Magnolia Scale Source: University of Kentucky
Magnolia Scale are native “soft scales. They are called soft scales because the females are hidden beneath a helmet-like soft leathery covering that t provides some protection. However, they are easily crushed. Rest easy, magnolia scale only infests members of the Magnolia genus. 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 infestation include star magnolia lily magnolia and saucer magnolia.
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 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. 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.
Life Cycle – 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 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). 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.
As with most soft scales, magnolia 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.
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. 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. For example, topical applications of pyrethroid insecticides (e.g., bifenthrin) targeting 1st instar magnolia 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.
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. Of course, as with any pesticide, it’s critical to read and closely follow product label directions.