Forest Protection - Insects

INTRODUCTION

Although small in size, insects have a very large impact on forest health in Canada. Insects can destroy more wood than forest fires. In 1995 over 1.5 million hectares (ha) of forest in British Columbia was attacked by insects as opposed to less than 100,000 ha of forest burned.

Many species of insects feed on trees or tunnel in wood to lay their eggs. The results can range from slowed tree growth to the death of the tree. At a forest level, insect damage can delay regeneration, shift species composition and affect the quality of water, wildlife and recreation values. In addition, insect damage adds to the fuel loading in the forest, increasing the fire hazard.

Protecting BC’s forests from pests is a responsibility shared among federal and provincial governments and forest companies. The federal government carries out overview surveys and research, while the provincial government carries out operational surveys, sets policy and administers control measures. Forest companies develop and oversee protection programs on the lands under their responsibility.

To explore the connection between insects and forestry, this article will explore the ecological role of insects in the forest, the damage caused by key forest pests, pest management concepts and how this information all comes together in forest management.

ECOLOGICAL ROLE

Insects are a part of the complex forest ecosystem. Like all parts of the ecosystem they have a role to play and they interact with many other components. This group of organisms is incredibly diverse and their ecosystem functions are equally diverse. The ecological role of insects ranges from benefactor to killer, with the beneficial insects being the most abundant.

Pollination is an important role played by some insects. Wasps and bees pollinate flowering trees and shrubs.

Speeding up decay is another insect function. Insects such as ants, termites and wood boring beetles bore into the wood of dead trees, speeding up the invasion of wood decaying microbes.

Insects speed up nutrient cycling within the soil. Insects such as collembolans, thysanurans, beetles, and flies feed on organic matter and fungi, speeding the flow of nutrients to the soil.

Other insects can act as predators and parasites of herbivorous insect pests. Under normal conditions these natural enemies control these pest populations.

Insects also act as food sources for many insectivorous birds, amphibians and mammals.

These multiple roles indicate the complexity of insect functions in the forest ecosystem. Insects are involved in the ecological processes of the forest, including in forest stability, succession and productivity.

Over time, the insect populations of the host tree, attacking insects and insect enemies fluctuate and end up regulating the composition and abundance of each. This impacts ecosystem stability.

The rate of forest succession can speed up or slow down based on insect action. Bark beetles attacking pioneer tree species, hasten the transition to climax. On the other hand, the fuel loading from the dead pioneer trees could increase the chance of a forest fire, which would move succession back to an earlier stage.

By feeding on unhealthy trees, insects help to re-cycle the nutrients from the dying trees to the healthy survivors. This maximizes the productivity of the average tree.

The number of beneficial or non-harmful insect species in a forest is large. They play many essential roles within the forest ecosystem. The number of insect species that cause problems for forest managers is quite small. However, their effect on the forest can be dramatic.

FOREST DAMAGE

An analysis of insect damage can be made considering the following aspects: how insects do the damage and how trees have evolved defences against insect pests.

Insects cause greater economic loss than any other pest group. The type and level of damage caused by herbivorous insects varies by species. Forest managers are concerned about damage such as loss of wood for products, decreased wood quality decreased forest values for wildlife, water and recreation, as well as increased fire hazard.

The damage to the individual tree may include growth reduction, reduced grade of timber, stem deformity, reduced seed crop and death of the tree. Where and how the tree is damaged depends upon how the insect feeds. The three main feeding styles of insect pests are:

Defoliators:

feed on foliage and buds by means of cutting and chewing mouth parts.

Woody tissue feeders:

Have similar mouth parts but feed on in the outer sapwood of the main stem, branches or roots.

Sucking insects:

Have piercing or sucking mouth parts for removal of sap from stems, needles or branches.

