Damage Factors Quercus wislizenii Abstract: The typical damage and the distinguishing characteristics of the injurious stage are described and illustrated for the following 11 important and/or common insects feeding on California oak trees, categorized as: Sucking insects: oak pit scales, crown whitefly; s- consuming insects: California oak moth, tent caterpillars, fruittree leafroller; Boring insects: western sycamore borer, oak twig girdler; Acorn insects: filbertworm, filbert weevil; and Gall insects: two-homed oak gall, dis- torted leaf gall. The management of these categories of injurious insects is discussed, emphasizing the philosophy and decision-making process. INTRODUCTION homelot may require an immediate spraying of insecticide, whereas such an oak in a wild, Insects are one of the important cate- forested area may require observation and the gories of "damage factors" affecting oaks. encouragement of natural predators and para- In California well over 125 insect species sites. The final part of this paper will have been recorded feeding on various parts discuss what man can do to regulate popula- of oak trees. Approximately 15 of these tions of these injurious insects. insects are of major importance to oaks; a few of these important insects are discussed For a more detailed account of Cali- here. fornia oak insects and their management, the reader may wish to write the author for a For the person managing oaks it is copy of Brown and Eads (1965). Most species logical to categorize insect pests by the mentioned here are well covered by Furniss part of the oak they feed on. This subdi- and Carolin (1977) which is available to most vides the large number of oak insects into foresters. groups of less formidable size, and in large part these groups are natural in that sucking pests are together, chewing larvae of moths SUCKING INSECTS and beetles are grouped, etc. Furthermore, control measures are frequently the same for Sucking insects include whiteflies, insects feeding the same way on the same aphids, leafhoppers, tree-hoppers, and scale plant part. Thus the sequence of insects insects. The long, stylet-like mouthparts of discussed here is as follows: sucking these insects are inserted into or near the insects; leaf-consuming insects; boring vascular system of the oak, generally the insects on trunks, limbs, and twigs; boring phloem, and the watery, sugar-containing sap insects in acorns; and gall insects. is withdrawn, sometimes in quantities that kill leaves and twigs. The plant cell struc- What the oak manager will do about ture is left intact. Often these sucking insects feeding on this tree may depend on insects inject into the plant some of their where it is. The infested tree on an urban saliva which may be toxic enough to kill tissue or cause it to be deformed. Sucking 1 insects seemingly have few defenses, except -'presented at the Symposium on the Ecology, their generally enormous reproductive powers. Management, and Utilization of California Honeydew may be dropped from infestations of Oaks, Claremont, California, June 26-28, some sucking insects, such as aphids and 1979. certain scales. Only two sucking insects are discussed in detail here: oak pit scales, L'~rofessor of Entomology, University of because of their great potential for harm, California, Riverside, CA 92521. and crown whitefly, because it is so common. Oak pit scales saliva, the bark around the scale begins to swell so that the mature, attached scale If the twigs and smaller branches of an appears to be residing in a pit. Three oak are leafless, and weakened or dead, like Asterolecanium species are present in Cali- those in figure 1, a close search should be fornia: 4. minus, A. quercicola, and A. made of the twig surfaces. If many shiny, variolosum with 4. minus being the most round, convex, yellowish-green, pin head- common. Insofar as known they have a single generation annually with peak hatch in late May. Of all the sucking insects affecting the oak tree, the pit scales can do the most harm because they frequently escape early detection, have a high reproductive poten- tial, and have toxic salivary secretions. Pit scale infestations can be devastating, particularly in addition to droughts. Crown whitefly One of the most common sucking insects, especially on black oaks, is the crown white- fly, (Aleuroplatus coronatus, Homoptera: Aleyrodidae). Superficially it appears like a salt and pepper encrustation on the bottom side of the leaves; but when magnified, as in Figure I--Note generally weakened condition figure 3, each immature whitefly appears to of these branches of valley oak from a be a flat, scale-like, black body from which severe infestation of an oak pit scale, issue several large plates of white wax, Asterolecanium minus. somewhat in the form of a crown. The adults sized bodies are found in dimpled areas of the bark, as in figure 2, then