Know Your Pests
Forest pests are an eclectic group. Tree “diseases” can be caused by fungi, bacteria, parasitic plants, insects, mammals, smog, chemicals, extreme temperatures, and other harmful agents.
Not all pests are villains
Most biologically caused diseases are a natural and essential part of a healthy forest. Insects, fungi, and bacteria all break down and decompose organic matter, releasing nutrients and creating soil humus. Insects are an important food source for wildlife, they pollinate forest plants and trees, and predator insects are critical to keep harmful insects under control.
What we consider disease is a disturbance in the normal functions of a tree. When this disturbance occurs sporadically it goes undetected. When it occurs over large continuous areas it may indicate a problem that should be addressed.
Boom and bust
Insect populations can quickly build into a devastating force. There is a natural boom-and-bust cycle for many populations. Population outbreaks are followed by a crash when the insects consume all available food, are weakened by disease, or devastated by natural enemies.
Insects actually destroy more timber annually than wildfire. However, they usually take their toll on individual or small groups of trees here and there, unlike the spectacular destruction of forest fires.
While insects may be the identifiable cause of tree injury and death, the ultimate cause is often poor tree condition due to drought or excessive competition for water, light, and nutrients; excessive water; or physical damage to the tree. Trees are constantly challenged by insects but generally only stressed or unhealthy trees succumb.
Trees are not as helpless against insect attack as one would expect. A healthy tree is able to defend itself from insect attack by producing pitch that drowns, or pitches out, the attackers. If you see pitch streamers, tubes, or granules, that is evidence that the tree is still alive and fighting.
Trees under stress, and those that are older or unhealthy, are more apt to succumb to insect attack. If there are numerous insect attacks with no evidence of pitch or resin, the tree is most likely dying. To be sure, look for beetle larvae under the bark at the site of an attack with no resin flow.
Bark Beetles and Engravers
Bark beetles and engravers are the most serious insect pests in California. They bore through the bark of most pine and fir species. These insects generally attack stressed or weakened trees.
They often work together: engravers may attack first, weakening a tree, followed by bark beetles which can kill the tree.
General reddening of the foliage in the tops of pine and fir trees is the most noticeable sign of successful bark beetle and engraver attack.
Insects that attack foliage can cause stress or even death by hindering photosynthesis. Most trees can tolerate partial defoliation, though this may make them more susceptible to bark beetle attack. Repeated or total defoliation can kill the tree outright.
Foliage insects can be found while they are feeding. 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.
Fungi: rots, rust, and root disease
Heart rots are the leading cause of wood decay. Look for large, shelf-like mushrooms (conks), on tree trunk or for mushrooms in the soil around the trunk. These are the fruiting bodies of fungi. When you tap on the tree a hollow sound indicates heart rot.
The fungi that cause the rot enter the tree through logging wounds, animal damage, or any injury that opens the inner part of the tree. Preventing injury is the most effective form of control.
The presence of conks indicates significant deterioration inside a tree which could cause it to be structurally unsound. Remove any trees that could fall on people or structures. In the forest, trees with conks should be left to become snags.
Rusts enter the tree through the needles. Rusts require an alternate host plant for the disease to complete its life cycle. Spores can travel hundreds of miles on the wind so the alternate host could be far away.
The most destructive rust in California is the white pine blister rust that infects sugar pine and others in the 5-needle pine group. Symptoms are random flagging (dead, brown-needled branches) in tree crowns. Close examination of these branches show a spindlelike swelling of the branch filled with orange spores. The alternate host for white pine blister rust is currant or gooseberry. Control was attempted by eradicating the alternate host, however, this proved ineffective.
Root diseases usually affect the whole tree uniformly. If tree leaves or needles are yellowing, dying, or falling for no apparent reason, root problems are usually the cause (but other causes such as mechanical damage to the roots, saturated soils, and foliage insects or rusts should also be considered).
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 (mushrooms) on and around the base of the tree. Bark beetles often successfully attack trees weakened by root disease.
Disease centers, in which a group of trees die, are another good indicator of root disease. Trees adjacent to dead trees are often infected though they do not show symptoms until the next year, when moisture stress finally browns the leaves.
Control root diseases by keeping the stand healthy. Salvage all the affected and surrounding trees because the disease may have spread through root contact.
Some root diseases become established when fungus spores land on and colonize a freshly cut stump. One particularly serious root disease, Heterobasidion annosum, can be prevented by powdering freshly cut stumps with borax, sodium borate. Another control method is to replant diseased stands with a 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.
Sudden Oak Death is caused by the fungus Phytophthora ramorum. Bleeding or oozing of a dark reddish-brown thick sap is the first symptom to appear on true oaks and tanoak.
Be familiar with your forest and notice any changes. If you see some suspicious activity—dead or dying trees, insect outbreak, mushrooms, etc.—you’ll want to figure out what is going on.
Collect specimens or take pictures of any potential problems. Diseases may be difficult to identify until they have damaged a tree to an obvious extent. Determine the part of the plant that is affected, any patterns to the damage, etc. If the cause of the disease is not readily apparent, start with the simplest possibilities: animal damage, frost, mechanical injuries, fire.
Generally, the best defense against insects and diseases is to maintain healthy trees.
Chemical or other insect control measures are expensive and often ineffective. While individual trees might be protected with insecticide treatments, only landscape trees are probably worth the cost and effort.
One approach to controlling forest pests is known as Integrated Pest Management (IPM), a strategy that considers the whole ecosystem. It focuses on long-term reduction of pest damage through a combination of techniques including biological control, habitat modification, changes in cultural practices, and resistant varieties.
With IPM, pesticides are used only after monitoring indicates they are needed according to established guidelines. Pest control materials are selected and applied to minimize risks to human health, beneficial and non-target organisms, and the environment. Treatments are chosen and timed to be most effective and least disruptive to natural pest controls.
IPM requires constant monitoring to determine whether a pest problem exists, and if the problem is intolerable enough to require treatment. In many cases the “no treatment” option is found to be the preferred, most cost-effective approach.
Learn about insects and other potential pests found in your area. Talk to your forester or a specialist at UC Cooperative Extension or CAL FIRE (see list on page 10). Developing strategies to prevent the occurrence or limit the effects of forest pests can be an exciting management challenge.