Defoliators are among the most destructive forest pests. More species of insects feed on the leaves than on any other part of the tree. Some mine the needles, others eat the entire leaf or suck the juices. Caterpillars, the larvae of moths and sawflies, are the most damaging of the insects. The defoliating larvae eat the tissue containing chlorophyll. Needles may be partially or completely eaten. These insects can defoliate an entire forest. The result of this kind of damage is reduced growth, deformation and a greater susceptibility to attack by other pests. If defoliation occurs over consecutive years, the trees may die. Since evergreens cannot replace their leaves as easily as deciduous trees, they are generally more seriously injured. Unlike many other insects, defoliators attack healthy trees equally with weaker ones.

Spruce budworm, Douglas fir tussock moth and hemlock looper are defoliators that attack conifers. The gypsy moth and tent caterpillar attack hardwoods. Outbreaks of defoliators are characteristically sporadic. They generally go through a cycle of build-up to extensive defoliation to an abrupt decline back to normal. This decline is usually brought about by disease, parasitic insects or climatic conditions.

Woody Tissue Feeders attack the bole of the tree, which results in reduced growth and often tree death. Most woody tissue feeders are bark beetles. The female adults find standing or downed logs and dig tunnels, severing the resin ducts of the tree. If the tree cannot repel the attack, the insect sends out powerful chemicals called pheromones to attract the males. Thousands of males and females will converge on the tree. The females then mate and dig out egg galleries in the dying tree. The larvae of these beetles feed on the innermost bark (phloem). As the larva feed, they girdle the tree, killing it from within. They cause significant damage in pine and spruce species.

Mountain pine beetle, spruce beetle, Douglas fir beetle, Englemann spruce beetle and weevils are examples of woody tissue feeders. Initially the insects attack unhealthy, weak and old trees, but as their numbers increase even a healthy tree cannot overcome them. Some bark beetles act as vectors of disease such as Dutch Elm Disease.

Ambrosia beetles are of economic concern and are wood destroyers of a different sort. They belong to the same family as bark beetles and have many of their habits but they do not feed on the wood itself. They cultivate and feed on a fungus that grows on the tunnel walls that they dig. The fungus further penetrates the sapwood, cutting off water flow. The fungus also creates a blue stain that degrades the value of the wood. Ambrosia beetles will attack living or dead trees or fresh cut logs.

Sucking Insects feed on plant fluids. These insects do not usually kill the tree directly. However, they do reduce the supply and movement of food and water within the tree, and so reduce growth. Tree diseases are also transported by these insects. The insects produce galls, swellings and deformities on the main stem and branches. These are formed from the injection of digestive juices into the host while the insect feeds.

Balsam woolly adelgid, coolly spruce gall adelgid, aphids, scales, and cicadas are examples of sucking insects.

It is worth noting that trees have evolved some defenses against insect attack. Some trees have static defenses such as physical or chemical barriers to limit insect feeding.

• Some trees have leaves with thorns that deter defoliators
• True firs can actually interrupt or delay insect development. They have a substance in their tissues that mimics the juvenile hormones of insects, thus preventing immature insects from developing to adults.
• Long-lived trees such as yews, red woods and cedars have effective chemical defenses as compared to short-lived trees.

Some trees have more dynamic defenses that are initiated by the insect attack.

• Conifers can mobilize chemical defenses to the attack site where the resin can kill or repel the insect. They "pitch out" or secrete a thick liquid into wounded areas. The resin contains terpenes or phenols which are toxic or interfere with bug digestion.
• Larch trees that are attacked by defoliators, will grow new needles that are smaller, tougher and contain lower concentrations of nitrogen, which is less palatable to the insect.
• Some trees (after defoliation) may create more tannins to make their new leaves more indigestible or even add enzymes to their leaf composition to interfere with bug digestion.

This kind of chemical defense is very costly in terms of energy. Stressed trees may not have the resources to launch these defenses. Tree defenses are a result of years of co-evolution with the insect. Trees that are attacked by introduced or exotic pests have not had a chance to adapt, and so are vulnerable.

PEST MANAGEMENT

Pest management is an integrated approach that avoids economic damage from insects and minimizes adverse side-effects to the environment. The comprehensive scope of pest management includes pest population reduction, applied ecology and economic and social acceptance. There are two main processes in pest management, prevention measures and control measures.

1. Prevention:

The concept of prevention centres on making the environment unfavourable to the pest.