most likely one of the oak pit scales, (Asterolecanium species, Homoptera: Asterolecaniidae) is present. The dimples or pits are especially noticeable when the scale is removed. When the young crawler scale first settles, the bark may be smooth, but with the insertion of the mouthparts and injection of its toxic Figure 3--Immature forms of the crown whitefly, Aleuroplatus coronatus, on the lower leaf surface of coast live oak. 8.6X. appear more like a conventional insect in being winged and free of the leaf. The tiny adults are especially noticeable around oak stands in the spring of the year when they fly in such numbers as to resemble the swirl- ing flakes of a snow storm. At such times Figure 2--Numerous adults and pits of the gol- this whitefly is a nuisance pest. But at den oak scale, ~sterolecaniukvariolosum~ other times when the immature forms encrust on two twigs of Ouercus robur. 1.7X. the leaf in high numbers it is not obvious what the damage is to the leaf or tree. Other sucking pests of oaks 5 years. These population collapses may result from a depletion of oak leaf food Oak treehovpers (Homoptera: Membracidae) and/or a variety of insect parasites and ~elatinou~bhitefl~ fileuro latus gelatin- predators (Harville, 1955) that build up -oms. Homoptera:-----I Aleyrodidae during an oak moth epidemic. They are also Stanford's whitefly (~etr&eurodei stanfordi, subject to a naturally occurring wilt dis- Homovtera: Alevrodidael ease. The California oak moth (Phryganidia Woolly oak aphid (~ie~o~h~liaquercicola, californica, Lepidoptera: Dioptidae) is also Homoptera: Aphididae) curious in that it is the only member of its Black-punctured kermes (Kermes nigropunc- family in the United States. Most of the tatus.. .- Homo~tera:Kermidael dioptids are tropical, but this one ranges Oak wax scale (Cerococcus quercus, Homoptera: into California, to north of San Francisco, Asterolecaniidae) and into Arizona. Ehrhornfsoak scale (Mycetococcus ehrhorni, Homoptera: Asterolecaniidae) The larva (fig. 4) when full-grown, is Oak lecanium scale (Lecanium quercitronis, about 25 mm long, without noticeable hairs, Homoptera: Coccidae) Oak scale (Quernaspis quercus, Homoptera: Diaspididae) Coast live oak erineum mite (Aceria mackiei, Acarina: Eriophyidae) LEAF-CONSUMING INSECTS This category of insects is readily noticed because the forms themselves are relatively conspicuous as is the type of damage they do. Also, their defecated droppings may be in such quantities as to appear in windrows on the ground and sound like a light falling rain. These insects eat the cell walls and watery cell contents of the leaves. The loss of leaf tissue reduces the food-making capability and slows the Figure 4--Fourth-instar larvae of the Cali- growth of the oak. All of the leaf blade may fornia oak moth, Phryganidia californica, be consumed, leaving only the ribs, or only on injured leaves of coast live oak. 0.9X. one leaf surface, usually the lower, may be removed in spots. This is referred to as and it has a large, brown head. The body is 'skeletonizing." Also there are a number of mostly black but may have longitudinal insects on oak leaves that may spend a part, stripes of white, red, and yellow. Although or all, of their immature stage mining be- each larva is relatively slow-moving, it tween the upper and lower leaf epidermis. Larvae of moths are the most common oak leaf consumers, although there may be stages of beetles or wasps that attack the leaves. Three of the most common and important leaf consumers are discussed here: California oak moth, tent caterpillars, and fruittree leaf- roller. California oak moth This voracious defoliator, also called the California oak worm, is perhaps the most important insect pest of oak trees during years of normal rainfall. During drought years the oak twig girdler probably equals it in importance. But it is typical of oak moth infestations to exist in extremely high Figure 5--Adult female of the California oak populations during one year; then these moth, Phryganidia californica; note populations will collapse and any stage of thread-like antennae. 1.7X. the insect will be difficult to find for 4 or consumes a great amount of oak foliage. The Fruittree leafroller adult female (fig. 5) is uniformly cream-tan except where the black cuticle is hairless on Archi s argyrospilus (Lepidoptera: the back of the thorax. The female has Tortrici-9- ae) is called "fruittree leafroller" thread-like antennae whereas the male's is since it is one of the most common and impor- feather-like; he also has a faint yellow spot tant pests of commercial fruits, especially in the center of each forewing. In most of pome fruit, but it has a wide host range and California there are two annual generations defoliates many woody ornamental plants. For of oak moth, but there may be as many as at least 30 years it has defoliated, with three in southern California.