Magnolia Scale

August 12, 2010 2 Comments

I would love to have a Magnolia tree in our yard, but we don’t have any place to put it on our slightly-less-than-a-quarter-acre-lot-that-already-has-seven-trees-on-it. 

For those fortunate enough to have a Magnolia tree, you may be dealing with an unfortunate aspect of that ownership.  Magnolia scale.  The Magnolia scale is the largest scale insect in Iowa. 

Adult females may reach nearly 1/2 inch in diameter when they are fully grown.  It is a shiny tannish brown and smooth.  As scales grow they are often covered with a white, mealy wax that is lost when the crawlers emerge.   The crawlers are the babies.

Magnolia scales have sucking mouthparts and feed on the sap of the tree, which stresses a heavily infested plant and can result in stunted growth, yellowish foliage, branch dieback or even the death of the plant.  Scales have also fed on Daphne and Virginia Creeper.  Magnolia scales produce large quantities of honeydew (which is a sugary excretion) that build up on the tree’s leaves and twigs.  Sooty mold is a black fungus that grows on that honeydew and turns the covered leaves and twigs black.  The honeydew also attracts ants, bees, wasps and flies.

The honeydew formed by the Magnolia scale.

The sooty mold that forms on the honeydew along with the adult females and the crawlers (babies).

From Ohio State University Extension, “the magnolia scale spends the winter on one to two year old twigs as tiny, dark-colored nymphs.” In the spring, with warmer temperatures, the scales start to suck the sap and molt once in May. This is the point where the males remain small (only about 1/8 of an inch) while the females get to their larger size.  (read more here).

If you are looking at adding a magnolia tree to your yard, check the new plant out carefully before you buy it.  Most of the scale infestations come with the plants with the large scale exoskeletons being left from the previous season.  If you find any of these remains, don’t buy the tree.

In order to control the scale, remove and destroy any heavily-infested branches and then treat with a contact insecticide which includes horticultural oil, insecticidal soap or other ornamental synthetic insecticide.  Or use a systemic insecticide (imidacloprid) in late August or early September.  A dormant horticultural oil can also be used in late fall or early spring when the tree is, obviously, dormant. 

-Horticultural oils at 1.5 to 2.0″ can be applied after the crawlers have settled into their feeding site in late August.  This can be very effective in reducing the scale population.  Make sure you thoroughly wet the stems and leaves. 

-Dormant oils can be applied in October to November and again in March to kill the nymphs that have overwintered on the stems.  Be sure to check spring buds for damage as they begin to swell. 

-Insecticide application can also be successful if they are applied when the insects are in their fresh crawler stage.. usually late August or early September.  If you apply the spray before crawlers appear or after they become dormant, it will have little effect on controlling the infestation.

Check with your local Extension office or favorite garden center for specifics on the oils or insecticides. 

At least this sounds a bit more manageable than those darn Japanese Beetles!

Dig it!

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  1. Terri says:

    Ilene, What is wrong with my hosta when they start getting lacey, holes in them. they start off every year so full and big and beautiful then all of a sudden they get holes in them. What kind of bugs do I have and can crickets do this to them?

    • Eileen Loan says:

      TT, chances are you have slugs. They love hostas and it has been moist enough to keep them from drying out. I talked about them in a post from June. You can find it here ( ) and maybe it will give you some pointers. Robins also poke holes in some flower plants, but I’m not sure if they are just being mean or going after the slugs and other critters crawling around on the leaves. I’m not sure that you can stop the robins since you need some insects for a healthy garden. Good luck!

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