This will keep the population low so outbreaks are less likely. Prevention is based on applied ecology which is the study of the relationship between the insect and its environment. The main method of prevention is the use of silvicultural practices.

Silvicultural practices can be used to change the structure of the forest in a manner that effects herbivorous insect populations negatively or the pest’s insect enemies positively.

Many insect problems result from forest stands that are stressed or unhealthy. Vigorously growing trees are less subject to insect injury than slow growing ones. The copious flows of resin and sap in healthy growing trees overwhelm phloem borers and the fast growth shortens each developmental stage of the tree, reducing the time it is vulnerable to insects targeting that stage.

Using silviculture methods to improve the health of the stand and increase the growth rate of the trees will help keep pests under control. Berryman 1 lists the following approaches:

1. Select tree species that are optimally adapted for the sites.
2. Use harvest methods adapted to the tree species that are being replanted and that mimic the natural disturbance patterns. Strong healthy seedlings are less prone to attack.
3. Remove diseased or unhealthy trees and logging debris that can act as breeding grounds for pests (salvage logging).
4. Encourage diversity of species. This helps to limit pest spread.
5. Use thinning, fertilization and prescribed fire to enhance vigour.
6. Prevent trees from becoming overmature by harvesting before this state is reached.

Good forest management equals good pest prevention. The healthy diverse forest that results from good forestry practices is better able to resist pest attacks.

2. Control

Once prevention has failed, control measures are required. There are two strategies for controlling an outbreak: a) containment and b) suppression.

Containment involves erecting barriers to the expansion of the pest population. Some examples of containment actions are using bait trees and harvesting a buffer zone. At a national level, quarantines and embargoes are used as methods of containment. Containment only works if the pest outbreak is clearly delineated. Success in containment is limited.

Suppression involves reducing insect damage by reducing the pest populations. Methods used include:

• chemical pesticides: highly regulated and controlled (usually only successful with certain insects, ie defoliators, and over time insects may become resistant).
• microbial insecticides: viruses/bacteria.
• behavioural chemicals: pheromones-attractants and repellents.
• genetic manipulation: releasing sterile males to control breeding.
• mechanical methods: harvest infected trees-then peel and burn, log and mill rapidly, submerge or water logs, host trees around infestation are removed (food removal), trapping.
• Biological control – see below.

Biological control refers to using natural insect enemies to control pest populations. Enemies can include:

• vertebrate predators such as birds or mice,
• arthrodpod parasitoids such as the tachinid fly,
• arthropod predators such as spiders and mites
• pathenogenic organisms such as bacteria

These enemies control their insect prey at different times and places in it's life cycle. This complexity suggests that preserving the diversity of natural enemies would be prudent. Enhancing enemy populations is not a one shot solution, as natural predator-prey cycles tend to adjust over time, making repeated interventions necessary. Foresters also look at what else is limiting enemy populations such as nesting sites for cavity nesting birds, mice and vole nest sites (leave debris) and undergrowth diversity.

Highly selective control techniques are preferred as these techniques target the specific pest species, with little effect on other components of the ecosystem. Pheromones and viruses are selective methods. The aim here is to drop the pest population down to a level where natural enemies can keep it under control.

Broad- spectrum control techniques, such as some chemical pesticides, may impact many non-targeted species and even stimulate the outbreak of a secondary pest. Forests are sensitive to persistent broad-spectrum pesticides. These are chemicals that do not break down into harmless substances but bioaccumulate in the food web. Unlike agricultural systems, forests depend on nutrient cycling, regulation by predators and host resistance that are natural parts of the system. Due to this important complexity, broad and or persistent control measures are very limited in their use today.

Pests can evolve resistance to repeated suppressive tactics. Agricultural use of chemical pesticides has resulted in resistant pest populations. Although the continuous use of pesticides in forestry is rare, this kind of resistance has developed in eastern North America where forests were sprayed in consecutive years for spruce budworm.

