Did your oak tree lose its leaves early this year? Did they just seem to be eaten away? They well may have been, by one or both of two common insect pests of oaks: the California tent caterpillar (Malacosoma spp.) and the California oakworm or oakmoth (Phryganidia californica).

Statewide, there were many infestations of these pests this year. Both pass through 4 forms: egg, larvae, pupae, and adult. The larvae, or caterpillar, is responsible for the damage to oak trees.

The young oakworm caterpillar is hairless and yellowish-green with dark stripes and has a head that appears too big for its body. Mature caterpillars are mostly black with prominent stripes. With warming temperatures and the new flush of leaves in spring, the tiny caterpillars begin feeding in earnest on the new foliage. You may see hundreds eating leaves, hanging on fine silk threads from oak trees, moving on the ground, even on your house! After about 6 weeks, the caterpillars become adults, a brown moth. The adults do not eat but live only to mate and lay eggs. This whole process, egg to adult, occurs two or three times per year. Depending on the duration of warm weather, oakworms can be active from early spring to late fall. Even into the fall, the U.C. Cooperative Extension Office in San Luis Obispo is still receiving inquiries about this pest.

Of the several kinds of tent caterpillars, the most common in the state is the California tent caterpillar, distinguished from the oakworm in being hairy, reddish-brown in color with black stripes, and having a small head. The tent caterpillar makes its living similarly to the oakworm, but it goes from egg to adult only once per year. Hence, infestations are usually over by mid-summer.

Although both insects may feed on some ornamentals, oak leaves are preferred. Oaks and caterpillars have lived together for a long time. Clearly, it’s not in the insect’s best interest to destroy its food source,oak trees. Defoliation that occurs concurrent with other natural stresses, such as drought, or with human-caused stresses such as over-watering of oak trees, root cutting, or soil compaction, could be the last straw. But normally, the oaks recover from even complete defoliation.

The defoliation and mess are about over for this year. The question now is whether the caterpillars will be back next year and can or should anything be done now or later to prevent their recurrence next spring! Both insects are cyclic. This year’s infestations lessen the likelihood of an infestation next year. Some indication of an outbreak next spring can be gotten from the number of eggs or small caterpillars on trees this winter and numbers of small caterpillars at the time of leaf flush next spring. Control measures may be advised if there are egg masses or caterpillars on most of your trees.

Fortunately, there is an effective, and fairly safe, control substance on the market for oakworms and tent caterpillars. Sold by the trade names of Dipel, Thuricide HPC, or Biotrol XK, this substance contains bacteria called Bacillus thuringiensis (BT). BT does not harm people, pets, birds, mammals, or most insects. When BT is ingested by caterpillars, they become sick and die. Don’t apply BT indiscriminately since it may harm honey bees which, of course, are beneficial. BT is sold at most farm supply stores. With help from tree-service people with the right equipment, it can be applied from the ground to one or several trees, or from aircraft over large areas. Importantly, control must occur when caterpillars are small (within 2 weeks of leaf flush in spring) and on a sunny day when they are active.

When deciding whether or not to use control measures, keep in mind that both insect pests are cyclic, especially the oak worm. An infestation one year makes it less likely for another the next. Numbers of egg masses on trees this winter and caterpillars next spring can be used to indicate whether control is necessary. Oak trees nearly always recover from defoliation, even when complete.

If you decide to control these insects be sure to follow the label and check any questionable legalities with your County Agricultural Commissioner’s Office. The University of California Cooperative Extension (UCCE) has several publications available on insect pests of oak trees. Contact your local UCCE Farm Advisor’s office for more information.

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