I visited a landscape in southwest Ohio yesterday with a lily magnolia that was heavily infested with Magnolia Scale (Neolecanium cornuparvum). This is one of the largest "soft scales" in Ohio with mature females measuring as much as 1/2" in diameter. This native scale has a strong affinity for non-native magnolias and associated hybrids. Native magnolias are more resistant perhaps because of natural defenses that developed because of 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. They 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. The first instar nymphs that hatch from the eggs are mobile at first (= "crawlers"), but become immobile once they insert their piercing-sucking mouthparts into stems. This is the overwintering stage.
Magnolia scale affects its host plants in two ways. Adult females and nymphs insert their piercing-sucking mouthparts into phloem vessels to tap plant sap. A substantial loss of sap from a heavy scale infestation represents a serious loss of energy resources to the trees. The associated physiological stress can produce leaf yellowing and loss, branch dieback and canopy thinning; even the death of entire trees. Indirectly, the stress can make trees susceptible to other problems.
The second way magnolia scale affects its hosts is a by-product of its "meat and potatoes" diet. Females and nymphs extract amino acids ("meat") and carbohydrates ("potatoes") from the phloem sap. However, there is a substantially larger amount of dissolved carbohydrates in the sap compared to dissolved amino acids. This means the scale must remove a large amount of the sugary sap to extract the small amount of amino acids. They discharge the excess sugar-rich sap in the form of a sticky, sugary fluid called "honeydew."
Magnolia scale is a prolific honeydew producer. The sticky honeydew may drip onto the leaves and stems of the host plant as well as plants beneath an infested tree, or onto sidewalks, cars, slow-moving entomologists, etc. The sweet liquid attracts a plethora of freeloading sugar-sippers including bees, wasps, flies, and ants. Indeed, the magnolia I visited yesterday was literally abuzz with insects on a sugar high.
The honeydew is commonly colonized by black sooty molds and while the molds do not harm plants, the sticky goo combined with the molds can produce an aesthetically unappealing mess. Of course, the occurrence of honeydew and sooty molds do not necessarily mean a soft scale is afoot. Many other sucking insects (aphids, planthoppers, etc.) also exude honeydew.
Current and Future Views
The current maturing females may be obscured by a heavy coating of white, waxy, flocculent material. This causes magnolia scale to be occasionally mistaken for other sucking insects such as mealybugs, planthopper nymphs, or woolly aphids. Eventually, the waxy coating thins to reveal the slightly convex, elliptically shaped pinkish-tan colored maturing females.
As the season progresses, the magnolia scale females will mature to reach their full size and "puff-up" to achieve their characteristic helmet-shape. The mature females have a smooth surface and are light tan to brownish-purple; they are often lightly coated in a white powder. Eggs are produced in late summer and remain beneath the female's body until hatching, which gives the appearance that the females are "giving birth" to live young. Egg hatch may occur continuously from early August into early October.
A Word from Management:
Magnolia scale infestations attract a wide range of natural enemies such as the notorious scale nemesis; the Twice-Stabbed Lady Beetle (Chilocorus stigma), or the peculiar looking lady beetle larva known as the Mealybug Destroyer (Cryptolaemus montrouzieri). Unfortunately, these bio-allies appear to have a limited effect on suppressing high magnolia scale populations. This may seem unusual for a native insect pest; however, remember that high magnolia scale populations are most common on non-native magnolias or their associated hybrids. It is speculated that the lack of defenses by the non-native trees may support such proliferation of the scale that populations simply overwhelm the ability for natural enemies to have a significant effect.
This means insecticide treatments may be necessary to support plant health. However, the extended period of egg hatch presents a serious challenge to using topical insecticide applications targeting the 1st instar crawlers in late summer to early fall; multiple applications and thorough coverage is required. This is particularly true for "horticultural oils" (e.g. summer oils). They can be highly effective if female scale bodies aren't stacked on top of the crawlers. Applications should be made in late summer, usually late August, and follow-up applications through the fall. 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 the neonicotinoid systemic insecticides imidacloprid (e.g. Merit) and dinotefuran (e.g. Safari). There are two effective "treatment windows" in Ohio: 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 only be made after flowering to avoid killing pollinators. Of course, as with all insecticide applications, it is critical to read and follow label directions.