Foresters usually use an integrated approach to pest infestations, utilizing several control measures. Preventive strategies are preferred as they are more permanent and they are compatible with the ecosystem-based forest management that is developing today. Unfortunately, population explosions do occur that are beyond the scope of prevention. Caution and foresight must be applied to any pest management strategy. Ecosystem, economic and social implications must all be considered carefully.

FOREST MANAGEMENT AND INSECTS

Poor forest practices can create an environment that enhances insect pest populations. For example, high levels of debris after logging, unhealthy seedlings from soil compaction or poor species selection all provide a ready food source for insects.

Good forestry is really a form of indirect insect control. Stressed trees are vulnerable to insect attack, so activities such as removing infected trees, mixed age stands, planting resistant species and thinning all help to keep trees vigorous and insect populations under control.

Detection is the first step in the forestry pest management process.

• Foresters are always on the lookout for symptoms of insect infestations such as defoliated trees, yellow or red tops or crooked stems.
• A careful examination leads to a diagnosis as to the pest involved.
• The next step is to make a prognosis – predicting the scope and seriousness of the insect problem. This requires detailed knowledge of insects and population ecology. There are forest entomologists and computer forcasting models that can help with this. Most forestry companies will bring in experts from the MOF and CFS to help with prognosis and control suggestions.
• The prescription or planned activities based on the information can be considered either prevention, or control measures, or a combination, depending on the level of immediate threat.
• If the situation is serious, all of the information gathered forms the basis for a management plan for long-term prevention and control.

Technology is providing increasing support for the forest planner.

• Aerial surveys help determine type and location of pests .
• Hazard ratings that result from survey data help to determine how resources should be directed.
• Computer models are being developed to determine how bark beetle epidemics will behave.
• Satellite technology is being used to track infestations over time. In the future it is hoped that satellite images will be able to detect trees that are stressed from a current attack, before the crowns turn red.
• CFS conducts pest surveys yearly throughout BC.

All of these tools help the forest manager to focus time and resources efficiently.

These tools and processes are helpful in managed forests. However, most of the forests in Canada are unmanaged. In truth, these forests are not totally natural, in the sense that fire suppression has altered the natural forest fire cycle. Fire plays a role in controlling pest populations by removing dead and unhealthy trees. This intervention of fire suppression has enhanced the environment for insect populations significantly.

SUMMARY

Insects play many important roles in the forest ecosystem, such as pollination, nutrient cycling and food sources. People sometimes label forest insects as "pests" when they compete with us for resources and negatively impact the economic or aesthetic aspects that we value.

The insect-pest problem is one of serious magnitude. Insects kill enough timber in the US each year to build 600,000 five-room houses. The numbers in Canada are also staggering. Insect infested areas are lost to us for decades in terms of the values we desired. They also act as a population centre for spreading the problem and they are an extreme forest fire hazard.

Pest management in a forestry context is much more complicated than in an agricultural setting where artificial inputs (fertilizers etc) help to keep the system going. Forest ecology must be understood and considered with care as the forest is dependent on the natural cycling of nutrients and other ecological processes. Applied ecology must therefore be the basis of forest pest management strategies, with its emphasis on natural enemies and environmental conditions to help keep damaging populations under control.

Bibliography

Principles of Forest Entomology. Barrows Knight, Fred. Heikkenen, H. J. 1980. Magraw Hill Book Company. NY. Forest Insects. Berryman, A. 1986. Plenum Press, NY. A Field Guide for Pest identification Canadian Forestry Service. 1992. Managing your woodlands. Cdn Catalogue in Publication Data A Field Guide for Site Identification. Land management Handbook 28 1994 Ministry of Forests. 1994. Cdn Catalogue in Publication Data

Ministry of Forests
Forest Health Survey Guidebook
Bark Beetle Web page
Gypsy Moth Web page
Forest Health Web site. November 2000.
NRCAN. Major Forest Pests.

Landscape Ecology and Natural Disturbance. Ministry of Forests Research Program. Parminter, John. 1997. Extension Note 10 From Ministry of Forests Web Site Introduction to Forestry. Sharpe,G. Hendel,G. Allen,S. 1996. McGraw-Hill Series in Forest Resources.