Insects actually destroy more timber annually than wildfire. Insects usually take their toll on individual or small groups of trees here and there, unlike the spectacular destruction of forest fires.
Insects are a endemic and necessary part of the forest ecosystem. Most insect species are somewhat beneficial. Insects speed decay of dead vegetation, releasing nutrients for other plants. Many forms of wildlife feed on insects. Bees and other insects pollinate forest plants. Some insects prey on other economically harmful insects.
Insect populations, however, can quickly build into a devastating force. Human attempts at insect control may actually prolong the epidemic. There is a natural "boom - bust" cycle for insect populations. The insect population builds up to point that food is available. Following the initial "boom" period, the insects consume all available food and starve or are weakened by disease. If the crash comes soon enough, then the trees might recover.
As with fire, prevention is the best insect control method. Integrated Pest Management (IPM), a term that describes sensible pest management using well timed suppression efforts based on the life cycle of the insects, combines prevention through good silviculture with quick reporting of outbreaks, and direct chemical and biological attack in response to suspected outbreaks. Healthy vigorous trees, promoted through good silviculture are your best defense against insect attacks. During harvest and pre-commercial operations, remove trees that have low vigor, as these are most prone to successful insect attack. Insects and disease infect trees through logging wounds. Plan all management operations to minimize damage to the remaining trees. Plant genetically improved seedlings that are resistant to insects and disease when available. Plant a variety of species where appropriate. Large areas of a single species are more susceptible to attack than stands of mixed species.
Bark beetles and engravers, though only about the size of a grain of rice, destroy the largest amount of timber in California of any insect. They bore through the bark. Females lay eggs in tunnels beneath the bark. The larvae tunnel through the tree eventually girdling it. Bark beetles attack trees near the base while engravers can be found near the top of the tree. Though these insects normally attack weakened trees, they may work together; engravers may attack first, followed by bark beetles, killing healthy trees.
Pitch tubes, which are small bubble-gum like blobs of sap and sawdust near the base of the tree, indicate bark beetle attacks. General reddening of the foliage in the top of the tree is the most noticeable sign of bark beetle and engraver attack. Positive identification can be made by removing a small piece of bark. The pattern of the tunnels made by the larvae are characteristic of the species of beetle.
Timber killed by bark beetles should be salvaged quickly to minimize spread, and to capture the tree's economic value before decay sets in. If the affected logs are unmerchantable, burn them or spray with an insecticide. Salvage timber in the winter when beetles are still in the bark. Generally keep your trees healthy and vigorous to prevent beetle attacks. Thinning dense stands of trees helps to maintain good live crowns, thus improving the vigor of the trees. Report any outbreak to your local CDF forester, farm advisor, or county agricultural commissioner.
Insects that attack foliage weaken the tree by reducing the amount of food the tree can manufacture. Most trees can tolerate partial defoliation, though this may make them susceptible to bark beetle attack. Repeated or total defoliation can kill the tree out right.
Foliage insects can be found while they are feeding. Also look for damaged leaves and needles. Egg masses, usually small pouches of webs attached to protected spots on the bark or under branches and leaves are easily detected.
Insects also affect twigs, buds, cones and roots. Look for a decline in tree vigor or damage to the specific part. Be familiar with your forest and notice changes. Respond quickly to insect outbreaks.
Learn about insect pests specific to your area. Insects have very specific requirements for build up to occur. Use strategies to prevent their occurrence or limit their effect. Talk to your forester, farm advisor or local CDF forester. Insects are an integral part of your forest. Learning about insects and developing strategies to balance insect populations with your objectives can be an interesting challenge in managing your woodland.
Forest diseases are caused by tiny organisms that live their lives at the expense of the tree. They survive by robbing the tree of nutrients and water. Like insects, diseases are a ubiquitous in part of the forest ecosystems.
Most of the general concepts used to control insects also apply to diseases. In fact, several diseases are actually transmitted through insects. Insect-transmitted diseases are usually managed by controlling the insect.
Prevention, through good forest management, is the best method for disease control. Chemical control, except in nurseries, Christmas tree plantations, or ornamental plantings, is rarely cost effective. Consult with your forester, farm advisor, or CDF forester to learn about diseases you should be concerned with, and strategies to prevent them. Christmas tree plantations may warrant more extensive control methods such as spraying fungicides.
Diseases may be difficult to identify until they have damaged the tree. Look for conks, shelf fungi, or mushrooms around the tree stem. Witches broom, erratic growth at branch ends, also indicates disease.
Dwarf mistletoe, a serious disease of western conifers, is recognized by its small scale–like leaves. Dwarf mistletoe also produces witches broom. Cut and remove affected trees that have evidence of dwarf mistletoe in the trunk of the tree. Pruning affected limbs may be sufficient in trees with dwarf mistletoe only in the branches.
Heart rots are the leading cause of wood decay. The causal fungi enter the tree through logging wounds, animal damage, dead branches, or any injury that opens to the inner part of the tree. Prevention of injury is the most effective form of control. Salvage damaged timber.
Root diseases, which spread from tree to tree by root contact are difficult to detect, and even more difficult to control. Look for fruiting bodies on and around the base of the tree. Trees may lose needles or change color. Bark beetles often attack trees weakened by root disease. Control root diseases by keeping the stand healthy. Salvage all the affected and surrounding timber because the disease may spread through root contact.
Disease centers, in which a group of trees die, are a good indicator of root disease. Root diseases often become established when fungus spores land on and colonize a freshly cut stump. One particularly serious root disease Fomes annosus can be prevented by covering freshly cut stumps with borax.
Another control method is to replant diseased stands with different tree species. Root diseases are typically species specific, affecting only one tree species. Harvesting the affected trees and planting another species could eliminate the disease problem.
Rusts are disease fungi that enter the tree through the needles, generate spores that infect another host, which then reinfect other trees. The most destructive rust in California is the white pine blister rust that infects sugar pine and other pines in the 5 - needle pine group.
Rusts require an alternate host for the disease to complete its life cycle. Spores can travel hundreds of miles, so the alternate host could be far away. As an example, white pine blister rust is an important disease affecting sugar pine. The alternate host for white pine blister rust is currants or gooseberries. Control was attempted by eradicating the alternate host; however, this proved ineffective. Some headway has been made in identifying the genetic mechanism that gives trees resistance to white pine blister rust. In the future, genetically resistant sugar pines may be commercially available to California landowners.
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Publication of this series was in part funded by the
California Department of Forestry and Fire Protection
under Contract numbers 8CA96027 and 8CA96028