Oaks 'n' Folks Newsletter Oak Pests by Bruce W. Hagen, CDF Urban Forestry Program California's native oaks live with a multitude of plant pathogens and arthropod pests (insects and mites) throughout their lives. As many as 850 insects and more than 380 diseases are known to occur on oaks in California (Swiecki, 2002). All life stages and virtually all parts of oaks are affected. While few of these pests cause serious damage in undisturbed oak woodlands, some can become rather destructive in the urban landscape. In general, the impacts of insects and disease are greatest on stressed and aging trees. Most serious pests are opportunistic, and flourish when the defense mechanisms of the tree is stressed. The most common environmental factors predisposing oaks to serious pest attack include chronic or acute drought, defoliation, frost, flooding, fire and injuries resulting in root loss. The native vegetation in an oak woodland ecosystem is well adapted to the climate, soil and other living organisms. Oaks have developed natural resistance to many native insects pests and pathogens. However some have become pests by developing tolerance to the plant's natural defenses. Some pests are very selective and feed only on a single species. A few non-specific pests such as oak root fungus and the fruit tree leafroller attack a wide variety of tree species. Many oak pests can attack, feed, and reproduce on healthy oaks. Only a handful of native insect and disease pests are capable of causing serious injury or mortality (Swiecki et al., 1990). Insects and pathogens can affect oak health, survival and regeneration, by interacting in complexes that exert an impact greater than any one pest. Mortality is typically the result of biotic and abiotic factors acting in concert, simultaneous or in succession. It primarily occurs as a result of catastrophic failure due to wood decay in the stem or roots, environmental degradation, energy depletion, girdling by borers and pathogen invasion. Pest abundance and persistence is largely determined by host nutrition, environmental conditions, and the prevalence of natural enemies. Furthermore, environmental conditions influence the distribution of insects and pathogens, and affect their rates of growth and reproduction. Outbreaks typically occur when the natural enemies are scarce or environmental conditions favor pest development. Adverse environmental conditions can suppress pest populations, but when favorable conditions return, pest populations typically rebound quickly because their natural enemies usually take longer to recover, giving the pest a short-term advantage. Therefore, outbreaks of native pests are typically sporadic and, short-lived.
Drought stressed plants, though, are not more susceptible to leaf- and sap-feeding insects as commonly thought. Such plants typically make lower quality hosts. (Herms, 1992). The leaves of water stressed plants are tough, contain less water and nitrogen and are less palatable (Kozlowski, et al. 1991). Severe stress, resulting from drought, defoliation, root loss, flooding, etc., increases susceptibility to secondary pests. Secondary pests, those that attack and kill or seriously injure stress-weakened plants are relatively common and extremely destructive in oak woodlands and managed landscapes. Such pests include bark beetles, borers (flatheaded, roundheaded, clearwinged and carpenterworms), root diseases, canker-causing pathogen, canker-rots, and root disease. Although such pests directly contribute to the dieback, and decline, they are not the proximate cause. Introduced pests are potentially destructive
because their new hosts often lack effective resistance. More importantly,
such pests usually arrive without their natural enemies. Oak pit scale
is an example of an introduced insect pest that can cause serious damage. Host nutritional suitability The reproductive success of pests is influenced by host nutritional quality, in addition to level of natural defensive compounds (allelochemicals), environmental condition and abundance of natural enemies. Consequently, the incidence and abundance of pests vary from year to year and throughout the tree species range. For example, host suitability generally increases as the nitrogen content increases and the allelochemical content decreases. Leaf- and sap-feeding insect pests typically lay fewer eggs on oaks with high carbohydrate reserves, and fewer larvae reach maturity. (Herms, 1997). Insect pests on trees with lower concentrations of allelochemicals generally produce more offspring. Most insect feeding and leaf disease damage is done early in the season when the foliage is young and succulent. Young leaves are comparatively more nutritious than mature leaves because they contain higher concentrations of water, nitrogen (amino acids and proteins) and other essential nutrients. Leaf tissue becomes less palatable and digestible to chewing insects, as concentrations of cellulose, lignin and tannins increase with age. The leaf cuticle that thickens with maturity is an important deterrent to insect feeding and certain pathogens. Young foliage also contains high concentrations of natural toxins, which inhibits feeding by non-specialized pests. Native oak pests, though, have developed a tolerance to these toxins and are able to feed and reproduce. They produce detoxifying enzymes and have developed other physiological adaptations to deal with the natural toxins. Leaf diseases are most prevalent