide range of natural enemies including lady beetles such as Sigil lady beetles (Hyperaspis spp.) and Australian mealybug destroyers (Cryptolaemus montrouzieri). Both have wool-coated larvae that are actually wolves in sheep's clothing.
These and other bio-allies can have a significant impact on maintaining magnolia scale infestations below noticeable levels on native magnolias. Unfortunately, they appear to have a limited effect on magnolia scale populations on non-native magnolias. It's speculated that the lack of defenses by the non-native trees may support such a rapid scale proliferation, the large numbers simply overwhelm the ability for natural enemies to have a significant effect.
This means other management tactics may be necessary to support plant health. A direct approach is to use physical removal. If trees are small and scale populations are low, a dish scrubber or bathroom scrub brush can be used to physically remove the females before they produce eggs at the end of summer.
Julie Crook (OSU Extension, Hamilton County) appears to have eliminated a substantial scale infestation on a medium-sized magnolia in her landscape by using this approach. Many of the images included in this Alert shows the infestation or close-ups are of specimens collected from her tree. However, sadly for me, almost no magnolia scale is evident on her tree this season.
Topical insecticide applications targeting 1st instar crawlers later in the growing season can be effective. However, the extended period of egg hatch presents a challenge and requires thorough stem coverage as well as multiple applications following label recommendations. Insect growth regulator products based on the active ingredient pyriproxyfen are effective and will not affect the beneficial bio-allies helping to keep scale populations in check.
"Horticultural oils" (e.g. summer oils) can also be effective on 1st instar crawlers; however, oils require direct contact and there is no residual activity. Thorough coverage is critical because oils only kill on contact. Spring applications can also be effective; however, there is a risk for damaging flower buds.
Control can be achieved with single applications of neonicotinoid systemic insecticide products with the active ingredients imidacloprid or dinotefuran. There are two effective "treatment windows" in Ohio. They are late summer to early fall, before settled crawlers stop feeding for the season, or sometime in May after overwintered nymphs start feeding. However, spring applications should be delayed until after trees have finished flowering to avoid killing pollinators. Of course, as with all insecticide applications, it is critical to read and follow label directions.