Bronze Birch Borer: A Scourge of White-Barked Birches, Or Not?
(the case for returning white birches to Midwestern landscapes)

Daniel A. Herms, Associate Professor
Department of Entomology, Ohio State University
Ohio Agricultural Research and Development Center
Wooster, OH

Wood-borers can be extremely destructive and difficult to manage, and bronze birch borer (Agrilus anxius) is no exception. However, research has shown that bronze birch borer can be managed effectively, and white-barked birch trees grown successfully in the Midwest, even in low-maintenance landscapes. Use of resistant tree species coupled with fundamental tenets of plant health care is the key to success.

Bronze birch borer is native to the hardwood and boreal forests of the northern United States and Canada, where its primary hosts are paper birch (Betula papyrifera) and grey birch (B. populifolia). Yellow (B. allegheniensis) and sweet birch (B. lenta) are attacked to a lesser degree. Over most of its range, bronze birch borer has one generation per year, although it may require two years to complete development in the northern reaches of its distribution. Adults emerge from infested trees beginning early May to mid-June (late May in Ohio), depending on locale and weather. The presence of small (1/8”), D-shaped emergence holes in the trunk and branches is a sure sign of infestation. Trees as small as one inch in diameter can be colonized.

Emergence of adults peaks about two weeks after it begins, and is largely complete after four weeks, but stragglers continue to emerge for another month. After emerging, adults feed on foliage for one or two weeks before becoming reproductively mature, but this defoliation is imperceptible. Eggs are laid on the bark surface or within cracks and crevices. Immediately upon hatching, larvae chew their way directly into the tree and begin feeding just under the bark on phloem tissue, where they form serpentine galleries. Larvae continue to feed through summer into fall, largely completing their development prior to overwintering. Pupation occurs in the spring, followed by adult emergence, thus completing the one year life cycle.

Host Impact
Bronze birch borer is commonly referred to as a cambial feeder, which is technically correct. However, the cambial zone is much too thin to be a sufficient resource, and they feed primarily on phloem tissue, which girdles the tree. Phloem girdling disrupts transport of carbohydrates from the canopy to the roots, which eventually starve. As the root system declines, water uptake is reduced, especially during periods of drought, resulting in gradual branch dieback from the top down as the canopy desiccates. Trees often decline over several years, but death can occur in one season when trees are overwhelmed by large numbers of larvae.

It is commonly stated that bronze birch borer colonizes the top of the tree first, and that infestations can be eliminated by pruning the dead branches. These are myths. Research has shown that bronze birch borer colonizes the entire trunk at random. Upper branches only die first. Furthermore, larvae develop only in living tissue. Hence, pruning dead branches has little value as a pest management tactic, but can improve the appearance of infested trees.

Borer-Resistant Birch Species
In 1979, Dr. David G. Nielsen initiated a long-term, large-scale study of birch resistance to bronze birch borer at The Ohio State University’s Ohio Agricultural Research and Development Center (OARDC) in Wooster. The study included seven species, and a total of 1400 trees planted in a randomized complete block design. Species tested included European white birch (B. pendula), mountain birch (B. pubescens), which is also native to Europe; two Asian species, monarch birch (B. maximowicziana), and Asian white birch (B. platyphylla); as well as three species native to North America, paper birch (B. papyrifera), grey birch (B. populifolia ‘Whitespire’), and river birch (B. nigra).

The results of this study were striking. By 1986, bronze birch borer had killed every individual B. pendula, B. platyphylla, and B. pubescens. Only 14% of B. maximowicziana survived 10 years, and all 200 were dead by 1999. The native species, however, fared much better. In 1999, after 20 years, 74% of B. papyrifera, 76% of B. populifolia ‘Whitespire’, and 98% of B. nigra were still alive.

The high survival rate of paper and Whitespire birches over the 20-year study is remarkable, as it occurred in the face of several severe droughts and a massive borer outbreak that killed 800 neighboring Asian and European birch trees, yet without the benefit of irrigation or insecticide applications. This strongly suggests that these native white-barked birches can be grown successfully in Midwestern landscapes with only minimal care. River birch showed no evidence of bronze birch colonization, confirming its immunity to bronze birch borer. Of the two the white-barked birches, paper birch proved to be horticulturally superior to Whitespire birch over 20 years, as it grew faster, had whiter bark and a more graceful, symmetrical growth habit.

These results stand in stark contrast to historic claims of bronze birch borer resistance in Asian white birch, as well as recently published claims of resistance in monarch birch, and underscores the pitfalls of conclusions based on anecdotal observations of a few individual trees. Claims of bronze birch borer resistance should be viewed with skepticism until they are rigorously evaluated in replicated studies that include susceptible species as a basis for comparison. Himalayan birch (B. utilis var. jacquemontii) has also been touted as borer resistant, even though it too has been observed to be severely infested.

