Oak watch
Publication Date: Friday Sep 20, 1996

Oak watch

If you have an oak tree, watch out for these three pests

by Scott Loosley

QI have a coast live oak in my yard that is about 30 feet tall and is next to my patio. While sweeping my patio, I noticed tiny little balls about 1/16-inch in diameter covering the ground. Figuring they must have come from the oak tree, I brought out a ladder to look at the leaves more closely and noticed a number of things. The tiny balls were attached to the leaves along with tiny star-shaped white and black specks and what looked like small (1/4-inch) white cocoons. Some of the leaves also have brown spots on them while others look fine. Now I'm concerned about the health of my tree. What should I do?

AYour oak has become a host to three of the more common pests that affect this tree in our area. The good news is that none of these pests is threatening to the tree's life, although they may contribute to an unsightly appearance in large populations. The three pests you have described are cynipid gall wasp, whitefly and oak ribbed casemaker. I'll give you a little information about each of them.

The little balls you were sweeping up from the patio are galls that are formed by the feeding of cynipid wasp larva. There are many different species of gall-forming wasps in the family Cynipidae that feed on oaks.

The type of gall is dependent upon the species of wasp, with many exhibiting very unusual shapes and colors. The small female wasp lays her egg in the leaf tissue. Fluids deposited with the egg cause the leaf cells to multiply abnormally, starting the formation of the gall. The egg hatches into a larvae and begins feeding on the leaf tissue. Secretions from the feeding larva continue the development of the gall, eventually encasing it.

The larva will feed from within the gall until maturing into an adult. It then will emerge from the gall and start the life cycle again. Most galls are not harmful to host oak trees and their populations can vary from year to year, so no control is recommended.

The second pest you observed on the oak leaf is an immature whitefly. Two of the more common species are crown whitefly (Aleuroplatus coronata) and Stanford whitefly (Tetraleurodes stanfordi).

The small white and black specks you found are an immature form, or pupae. This stage of the pest is immobile and the white substance is a waxy covering, which somewhat resembles a crown. The whitefly will overwinter in this stage until it hatches into an adult in the spring. The adult feeds on the tree by sucking the sap, but apparently they do little damage. The appearance of sooty mold, a fungus that grows on the excrement (honeydew) of the whitefly, is probably the worst aspect of this pest. Beneficial insects generally keep whitefly in control and the use of pesticides are not recommended.

The third pest of your oak tree has the greatest potential for damage when in large numbers. The oak ribbed casemaker (Bucculatrix albertiella) is a type of leaf skeletonizer, feeding on leaf tissues between the veins, eventually causing brown patches.

The adult form of this insect is a mottled white, brown and black moth. Eggs hatch into larvae that feed inside the leaf. As the larvae develop, they feed externally on the lower surface of the leaf. The skeletonized portions then turn into brown patches. The larvae then spin white, cigar-shaped cocoons with distinct longitudinal ribs.

There are usually two generations of this pest each year, one in spring and the other in summer. Small infestations in the landscape are generally tolerable, but larger populations may require the application of a summer-weight horticultural oil. The application should thoroughly cover the lower leaf surface when the larvae are feeding. Monitoring of the population is critical for the proper timing of application.

Luckily, none of these pests causes severe problems for your oak. Since most insects attack stressed plants, you may want to think about the cultural practices you are using for your tree. A healthy tree is more likely to resist infection from insects and pathogens in the future.

Scott Loosley is the horticulturist at the Gamble Garden Center in Palo Alto. Send questions to Loosley care of Palo Alto Weekly, P.O. Box 1610, Palo Alto, CA 94302.



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