The Purdue Plant and Pest Diagnostic Laboratory

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May 23, 2012

Sap-Sucking Bugs Make a Mess of Indiana’s State Tree

Cliff Sadof, Department of Entomology, Purdue University

Across Indiana this week, homeowners with tulip poplar trees could be finding their cars and landscape plants covered in a mysterious sticky film. This time of year tulip poplars are easily recognized by leaves whose shape resembles a tulip and the two inch long lime green and orange flowers in the tree canopy.

The culprit in this sticky mess could be the tulip tree scale insect, a small sap-sucking bug that feeds on the trees’ branches.  Declared as the state tree of Indiana in 1931, tulip poplars are well-regarded and widely planted for their beautiful flowers and form, shade, and rapid growth.  The tree’s sap carries vital nutrients, and feeding by the insect can cause damage to the trees in addition to the unsightly mess.

In most years, native parasites and predators keep this insect from becoming a pest. For some unknown reason, this year the scale population has reached record numbers on tulip and magnolia trees.  As the bugs feed on the branches, they excrete a sticky waste product called “honeydew,” which rains down out of the infested tree canopy and onto objects below.  The honeydew is a favorite food of other insects such as ants and wasps, as well as a fungus called sooty mold.  Sooty mold growing on the honeydew often gives vegetation under infested tulip trees a black moldy appearance, but is usually an aesthetic problem. This year, the prolonged drought and heavy infestation may act together to kill some trees. Also, heavy accumulations of honeydew at the base of trees have been causing dangerously slick driving conditions on some roads.

Heavy tulip tree scale infestations can cause yellowing and drop of leaves, branch death, or whole tree mortality in smaller trees. Homeowners with tuliptrees can recognize tuliptree scale on sticky trees by looking mottled orange bumps about 3/8” long left by last year’s female scales. This year’s scales are black are only about ¼” long at this point. They have yet to mate and plump up with this year’s eggs.   

To save trees homeowners should water trees during the drought.  This will help trees survive the infestation. Certain insecticide treatments can help stop the flow of honeydew, by killing the scale insects. A soil-applied systemic insecticide, such as imidacloprid, applied with a watering can within one foot of the tree trunk can provide some control if applied now.  However, it won’t work if applied to bone dry soils.  The soil will need to be irrigated before applying the product. It will take at least 3 weeks to get up into the tree canopy by which time the flowers will have fallen off the trees… diminishing the impact on bees.  Although foliar sprays can be effective most homeowners lack the equipment they need to reach the tree tops. These products are best applied by a tree care professional.

Professionals have the capacity to kill the scales more quickly by using a variety of tree injection techniques to deliver imidacloprid, dinotefuran, or emamectin benzoate. This strategy could be useful in situations where traffic safety is an issue.

The following resource provides more information on control options and some background on scale insects: E-29-W Scale Insects on Shade Trees and Shrubs (pdf file).

Youtube video for homeowners on how to apply imidacloprid,

And what to expect when a professional comes to inject your trees.


HORT ALERT: 05-23-2012 White bugs (pdf file) - Larry Caplan, Extension Educator, Vanderburgh County

Click image to enlarge

Tuliptree scale

Tuliptree scale

Tuliptree scale

Tuliptree scale

Tuliptree sooty mold

Tuliptree sooty mold

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service