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Oak worms: Advice for dealing with the pest

While oak worms can cause serious defoliation of native coast live oak trees, the leaves typically grow back.
While oak worms can cause serious defoliation of native coast live oak trees, the leaves typically grow back.

A third dry year on the Central Coast may bring an abundance of oak worm activity to our area. Not to worry. Our Master Gardener volunteers receive many calls regarding this subject and offer the following advice on dealing with them.

Although the worms can cause partial or even complete defoliation, new leaves will grow back. Some defoliation may actually benefit trees during low-rain years. Fewer leaves means less water loss through transpiration. Oak trees and oak worms are both native to California’s oak woodlands and have learned to live together. While the worms are capable of eating themselves out of house and home, Mother Nature typically prevents such things from happening. Therefore, a large outbreak is not likely to occur year after year, allowing trees time to recover.

The caterpillars feed from about April to September, and this year could see a large population of worms. The relatively dry winter followed by an early spring has resulted in a high survival rate of overwintering eggs.

This summer and fall could bring a large infestation. The good news is that the population of natural predators increases as their food source increases. The most important predators are parasitic wasps, the spined soldier bug and two tachinid parasitic flies.

There is no practical way to completely eradicate the worms. Broad-spectrum pesticides would not only knock down the worms but their beneficial predators as well. Also, because of the size of most oaks, it’s difficult and expensive to treat one tree, let alone the entire oak woodland. Therefore, chemical treatments are not recommended.

If you’re worried about the rest of your yard, the caterpillars usually eat only coast live oak leaves and will rarely feed on other plants. During an especially heavy outbreak, they may feed on non-oak species, but only temporarily. The worms may also fall onto or into your home; however, they won’t cause any damage, even though stepping on them makes a gooey mess!

For more information, visit Current Topics of Interest on the Master Gardener website: http://ucanr.edu/sites/mgslo.

Or, call your UCCE Master Gardener Helpline in Templeton at 781-5939; San Luis Obispo, 434-4105; or Arroyo Grande, 473-7190.