As the summer comes to a close, acorns are maturing around the valley. It is no coincidence that people have been asking about the sticky mess under the canopies of their native oak trees, especially coast live oaks. It happens every year. Large droplets of sticky sap accumulate on patios, outdoor furniture, and whatever happens to be under oak tree branches. In most cases, it is a result of an infection in the acorns.

There are many causes for fluids oozing from oaks, but they have something in common: wounds. In response to wounds and infections, trees exude fluids laden with protective phytochemicals. Some of the organisms involved may include Phytophthoras including P. ramorum (Sudden Oak Death disease), foamy canker, wetwood infections, various sucking insects and borers.

None of those are responsible for the mess I’m talking about here. The clusters of large droplets are the typical sign of “drippy nut disease.” Seriously. That is straight from Oaks in the Urban Landscape: Selection, Care, and Preservation (University of California, Publication 3518)

As the acorns begin to grow, various insects, including three species of Circulio filbert weevils, the filbert worm and possibly other insects, use them as a source of food for their offspring.

The filbert weevil chews a hole in the acorn and deposits eggs inside. Grubs hatch from the eggs and feed on the carbohydrate and oil-rich endosperm.

The filbert worm is the larva of a small, rusty brown moth (Cyadia latiferreana or Melisopus latiferreanus, depending on the reference.) The adult female lays eggs on the surface of the acorn. After they hatch, the larvae chew into the acorn to get at the food source.

Whatever the cause of the wound, the acorn becomes infected with a bacterium, Erwinia quercina. The infection is limited to the acorns. It does not infect the tree.

The result can be an 80% reduction in the number of viable acorns but there is no direct harm to the tree.

The problem has been persistent and seems to be increasing. For years, I have been telling people there is no proven prevention. And I have become concerned that we are losing too many acorns.

References (“USDA Field Guide to Insects and Pathogens of California Oaks” and U.C. IPM Online) offer no insecticide treatment to cure or prevent the disease, but checking with Hit Men, a well-known local pest control company, I discovered they have developed a program that is proving to be effective. It was researched and implemented by manager Rick Coffman.

In a phone conversation, Coffman said they have been achieving 75 to 100% control of the problem for most of their customers since they began the treatment program over three years ago. After researching the life cycle of the filbert weevil, Coffman devised a way to break its life cycle using a proprietary soil treatment. The weevil over-winters in the soil and the treatment kills them before they infest the young acorns in spring.

Their customers have noted some effectiveness in the first year and increasing effectiveness in the second and third years. The treatment is applied in February to March. Heavy spring rainfall interferes with effectiveness and paved root zones prevent effective treatment Coffman said. To help with effectiveness, he advises cleaning up infested acorns in the Fall.

As to the mess this year, the residue can be washed away (repeatedly) before it dries and outdoor furniture can be covered with tarps until the dripping stops when the acorns fall.

Handpicking the infected acorns is not a practical solution, but pruning can help. Remove or reduce the number of branches growing over surfaces to be protected, but beware of disfiguring a tree for the sake of avoiding a seasonal mess.

As to the new treatment, keep in mind it is preventative. If you are interested, plan ahead.

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Bill Pramuk is a registered consulting arborist. Visit his website billpramuk.com, email questions to info@billpramuk.com or call him at 707-363-0114.