Pests That Attack California Oaks

Oaks are famous for the food they provide local wildlife. In fact, oaks help to sustain everything from squirrels and jays to black bear and deer. But while most animals primarily feed on the acorns of oaks, other animals – primarily several types of insects – feed on the leaves and other parts of oak trees.

These pests often cause significant damage to living oaks, and they may even cause enough stress to kill the tree outright. Others may not cause substantial damage via their feeding activities but still represent a threat, as they can transmit fungi, bacteria and diseases to the trees on which they feed.

Some of these insects can be managed, while others are more difficult to control. But in all cases, you’ll have a better chance of saving your oaks if you solicit professional help quickly before the infestation grows out of control. To help you do so, we’ll discuss some of the most important pests of oak trees below.

1. California Oakworm
California oakworms are 1-inch-long, when fully grown, black-and-yellow caterpillars who are found throughout the coastal regions of our state. Their feeding activities cause severe defoliation, but fortunately, they rarely kill the trees they infest. Close inspection of oakworm-damaged leaves will reveal that the bottom surfaces of the leaves have been eaten, but the top portions are left intact (although the top portions of the leaves will turn brown).

2. Fruit-Tree Leaf Roller
The fruit-tree leaf roller is found throughout California, and it attacks almost all of our native oaks. These small, light-colored caterpillars feed on new leaves in the spring. They also roll the leaves up into bizarre shapes, which serve as their cocoons. Most healthy oaks can survive low-level infestations, but trees that have been stressed by drought or previous attacks may decline and eventually die.

3. Tent Caterpillars
Tent caterpillars are 2-inch-long caterpillars that are usually covered in tufts of hair. They are named for their tendency to construct large silk “tents” in oaks and fruit trees. Fortunately, tent caterpillars rarely cause significant stress for oak trees, although some homeowners consider them to be nuisances. Some people also find their nests to be eyesores.

4. Western Tussock Moth
Named for the white hair tufts on their backs, larval tussock moths (caterpillars) feed on oaks and a variety of other tree species in California. These pests often cause serious defoliation. In addition, these caterpillars also cause problems for people, as they produce quite a bit of frass (droppings) and their hairs can be irritating to the skin.

5. Gall Wasps
Gall wasps are small insects who deposit their eggs in the tissues of oak trees (and many other species). This causes the oak tree to create a swollen, typically brown chamber in which the eggs and larvae will develop. Some gall wasps are capable of depositing eggs in many different oak species, while others have evolved to favor a single one. Although the galls they create often cause homeowners quite a bit of concern, gall wasps rarely cause damage to the trees they use. Rarely, they’ll cause damage to individual branches or twigs.

6. Oak Pit Scale
Pit scales are small insects that cause depressions to form on the surfaces of branches and twigs. The actual insects are barely more than 1 millimeter in length, so they’re hard to spot, but their pits are often quite apparent. Pit scale insects are found throughout California, although they rarely reach high population densities in forested areas – they more commonly infest isolated trees in home landscapes. Pit scales typically prefer deciduous oak species to live oaks, and they can cause serious damage to trees.

7. Bark Beetles
There are a few different oak bark beetles that attack California’s oaks, but they all exhibit relatively similar biology and feeding habits. These beetles, which may reach about 2 millimeter in length, feed on the inner bark of the trees by tunneling through the outer bark. They also lay their eggs inside these tunnels. While the beetles can cause damage and accelerate the decline of stressed trees, the biggest threat they pose is their tendency to be associated with the pathogen responsible for sudden oak death.

8. Carpenterworm
Carpenterworms are off-white to pinkish-white caterpillars (larvae) that can reach about 2.5 inches in length. They are found throughout the state, and they attack ashes, aspens, willows and elms in addition to oaks. These larvae, which feed on the inner bark of trees, create small tunnels in the bark, which are about ½ to ¾ inches in diameter. Carpenterworms may cause branch girdling themselves, but they may also make trees vulnerable to fungal attacks, thanks to the tunnels they make.

If you think that one of your oak trees is suffering from an insect infestation, you’ll need professional help, so give your friends at Evergreen Arborist Consultants a call. We’ll have one of our certified arborists visit your property, inspect your trees and identify the insects that are causing the problem. We’ll then recommend a prudent course of action that may save your tree and prevent the insects from infesting your other trees.