Courtesy Meredith Seaver, USU Extension

We just moved to Spanish Fork from Colorado Springs. The tree that we have in front of our home, which is on the south side, has a drippy sap coming from it. It looks like corn syrup with a gooey sugar-type substance along the branches of the tree. I am not sure what variety this tree is, but it really doesn’t have a pretty shape at all. It almost looks like something that would grow wild along a riverbank. It almost looks like an insect or disease has taken it over.

Your tree looks like an elm, possibly one of the new hybrids. Unfortunately, this tree is heavily infested with what looks like European elm scale. I also see some minor elm leafminer damage on the leaves.

The leaf damage from elm leafminers (the larvae of a sawfly) is a cosmetic issue as leafminer feeding doesn’t affect the health of the tree. The scale infestation, however, is a serious problem for the tree.

Elm scales are small, soft-bodied insects that feed on the nutrient-rich sap just under the bark and on the leaves of elm trees. They are common on elm trees but can occasionally be found on Zelkova trees, which are related to elms. These insects are responsible for the sticky, dripping honeydew you’ve described. The scale insects use their straw-like mouthparts to pierce the bark and remove the sap. Once these insects begin feeding, they remain attached to the tree for the rest of their lives. Their feeding can reduce the vigor of a tree and cause leaf drop and even branch dieback. This may account for the uneven appearance of your tree, and a little pruning attention from a certified arborist could improve things.

Elm scale can be difficult to control because females can lay hundreds of eggs each season. They also have some protection from insecticidal sprays because of the waxy coating on their bodies. That’s not to say the situation is hopeless, though, because there are a couple of times during the year when control can be successful.

Courtesy Meredith Seaver, USU Extension

Because elm scale are sap-feeders, they are susceptible to systemic insecticidal soil drenches in the spring when leaves are expanding. Tree and shrub insect control products with the active ingredient Imidacloprid are available to homeowners and effective when applied properly. This soil drench can be helpful in controlling elm leafminer, too.

Applying a 2% dormant oil spray before bud break in the spring can smother overwintering scale. Making this a yearly control practice can help your tree avoid serious infestations.

It’s too late for either the systemic soil drench or the dormant oil spray, but there is a control window opening now. Egg hatch is beginning now in some parts of Utah County, and it will last for about four weeks. The hatchlings, called crawlers, are more susceptible than adult scales to oils, insecticidal soap and insecticides. Crawlers will be out moving on the tree to find feeding sites on young twigs and the undersides of leaves where they will spend the summer.

If you want to avoid harsh insecticides, insecticidal soap or 1% horticultural oil applied about every five days during the crawler season will be effective. Make sure your tree isn’t drought-stressed and do the applications when temperatures will be below 85 degrees. A product with the active ingredient azadirachtin is another soft control option.

Products with the active ingredient gamma-cyhalothrin, lambda-cyhalothrin or permethrin are conventional insecticides that are effective for control during the crawler stage. You’ll need to make two applications spaced two weeks apart if you use these conventional insecticides.

Healthy, vigorous plants are always more resistant to pest and disease problems. Give your tree good care with deep, infrequent watering (only once a week during the summer and less often in the spring and fall) and frequent inspections to catch any other pest or disease issues early. Most ornamental trees don’t need regular fertilizer applications, but if your tree has lost some vigor, an early spring application of nitrogen may be helpful.