The magnolia scale, Neolecanium cornuparvum, is one of the largest and most conspicuous scale insects known to occur in Ohio. Adult females may reach nearly ½ inch in diameter when fully grown. This soft scale is shiny, tan-brown and smooth. As the scales grow, they are often covered with a white mealy wax. This wax is lost at the time that the crawlers emerge.
As the name implies, this insect is primarily a pest of various species of magnolia. Saucer, star and lily magnolias are most commonly attacked and can be severely infested. Other species of magnolias such as cucumbertree and southern species are occasionally attacked but are rarely damaged as a result. This scale is occasionally confused with the tuliptree scale, which is another large, soft scale that infests tuliptree (yellow poplar) and magnolia.
Magnolia scales have sucking mouthparts. Damage occurs as these insects remove plant fluid from their host. Heavily infested branches and twigs are weakened and growth is stunted. Additionally, development of leaves may be halted, resulting in a thinning canopy. Under a continuous and heavy infestation, even mature trees may be killed.
Like most soft scales, the excess plant sap is excreted as a sweet, sticky material called honeydew. As the scales continuously feed, the honeydew drips onto the nearby foliage and branches. An unsightly dark fungus, called black sooty mold, colonizes the honeydew. Generally, this mold is harmless and is only considered a nuisance. When the sooty mold completely covers leaves, photosynthesis may be hindered. The honeydew and sooty mold can foul park benches and sidewalks under infested magnolias. The honeydew may also attract ants, bees, wasps and flies that feed on it.
Description and Life Cycle
The magnolia scale spends the winter on one- to two-year-old twigs as tiny, dark-colored nymphs. As temperatures warm in the spring, the scales begin to suck sap. The nymphs will molt and begin feeding in early spring. Growth is rapid during this time. The males remain small, about ⅛ inch, and they soon turn a translucent white. Males emerge as tiny, pink to yellow gnatlike insects with two long, waxy threads extending from the tip of the abdomen, which may aid in flight stabilization. At maturity, these fragile males do not feed; they simply mate, then die. The females continue to expand as they feed, and by early June they have turned a brownish-purple color. By early summer, the yellowish-brown females are covered with a powdery, white waxy coating. By late July and August, the adult females begin to give birth to their young, known as crawlers. The tiny, mobile crawlers move around until they find a suitable feeding site on which they settle down, feed and remain throughout the winter.
|Magnolia scale nymphs.||Adult male among nymphs.|
Though there are several predators and parasites known to attack this scale, they are rarely effective at suppressing populations, especially on smaller magnolias.
Strategy 1: Pest-Free Plants
Most of the magnolia scale infestations are already present on the tree, so careful inspection of the branches is imperative upon purchase. The large scale exoskeletons often remain from the previous season. Any plants with these remains should be avoided.
Strategy 2: Summer and Dormant Oils
Horticultural oils, often called summer oils, at the label rate applied after the crawlers have settled in late August can be very effective in reducing the scale population. Be sure to thoroughly wet down the stems and leaves. Dormant oils can be applied in October through November and again in March to kill the overwintering nymphs located on the stems. Be sure to check the spring buds, as some damage may be caused on the flower buds if they have begun to swell.
Strategy 3: Standard Chemical Control
Magnolia scale can be satisfactorily controlled with a variety of insecticides if applied when the insects are in the freshly settled crawler stage. This is usually in late August to early September. Sprays applied before the crawlers are present or after they have become dormant in the overwintering stage will have little effect. When applied in early May, soil drenches with systemic insecticides have provided excellent control. Always read and follow label instructions.