Magnolias are one of the most popular ornamental trees across the U.S. Known for their spectacular flowering plants, they come in a variety of shapes and sizes, from small shrubs to medium-sized shade trees.
For the most part, these trees are pretty safe choices for Illinois gardens: they adapt well to the climate, flower profusely every year, and tolerate a fair range of conditions.
However, in recent years, we have noticed an increase in reports of one of the major pests in Illinois: the magnolia scale insect. The magnolia scale insect (Neolecanium cornuparvum) affects nearly all magnolias. Although invasive magnolias and hybrids tend to be susceptible, it has also been reported in native species.
Small infestations may not have much impact on the health of your tree, but in large populations, these pests can seriously affect the health of your tree and even cause death. Because the pests feed on sap, the more sap they consume, the less energy your magnolia tree has to survive.
Magnolia scale insects are a type of soft scale insect that develop an exoskeleton or armor when they reach adulthood, but lack the hard, protective cuticle that hard scale insects produce. Instead, they have a soft, almost fuzzy cuticle made from a waxy secretion that serves the same purpose as the cuticle of hard scale insects in protecting the insect from predation. However, these pests are often not recognized as scale insects because of their fuzzy appearance.
Scale insects are unique in that, as adults, they are sedentary, whereas their juvenile stages are quite mobile. They are born in August and September as tiny, six-legged crawlies, and, as their name suggests, they crawl along twigs in search of food. When they find a suitable spot, they settle there to spend the winter.
In spring, the larvae become active again and move around to feed. As spring progresses, the larvae settle in one location where they grow and mature into adults. Males eventually emerge as gnat-like insects, mate, and die. Females turn from white to a brown-purple color, begin to produce waxy hairs, and continue to grow and enlarge by this time of year.
The females give birth to larvae in late summer or early fall, and the life cycle continues, but adult females die after giving birth, so the population is sustained by overwintering larvae.
There are many ways to control magnolia scale insects, but effective control requires understanding their life stages. The easiest way to control small populations on small trees is by hand removal in June and July when the females settle in to feed and create protective cover. The insects can be scraped off the bark with a fingernail or a small scraper, causing minimal damage to the bark.
If the tree has a large crown, you can blast away a significant number of adult insects with a hose. Care must be taken not to break up the bark, so don't use a pressure washer; however, most garden hose nozzles on a more concentrated setting will effectively remove scale insects. This method may need to be repeated for several years to reduce the scale insect population, but it is one of the easiest and cheapest methods of control.
The bugs can be killed with insecticidal soap or horticultural oil when they emerge in August or September, or effectively controlled with a pre-budbreak application of horticultural oil in early spring, but these methods are contact treatments that require thorough coverage of all twigs and trunks, may require specialized sprayers, and are best left to professional tree care professionals.
As a last resort, a systemic insecticide can be injected into the tree a few weeks before the caterpillars become active in the spring. Timing will vary depending on spring weather, but applications are usually made in April to allow the plant plenty of time to absorb it before spring caterpillar activity. Again, this treatment is best left to the professionals.
In many cases, scale insect infestations can be managed with mechanical removal if caught early, so be sure to inspect your magnolia trees regularly, especially this time of year when the females mature and become easier to spot. Promoting tree health by mulching to the drip line and watering during hot periods are also effective strategies for both preventing and treating infestations.
Ryan Pankau is a horticulture educator for UI Extension serving Champaign, Ford, Iroquois and Vermilion counties. This column also appears on his “Garden Scoop” blog at go.illinois.edu/gardenscoopblog.
