Protecting Future Forests From Insects

William J. Mattson, principal insect ecologist, North Central Forest Experiment Station, Forest Service, East Lansing, MI.

Protecting our world's future forests from insects may be far more important than it is today because burgeoning world populations will place much greater demands on forests for both recreation and raw materials. As a result, a unit of loss to insects in these more precious forests will be far more costly to society than today.

To combat future insect problems, innovative insect management methodology that stresses prevention rather than cure, and, at the same time, is inexpensive, long lasting, and environmentally safe must be developed. Future research on forest insects will focus on:

Developing and enhancing inherent plant resistance;

Deploying resistant plant varieties in a manner that minimizes the evolution of adaptations by insects to overcome resistance;

Employing sophisticated integrated pest-management strategies that more effectively use inherent plant resistance, natural enemies, pesticides-biocides, and behavior-modifying practices that reduce insect host finding and acceptance;

Developing more accurate knowledge about the relationship between crop losses and insect abundance so that forest managers know precisely when it is necessary to suppress insects;

Maximizing the activities of all those beneficial insects that contribute positively to the vital process of forest ecosystems.

Tree breakage and death are common after fusiform rust infection sweeps through a stand. This 14-year old slash pine plantation shows heavy damage.

Developing and Enhancing Inherent Plant Resistance

Increasing basic understanding of how plants resist insects will continue to be an important area of research because plant resistance is clearly the ideal method for combating insect problems. Once developed, it is inexpensive to employ, it is environmentally safe, and can be long lasting if used wisely.

Revolutionary developments in methods for altering plant genomes will allow for more rapid development of resistant plants through insertion of those genes affecting the expression of resistance traits. Moreover, mass propagation of these new, more resistant plants through new methods such as tissue culturing and optimal growth techniques will speed up their widespread planting in the field.

Once the mechanisms of resistance have been identified, it will be possible to enhance their effectiveness by planting trees carrying these desirable traits in those particular soil and climate combinations where their expression will be maximal. Furthermore, if expression of resistance also is affected by plant age and other factors such as moisture stress, this information will be valuable to forest managers in determining the need for insect suppression when plants may be temporarily susceptible.

Successful Use of Resistant Plants

The measure of success in resistant plants is their ability to hold up at least until they are ready for harvest. In other words, the goal is to prevent insects from evolving counter adaptations that allow them to overcome the resistance. Deployment has at least two components. One is the diversity of the tree varieties or the number of different kinds of resistance simultaneously employed against the insect. The second is the density and dispersion of these different varieties. For example, how many trees are planted per unit area, and what is the pattern of planting these different varieties? Should they be randomly mixed, grown in checkerboard-like patches, or in some other pattern?

The stability of resistance its life of usefulness against a pest insect depends on how the resistant plant varieties are deployed in the forest. For example, plant pathologists are learning how multi-lines of grains lower losses to pathogens (disease-causing agents). Likewise, agricultural entomologists are learning how the deployment of various varieties of resistant wheat affect the Hessian fly.

Improved Integrated Pest Management

The integrated management of tree pests such as insects is a valuable concept that has not yet been well grounded in practice. In the future, however, it will be standard operating procedure for dealing with both insect and pathogen pests in forest environments. Such integrated pest management (IPM) will start with the close matching of resistant trees to the climate and soils where they are best adapted. It will be followed by sophisticated planting strategies that mix varieties of resistant trees at spacings and clusterings that disfavor the principal pests.

Next, IPM will employ practices that enhance the effectiveness of natural enemies (parasites, predators, disease organisms) of the principal insect pests. For example, it may be necessary to establish a nectar-bearing plant species among the forest trees for the purpose of feeding parasites and predators to lengthen their lifespans and increase their effectiveness in killing the pest insect. Judicious use of pesticides and biocides that are more pest specific and safer in the environment also are part of the bioengineering of more virulent insect pathogens such as viruses and bacteria may contribute significantly to this tactic. Likewise, the bioengineering of more resistant plants whose biochemical compositions will lower the immunocompetence and suppress the detoxification systems of pest insects is also part and parcel of this tactic. Furthermore, greater use will be made of behavior modifying tactics such as:

Chemicals, colors, and shapes that attract pest insects to traps;

Chemicals that repel and disrupt pest insects in their host and mate finding activities;

Modifications in tree and forest structure so that light, temperature and other conditions are nonoptimal for insect oviposition, feeding, mating, and resting.

Relationship Between Losses and Pest Abundance

To successfully manage pest problems, knowing the relationship between crop losses and pest densities is necessary. This information is not yet available in a substantial form for any major forest insect. Why? Because in the case of trees, which can live for hundreds of years, it is costly to measure insect density and the concomitant impact over the long term.

In the future, such research may be easier to do because most future forests will be grown for shorter periods before they are harvested. It als, may be easier to measure insect densities as new insect monitoring techniques such as pheromone traps are more effectively employed. Currently, these techniques show great Promise for estimating insect densities, especially when populations are seemingly sparse. Research on novel methods for monitoring and measuring insect populations will be as important in the future as it is today.

Research into the impact of insects on forest resources may be more appropriately done through computer simulation studies with supporting research in key areas of plant nutrition and physiology and silviculture. Developing realistic models of insect impact on forests undoubtedly will become a major thrust in future years and will serve to energize and focus experimental research on the relationship between tree losses and pest density.

Maximizing Effects of Beneficial Insects

Although one usually thinks of insects as detrimental to forest ecosystems, quite the opposite is true. There are far more beneficial and innocuous species than pests. In the future, there will be even greater need to know about the beneficial contributions of insects and the means for enhancing them.

Parasites and predators of pest insects have already been identified as one group of beneficial insects whose biology, ecology, and effectiveness must be enhanced.

Another group often overlooked is the pollinating insects in forest ecosystems. The seed crops of many temperate and tropical deciduous trees, shrubs, and herbs are vitally dependent on adequate populations of pollinators. Because some forests may be more important for their recreation and wildlife resources than for their wood, berry-producing trees and shrubs and their pollinators should be studied.

Soil invertebrates as a group, along with soil micro-organisms, release nutrients and energy that are trapped in the organic debris that accumulates on the forest floor as litter. Without their activities, forest growth would decline and eventually come to fluctuate about a much lower average level of productivity. So their contributions need to be much better understood and managed. Methods for deciding whether soil communities are indeed fully stocked with the proper numbers and kinds of different organisms to get maximal nutrient and energy turnover are needed. For example, are there adequate numbers of large, soil-litter mixing species such as the various kinds of worms, craneflies, termites, and so on? If not, should some new species be introduced to the forest system?

Finally, the role of invertebrates in forest stream and river systems needs to be better addressed. They, along with micro-organisms, are the foundation for all sport fishery resources in our Nation's forests.