Chinese hackberry

Chinese hackberry leaves with sticky honeydew and Asian wooly hackberry aphids.

Bill Pramuk photo

In my previous column, I discussed the recent increase in limb failures in trees of certain species that responded to last winter’s heavy rainfall by producing heavy loads of new growth.

Here are two examples of a different kind of result:

1. The route of our daily, early morning walks follows neighborhood sidewalks, passing under numerous trees. Some of these are Chinese hackberry trees (Celtis sinensis) a sturdy deciduous shade tree resembling its close relatives, the elms.

During the recent years of drought, I had often wondered why the neighborhood hackberries were so clean. That is, they showed no sign of the notorious pest, the Asian wooly hackberry aphid (Shivaphis celti). It is a sucking insect that became a rampant pest from Florida to Texas and north to Tennessee after its introduction to the U.S. in the 1990s. It appeared in California in 2002, causing a widespread mess in most areas where hackberries are grown, including Napa.

As the aphids suck sap from the leaves, they excrete the excess “honeydew,” which drips onto surfaces under the tree canopies.

This summer, the hackberry canopies are lush with vigorous growth, the sidewalks under the trees are dark and sticky with honeydew and the leaves show the typical signs of the wooly aphis. The correlation: Plenty of sap in vigorous new shoots feeds and allows the aphid population to increase.

The University of California Pest Notes (#74111, June 2005) advises: “Avoid fertilizing hackberry unless nutrient deficiency has been diagnosed. Excess nitrogen has been shown to increase aphid numbers on certain plant species.”

I am pretty sure the trees, growing in curb strips, did not receive any fertilizer, but they did receive a big boost from the generous rainfall.

2. On a property I have visited regularly for a few years, there are a couple of unusual elms I tentatively identified as rock elms (Ulmus thomasii). I have been pleased to see that they have not shown signs of wilting and dieback caused by Dutch Elm Disease, nor the skeletonized foliage resulting from Elm Leaf Beetle infestation. Both problems are common in the Napa area.

This spring and summer, the trees have shown phenomenally lush growth. Like the hackberries, they have not received fertilizer. Abundant rainfall last winter and spring is the most likely cause of the lush growth.

And, like the hackberries, these trees began to show shiny, sticky honeydew on the leaves and on the asphalt driveway under the canopies.

A close look at the new shoots revealed a heavy infestation of scale, a different kind of sucking insect, (species not determined) on the stems.

The trees look as healthy as can be, and the owners prefer to avoid the use of insecticides as much as possible. Although many tree diseases have insect vectors, as far as I have been able to determine, scale insects do not vector elm tree diseases. So, in this instance, I see no urgent need for intervention.

Going back to the hackberry wooly aphids, University of California IPM

discusses management of this pest in detail.

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Regarding damage, it says: “This aphid is a pest because it produces copious honeydew excretions upon which blackish sooty mold grows creating a sticky mess on leaves and surfaces beneath infested trees. However, no long-term or serious damage to hackberry trees has been found after years of aphid infestations. Insecticides apparently are not warranted to protect the health or survival of infested trees but are applied when honeydew excretions are intolerable to people.”

Where the sticky mess is intolerable, UC offers the following options and caveats:

— Biological control with insect predators such as lady beetles and lacewing larvae: In many areas in California, these do not provide adequate control.

— Non-residual contact insecticides such as spray oils, soaps, pyrethrins: Follow label directions. Take steps to conserve natural insect predators and avoid unintended consequences of insecticide over-spray and runoff.

— Dormant oil spray before leaves emerge in spring: Unlikely to give complete control and will not kill aphids that fly in afterward.

— Systemic insecticides: If properly applied, these can provide an entire growing season of control. Systemics can get into other plants and poison beneficial insects.

— Injection: Avoid this on hackberry. It can spread a certain, unexplained disease.

— Residual sprays: Persistent insecticides such as carbamates, organophosphates, and pyrethroids are not recommended for aphid control in landscapes. They are toxic to beneficial insects, may cause outbreaks of spider mites, and may runoff into waterways where they kill aquatic organisms.

Bill Pramuk is a registered consulting arborist. Visit his website, Email questions to or call him at 707-226-2884.