Garden Detective

What is attacking beloved lemon tree?

The gold-spotted oak borer poses a threat to California’s oak trees. The pest, prevalent in San Diego County, is moving north up the Golden State.
The gold-spotted oak borer poses a threat to California’s oak trees. The pest, prevalent in San Diego County, is moving north up the Golden State.

Q: An old Eureka lemon tree is showing on the trunk over an 18-inch area a foot or so above the ground gobs of what looks like sap that has oozed out. I was told by a neighbor that this was evidence of a tree borer. I went online to learn that one scrapes off the gooey stuff to find a hole, and with a hypodermic needle, one injects kerosene. I did this, but substituted alcohol. What is your opinion? All the neighbors love the tree because it supplies copious amounts of lemons to everyone, so I’d sure hate to lose it. I might add that the tree was heavily damaged in the deep freeze in the late 1980s but seems to be holding up just fine despite that.

Don Gibbs, Davis

According to UC master gardener Carol Rogala, holes in the bark, stains or oozing liquid on trunks and/or limbs and sawdust-like powder in bark crevices are common damage symptoms caused by borers.

A number of different types of insects may bore into tree trunks and branches in their larval stages, producing sawdust or sap-filled holes and weakening trees.

UC Cooperative Extension research does not support the use of either kerosene or alcohol injected into tree bark for management of borers.

Most borers can successfully attack only trees that have been stressed by under- or over-irrigation, disease, lack of proper care, or injury by mechanical equipment. Usually by the time the tree is infested with borers, there is little you can do to manage them other than improve tree vigor, prune out infested branches or remove the tree.

Insecticides are occasionally used to prevent infestations of bark beetles on high-value trees or to manage certain clearwing moths.

Effective management practices vary according to species. Confirmation of species requires finding the insect, although knowing symptoms and host plant species can help.

Many tiny holes in tree trunks and branches may indicate bark beetles; larger open tunnels filled with sawdust-like frass indicate clearwing moths; and flatheaded or roundheaded borers leave wet spots and dark stains and D- or 0-shaped emergence holes.

Prevention is the most effective method of management, and in many instances the only form of control. Protect tree roots and trunks from injury, as well as protect trees from sun burn or sun scald and other disorders.

Additional information on bark beetles, borers and clearwing moths can be found at


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