By: Sandy Feather ©2009
Penn State Extension
Q. We have had a magnolia tree near our front door for over 20 years. Last summer, it started to drip sap all over the place, and there appeared to be some kind of mold on it. There seemed to be a lot of bees hanging around it, too. This has been such a lovely tree for us that I would hate to lose it. Do you have any idea what is going on?
A. It sounds as though your tree is infested with magnolia scale. It is a common pest of saucer magnolia (Magnolia soulangiana), star magnolia (Magnolia stellata), and cucumbertree magnolia (Magnolia acuminata). You may not have noticed the insects on the smaller twigs – they look like little white bumps. It is understandable that gardeners have a hard time believing these “bumps” are living insects since they do not move.
Once the maturing scale insect inserts its mouthparts into a twig or small branch, it spends its entire life in the same place. Magnolia scale (Neolecanium cornuparvum) is the largest scale insect in the United States.
The tree is not dripping sap, but the sticky substance you took for sap is known as honeydew, a polite term for the excrement produced by the insects feeding on your tree. The bees (and probably flies and ants) are there to consume this sugary substance.
Adult scale shells lined-up on this Magnolia branch, notice the black sooty mold from the honeydew.
Magnolia scale resides on tree year-round, but goes unnoticed until it starts producing large amounts of honeydew. Honeydew production is direct result of this insect’s sugary diet. Scale insects feed with piercing-sucking mouthparts. In the case of soft scale insects, such as magnolia scale, they insert their mouthparts into the portion of a plants’ vascular system responsible for transporting the carbohydrates and sugars produced by photosynthesis.
Honeydew can be a real annoyance when it drips on the lower leaves and anything unfortunate enough to be under the magnolia. It also attracts nuisance insects such as yellow jackets, ants and flies. To make matters worse, a black sooty mold fungus often grows on the honeydew. Although it is unattractive, sooty mold does no real damage to the tree – it is simply growing on the carbohydrate-rich honeydew.
It is helpful to understand this pests’ life cycle in order to control it effectively. Magnolia scale warrants control, not only because of the mess it creates, but also because it can cause severely infested branches to die. They are usually present in large numbers, and it does not take long for a sizeable population of magnolia scale to build up. Adults are covered with a waxy covering that makes them quite impervious to insecticide applications, so you have a narrow window of time when the young nymphs (immature insects) are susceptible. Newly hatched magnolia scale nymphs are also known as crawlers because it is the only point in their life cycle when they are mobile. There is one generation of magnolia scale a year in our climate.
Scale crawlers hatching out on a heavily infested Magnolia branch
Controlling magnolia scale is a two-step process. First, apply a horticultural oil spray just before the tree leafs out in spring to kill the overwintering nymphs. They are quite small and difficult to see, but they are present and appear as small, gray dots on smaller twigs. They are visible to naked eye when you know what you are looking for –you may find it helpful to look with a magnifying glass. Horticultural oil suffocates the pests, rather than poisoning them, and will greatly reduce the number of young produced in late summer. Horticultural oil is more refined and lighter than dormant oil (Volck oil), and there is less chance of burning tender new foliage if you slip up and spray as the tree begins leafing out in the spring.
The timing for the second step is critical because the newly hatched crawlers are very susceptible to insecticide applications, including environmentally-friendly products such as insecticidal soap. Magnolia scale is in its crawler stage in late August and early September in our area. Of course, insects do not live by the calendars that govern our lives; their development is based on temperature and moisture. It is always best to check and make sure you see active crawlers before applying insecticide sprays to control them. The crawlers are quite small, but can be seen upon careful inspection. A hand lens or magnifying glass will make them easier to see. They will appear as small, reddish specks moving around acitvely. Begin checking your magnolia in late July. Concentrate your scouting on last year’s and the current year’s growth. Make your first insecticide application when you first see the small crawlers moving around.
Beautiful Saucer Magnolia! Saucer Magnolia blossoms against a bright blue sky
Sevin (carbaryl), Bayer Advanced Power Force Multi-Insect Killer (cyfluthrin), horticultural oil, Bayer Advanced Garden Tree & Shrub Insect Control (imidacloprid), insecticidal soap, BioNeem (neem), and malathion are all registered for control of magnolia scale crawlers. Depending on the insecticide, you may need to make repeat applications at the intervals recommended on the product label.
Bayer Advanced Tree and Shrub Insect Control has shown to be very effective in controlling soft scale insects, including magnolia scale. This product is applied as a soil drench, rather than sprayed onto the tree. It is absorbed by the roots and moves through the tree’s vascular system. It is best applied in early spring so that it has time to move into the tree canopy where the insects are feeding. Be sure pull back any mulch and apply the product to bare ground within 18 inches of the trunk. Water it in after application and replace the mulch. One application a year is all that is necessary with this product.