Honeydew gatherers

By Jeff Burbrink
Extension Educator, Purdue Extension Elkhart County

GOSHEN — Have you ever parked your car under a tree to keep it cool on a hot summer day, only to have a lot of tiny drips of sap all over the car? If so, you are in good company.

That sticky stuff is called honeydew and it is secreted by several types of insects that feed on tree leaves and stems. The aphids and scales creating that sticky mess suck the juices from the tree in a manner similar to the way a mosquito takes a sample of your blood. The excess juice passes through the insect’s body, so in a way, that sticky stuff is insect poop. You’ll never look at those drips in the same way.

The tree species I hear the most complaints about is tulip tree, but maples, hawthorn, walnut, pines, spruce, elm, oaks, honey locust and linden all get votes for the stickiest trees in northern Indiana.

What makes the situation even more irritating is that the sap is high in sugar. The sugar makes a great food for several types of molds. The molds are typically black in color. Anything under the tree that is dripped upon can eventually turn black from the sooty mold. If thick enough, leaves and flowers of plants covered with the sap and sooty mold can die. Swing sets, lawn furniture, decks, air conditioners, rocks, window sills, sidewalks, virtually anything outdoors, can be blackened by the moldy growth on the honeydew.

What to do about it?  For shorter trees and shrubs, a strong blast of water can be enough to knock the aphid population back for a short time, and it may wash away some of the honeydew and sooty mold, but it will not phase the scale insects, which are firmly attached to small limbs and twigs.

Power washers and soap can scrub picnic tables and other solid objects that have honeydew and sooty mold built up on them, but take care not to power wash tree bark or other plant parts. Power washers have been known to blast bark from trees, doing far more damage than the honeydew itself.

Insecticides sprayed on the trees may work on the aphids and scales, but only when the scales are in their “crawler” or juvenile stage of growth. The mature scales develop a waxy coating on their back, which repels most of the water carrying the insecticide. Most of us do not own the equipment needed to spray high into a tree canopy, so hiring the job out is probably the only good way to treat most tall trees.

There are also some systemic products you can use on trees for some control of aphids and scales. The most accessible product for homeowners is imidacloprid. It is used as a soil drench around the base of trees. The dose is based on the diameter of the tree. Once the tree exceeds 20 inches diameter, it does not work as well as the professional treatments. It is also important for you to pull organic matter like turf grass or mulch away from the base of the tree before doing a drench because the organic matter will neutralize some of the effect of the insecticide.

Professional tree care people have access to other products that may be more effective with larger trees. Some of these are sprays and drenches, and some are injectables, administered through tiny holes drilled into the tree. These professional treatments may give people sticker shot, but they are more reliable than the homeowner products.