Home & Garden

Oakworms are a nuisance but not deadly to trees

Q. Won’t my oak trees die if caterpillars eat most of their leaves? — Lynda H., Paso Robles

A. Some years their numbers seem to equal those of a plague of locusts; in other years, they’re hardly noticeable.

Either way, the feeding habits of the California oakworm often evoke concern, or even downright alarm, among homeowners who are fortunate enough to have stately oaks in their landscape. Oakworms can completely defoliate the oak trees in an entire neighborhood when numbers are of biblical proportions.

So what should you do if your oak trees fall victim to vicious predation? Usually it’s best to just grin and bear it. The oakworm, the larva of the oak moth, is a pest that is part of a natural life cycle that includes our beloved oak trees and the California oak moth. (For more detailed information about the California oakworm visit: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7422.html.)

Most years the oakworm, which is actually a caterpillar, feeds only on oak leaves. As it would not be prudent for a pest to completely destroy its only food source, healthy oak trees will survive even the most dramatic defoliation.

Keeping your oak trees hale and hearty is key to their surviving an oakworm infestation.

Oaks can be adversely affected by many factors, but the leading stressor is improper care of their sensitive root system.

You can avoid this by keeping a few things in mind. First, avoid overwatering. Once established, under nor mal weather conditions, native oak trees should only receive water from annual rainfall. During times of severe drought, however, it is necessary to provide some water during winter and spring, but only in amounts that simulate normal rainfall. Also, protect the area under and around oak trees during construction or landscape projects, and avoid altering or disturbing the natural terrain in those areas. Soil impaction caused by heavy foot traffic or machinery, grade changes, trenching, and paving can all cause stress that weakens your trees.

Remember, a healthy tree is the best defense against an army of oakworms. The University of California Agriculture and Natural Resources (UCANR) Department offers a free guide with valuable information about caring for oak trees at: http://anrcatalog.ucdavis.edu/pdf/21538.pdf.

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