Fall Care for Perennials
by Pam Wallace, UC Master Gardener

Live Oak Woolly Leaf Galls
Wherever oak trees occur, you'll find a group of small insects called gall makers living with them. The majority of gall makers are tiny wasps, but in some cases flies are responsible. These insects cause deformities, known as galls, of various shapes, sizes, and colors on leaves, twigs, bark, flowers, buds, acorns, and even the roots of oak trees.

Each gall maker creates its own distinctive gall using chemicals from its body and from the tree. The Live Oak Woolly Leaf Galls are unusual. They look like tan, furry balls growing on the back side of a green leaf. They are soft and fuzzy to the touch. It is hard to imagine that a little insect lives inside. The majority of oak leaf galls do little or no harm to their host tree. They are just living inside their little galls minding their own business. Take an autumn walk in Live Oak Park in Tulare and you'll see lots of the woolly leaf galls.

Asian Woolly Hackberry Aphid Takes Residence in California
The Asian Woolly Hackberry Aphid was discovered in California for the first time this summer. In August several cities in the Valley started to complain about the sticky mess under their hackberry trees. Closer inspection revealed that a small aphid, about ¼-inch long and covered with white, fuzzy wax was secreting honeydew as thick as maple syrup. The aphid originated in Asia but it has been in Florida for several years. This aphid is easy to see and infests both upper and lower leaf surfaces. Like many other aphids the females give birth to living young all summer long, so populations can build to very high levels by late summer.

While the honeydew is a great nuisance, control is probably not needed to protect the long-term health or survival of otherwise healthy trees. This is based on experience with other woolly aphid pests of trees, like the woolly ash (tree) aphid, and on reports from the Southeast where they've experienced the Asian woolly hackberry aphid.

Aphid control in other locations: In response to complaints, the City of Modesto is spraying trees with insecticidal soap. This is more to wash the leaves and remove honeydew than to control the aphids. And they are advising citizens that they will probably return with Merit soil treatments in late winter.

The City of San Mateo says that insecticidal soap spraying on their downtown hackberry appears to provide control and is satisfying the businesses there.

Limit foliar spraying to low-toxicity materials such as insecticidal soap or oil. Even though these materials are not as effective as harsher insecticides, they are safer in the environment. Even insecticides won't work as well as they should because it is too hard to get thorough coverage of waxy-covered insects on the underside of the leaves in a mature hackberry tree. Spray drift becomes more of a hazard than killing aphids.

Don't inject or implant anything into rows of hackberry trees. Chinese hackberry trees near Sacramento are susceptible to an undefined tree killing malady. The suspected cause is a vascular wilt pathogen, which may be mechanically spread by unsterile tools that contact internal parts of multiple hackberry trees. It would be unfortunate if implants or injections to provide short-term control of a nuisance aphid honeydew problem ended up killing trees. A single tree in a backyard would probably respond to a tree injection, but it would take several weeks to be effective.

A soil application of systemic insecticide (imidacloprid, sold as Merit) might be effective, but this is based on information from other situations. Late winter to early spring (around the time of flush of new leaves) is believed to be the most effective soil treatment time.

For now, consider tolerating the problem. I inspected several young trees this summer and noticed beneficial lacewing eggs all over the infested trees. When a lacewing egg hatches, the larva feeds on the aphid. By fall the woolly aphid populations had declined dramatically.

Hackberry trees are losing leaves now. If you plan to take action, consider treating in late winter or early spring as suggested above. From Florida where this pest has occurred for several years, heavy infestations of aphids are nothing more than a nuisance, causing little if any damage to trees.

November 14, 2002