Feature Article - Trees and Pests
By Bruce W. Hagen
Trees are subject to insect or disease pests for a number of reasons. They may be inherently susceptible, or in a vulnerable condition (predisposed by stress), at a favorable stage of development (succulent foliage), or pest development may be favored by the current weather conditions. For example, leaf diseases such as anthracnose, leaf spots, needle casts, and rusts, and are prevalent on developing tissue during rainy or humid periods. The level of damage will depend on the host-plant resistance, the nutritional content of the host tissues, the absence or abundance of natural enemies, the level of pest virulence or aggressiveness, or any combination of these factors. Most pests are fairly host specific, usually attacking one or several species within a single host genus. Only a few pests have a broad host range.
Native versus introduced
Native pests can, at times, become numerous and destructive in response to favorable environmental conditions, high host populations, or when their natural enemies are scarce. Native pests outbreaks are typically short-lived and sporadic. However, serious and widespread damage can occur if environmental stress becomes severe during the outbreak. For example, FTC defoliation and prolonged drought can lead to widespread oak mortality; bark beetle outbreaks will also occur during prolonged droughts; woodborers and canker diseases are also more prevalent on drought-stressed trees.
Introduced pests are generally more destructive because their hosts in this country often lack effective resistance and, more importantly, they arrive without their natural enemies. Notable examples of introduced pests include Dutch elm disease, oak wilt, butternut canker, white pine blister rust and gypsy moth. Most introduced pests are not easily managed, even with pesticides. At best, pesticides offer only a temporary solution. Biological controls, the successful introduction of effective parasitoids and predators is the best long-term solution for managing introduced pests. The most practical means to deal with potential introduced pests is through prevention-exclusion, and regulatory control. Once a pest is introduced, control practices, population management, and proper sanitation are used in combination the reduce losses.
Abiotic factors causing tree stress include drought, poor soil aeration, flooding, poor drainage, excess irrigation, irrigation temperature extremes, mineral imbalances, air pollution, wind, shade, etc. Biotic factors causing stress include defoliation (insect or disease), excessive pruning, root severance, competition, age, girdling, roots, etc. Stress has a major impact on growth but photosynthetic capacity remains largely unchanged until stress becomes severe. At this point, both growth and photosynthesis are impacted. Severe stress leads to energy depletion and suppressed levels of natural defensive compound and increased susceptibility to opportunistic pests. Severely stressed trees are generally poor hosts for sap- and leaf-feeding insects because of low tissue water content, poor nutrition, and higher levels of defensive chemicals.
A common misconception is that unstressed trees are more pest resistant than stressed trees. While it is true that severely stressed trees are more susceptible to opportunistic pests, unstressed trees may not be more resistant than moderately stressed trees to pests. The notion that stressed trees are more susceptible to pests is based on the assumption that stressed plants have less energy to allocate for defense. Numerous studies have shown that moderate stress typically enhances pest resistance by increasing energy reserves and the production of defensive chemicals (allelochemicals).
The "attractiveness" or "palatability" of a plant to a pest is determined not only by the concentration of toxic or inhibitory compounds but also by the amount of available nitrogen in their growing tissues. A pest's survival and abundance depend on the relative availability of these compounds. Nitrogen, an essential compound of protein, is a limiting nutrient for most plants, insects and fungal pathogens. Availability of nitrogen begins to decline with maturity. Phloem-feeding insects, such as aphids, psyllids, whiteflies, and scales, feed on nitrogen-rich sap in the phloem moving to new growth.
Young leaves are more suitable or nutritious to the insects and pathogens that are adapted to their host's defensive chemicals because they contain higher water and protein concentrations and because their protective leaf cuticles are not fully developed. As leaves age, their nutritional quality declines and their toughness increases, as do concentrations of other allelochemicals such as tannins and lignins. Leaf tissue gradually becomes less digestible to many chewing insects as a result of increasing levels of lignin (lignin, a natural component of wood and leaves, stiffens wood and toughens leaves). Feeding typically lessens and insect populations diminish as leaf tannins and other defensive compounds increase. However, some pests, such as conifer sawflies, prefer mature or over-mature foliage because some defensive compounds may decrease with age.
Many serious pest problems are more prevalent in older trees. As trees age, their mass (respiratory demand) increases, while their photosynthetic capacity (foliar surface area) changes little or decreases. Ultimately, energy demand for life-sustaining processes exceeds energy production. When that occurs, energy for defense is largely diverted to meet metabolic demand and maintain growth. Thus, tree health and resistance to pests decrease with age. Management practices should reflect these changes.
Assessing tree health
Tree health and pest resistance depend on photosynthetic output and adequate energy reserves, which allow for replacement of lost or damaged tissue, effective response to invading pathogens (compartmentalization), and the production of allelochemicals that increase pest resistance.
Tree health usually can be assessed by overall appearance: color, density, and retention of the foliage, shoot length, annual increment, live-crown ration, rate of wound closure, and relative freedom from serious pests. Rate of growth, however, is not always a reliable predictor of pest resistance. It is a mistake to assume that rapid tree growth is a sign of good health. Contrary to what most people believe, rapidly growing trees often are less tolerant of environmental stress and more susceptible to pest problems than slower-growing trees. The reason is that in rapidly growing trees, much of the available energy is used for leaf and shoot growth rather than for root growth, accumulation of energy reserves, and production of defensive chemicals. Low to moderate stress, which slows growth without significantly impacting photosynthesis, may result in increased carbohydrate storage and improved pests resistance. Symptoms of severe stress, though, include chronic pests, secondary pests, thinning crowns, off-color foliage, epicormic sprouts, dieback, and decline.
This article was excerpted and edited from "A More Proactive Approach to Pest Management" by Bruce W. Hagen, California Department of Forestry. His original article appeared in the April 2004 issue of Arborist News, a publication of the International Society of Arboriculture.