in the spring when the foliage is most susceptible, and rain and mild temperatures favor spore production, dispersal, germination and tissue damage. Cool, wet springs also favor disease development by slowing shoot elongation and leaf development, thus extending the period of time that the leaves are succulent and susceptible to infection. Powdery mildews, however, can develop on mature leaves when environmental conditions are favorable. Common insect pests Defoliators, predominantly moth larvae, cause the most obvious injury. Their outbreaks are typically sporadic and short-lived. Nonetheless, total or significant defoliation can slow growth, retard wound closure, impair response to wood decay pathogens and increase susceptibility to secondary pests (Wargo, 1978). Repeated defoliation, though, can affect tree health and even survival, particularly when combined with severe drought. Sucking insects often occur in abundance on oaks, especially on the new growth. Symptoms include sticky, blackened, curled, yellowed or spotted foliage, dieback, and decline. Among the sap-feeding insects, the foreign oak pit scale is the most destructive, particularly during drought years and in urban landscapes. While, boring insects (oak bark beetle, ambrosia beetles, oak twig girdler) attack severely stressed trees, girdling branches or killing areas of the trunk. Others bore into the wood, weakening tree structure and facilitating decay-causing pathogens. There are more than 200 species of gall-forming pests on native oaks have been described. Galls are abnormal, insect or mite induced growths that provide both food and shelter for the pest. They vary greatly in size, shape, color and location, and can be found on leaves, flowers, acorns, buds, twigs, branches and roots. Although certain species can kill branches or portions of leaves, they seldom cause significant injury.
Fungal diseases play a major role in the growth, survival and regeneration of oaks throughout California. Wood decay, cankers, canker rots and root disease are important causes of mortality in rangeland oaks (Swiecki et al., 1990). Canker-causing fungi kill the bark and cambium of trees, frequently girdling and killing branches. Some also decay the sapwood below. Symptoms usually appear as dead, sunken areas in the bark. Canker rots, on the other hand, are caused by wood-decaying fungi that ultimately spread to and kill the cambium and phloem, giving rise to cankers. Furthermore, they cause a white rot of the heartwood, frequently leading to limb and trunk failure. By comparison, brown rot of the heartwood is quite common in oaks and is typically caused by the sulfur fungus. Root and butt rot (wood decay) is especially important in the decline, failure and death of oaks. The single most important root pathogen of landscape oaks is oak root fungus (Armillaria spp.). Crown rot (Phytophthora spp.) is also a destructive root pathogen affecting landscape oaks in irrigated settings. Trees receiving frequent irrigation and those in poorly drained sites are most susceptible. Irrigation releases an important constraint that dry soil normally provides on the development of certain root diseases. Although foliar diseases (e.g., anthracnose, powdery mildew) are important in the urban landscape, they appear to have little impact on rangeland oaks. Damage ranges from leaf spotting to killing of leaves and shoots.
Leafy mistletoe is a parasitic plant that obtains water and minerals from its host but produces most of its own carbohydrates. Heavily infested tree branches can die because they are unable to regulate water loss from the mistletoe but in most cases mistletoe does not present a problem for oak trees and should be left for birds that eat the berries. Cankers, though, can result when mistletoe plants die, leaving the surrounding wood open to wood decay pathogens. In summary, while most insect and disease pests of oaks cause minor injury, their effects may be cumulative. Insects and diseases frequently exert their impacts as an interacting complex influenced largely by environmental conditions and tree vigor. Dieback, decline and sometimes death may result. Pest problems, increasing age, environmental stress, construction injury and harmful cultural practices are all important factors affecting the health, growth, longevity and survival of oaks. Healthy, stress-free trees are more resistant to most pests and are more tolerant of the injury caused by more serious pests. References Herms, D. A., and W.J. Mattson. 1992. The dilemma of plants: to grow or defend. Quarterly Review of Biology, 67 (3) 283-335 Kozlowski, T., P, J. Kraemer and S. G. Pallardy, 1991. The Physiological Ecology of Woody Plants. Academic Press Inc., New York. 657p Swiecki, T. J. and E. A. Bernhardt, and R. A. Arnold 1990. Impacts of diseases and anthropods on California's rangeland oaks. California Department of Forestry and Fire Protection, Forest and Rangeland Resources Assessment Program. Swiecki, T. J. 2001. Observations and comments on oak and tanoak dieback and mortality in California. An on-line publication: Phytosphere.com/tanoakobservations/tanoak.html Swiecki, T. J. and E. A. Bernhardt. 2002. A delicate balance: Impacts of disease on the health of California oaks. This article is based on a paper publisheded in Fremontia 18:58-63 Wargo, P. M. 1978. Insects have defoliated my tree–now what's going to happen? J. Arboric. 4:169-175
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