The high level of resistance in the North American species, and the great susceptibility of the Asian and European species may be explained by their evolutionary history. The native birch species would have been driven to extinction by bronze birch borer, which is also native to North America, had they not evolved strong defenses. On the other hand, since the Asian and European species have not evolved with bronze birch borer, natural selection has not equipped them with defenses specifically targeted against bronze birch borer. If bronze birch borer was introduced accidentally to Asia or Europe, it would have the same devastating impact on birches there that its close relative, the emerald ash borer (Agrilus planipennis), is having on native ashes here in North America. A clear pattern is beginning to emerge regarding tree resistance to wood-borers: where there is no coevolutionary history, there is no resistance.

Environmental Stress and Tree Defense
In North American forests, bronze birch borer acts as a secondary, opportunistic insect that only colonizes trees weakened by biotic or abiotic stress, which is thought to weaken the natural defense system of trees. Although relatively little is known about mechanisms of borer resistance in deciduous trees, which generally lack the resin systems of conifers, it has been hypothesized that deciduous trees resist wood-borers by means of rapid wound-induced responses of phloem tissue. Phloem responses to feeding (and other) injury consist of two phases: (1) immediate synthesis of specific, low-molecular-weight secondary metabolites and defensive proteins, which may slow the growth of the invading organism; and (2) subsequent formation of wound periderms, which isolate the wound, inhibit the spread of the colonizing organism, and reestablish phellogen and cambium integrity.

Wound periderms ("callus" tissue), consist of the phellogen (a zone of meristematic cells), phelloderm (parenchyma cells formed inwardly from the phellogen), and phellem (cork-like tissue formed outwardly from the phellogen). As periderm cells differentiate into xylem, phloem, and cork, only the meristematic cells of the cambium and phellogen continue dividing. Phellem cells accumulate large quantities of suberin (a heteropolymer composed of aliphatic components attached to a phenolic matrix), lignin, and other phenolic compounds on their cell walls, and subsequently die. As the phelloderm gives rise to new phellem, layers of dead phellem are compressed to form the corky periderm surface.

Wounds created by larval feeding stimulate wound periderm formation which may encapsulate and/or intoxicate small larvae within suberized tissue with high secondary metabolite concentrations. In effect, larvae may be in a developmental race against the tree. Larvae move through the phloem only as fast as they can feed. Wound-induced accumulation of defensive metabolites may slow larval development, facilitating encapsulation of larvae by rapid callus formation. Implicit to this hypothesis is the prediction that the rate of wound periderm formation necessary for resistance is a threshold value equal to the maximum rate of larval movement through the plant.

An experimental field test was consistent with this hypothesis. Experimental drought stress decreased resistance of paper birch to bronze borer, which was highly correlated with rate of wound periderm formation. Furthermore, the relationship was described by a negative power function, which is consistent with a threshold response. Resistance disappeared as rate of callus formation dropped below 0.02 mm/day. Rate of wound periderm formation was also correlated with rate of net photosynthesis, dropping exponentially as photosynthesis rate declined, with the rate dropping below 0.02 mm/day only in severely stressed trees with very low rates of photosynthesis. Even moderately stressed trees maintained high resistance to bronze birch borer.

These patterns are consistent with previous assertions that strong defensive reactions depend on rapid transport of current photosynthate from the canopy to the trunk, as these responses quickly deplete local energy reserves. Stress may compromise borer resistance by limiting the flux of energy necessary to drive these responses. Serpentine welts or ridges visible on the bark surface resulting from callus formation are evidence of a strong wound response. Failure of emergence holes to form in the vicinity of bark ridges is a common occurrence, indicating that larvae died before completing development. If callus tissue forms slowly, larvae can feed fast enough to stay ahead of the wound response.

This hypothesis was supported by a direct experimental test in which we found that trunk girdling, which interrupts downward translocation of photosynthate, dramatically decreased the rate of wound periderm formation and bronze birch borer resistance in the portion of the trunk below the girdle. However, wound periderm formation and borer resistance were actually increased above the girdle, possibly due to increased availability of photosynthate.

European and Asian birches also produce wound-periderm tissue in response to bronze birch borer attack, yet are killed anyway. Clearly, production of wound periderm formation is itself not sufficient for the expression of resistance, perhaps because larvae can move fast enough to escape the response in the absence of specifically targeted toxic metabolites. Because of the intimacy of interaction between trees and wood-borers, coupled with the debilitating effects of phloem destruction, trees should evolve specifically targeted, potent defenses against wood-borers. For example, insects (and pathogens) that colonize the vascular tissues of trees elicit rapid accumulation of low molecular weight, antibiotic compounds (phytoalexins).

Management of Bronze Birch Borer
Plant health care is the key to effective bronze birch borer management. Landscape design that considers host plant resistance and ecological requirements of the tree is the critical first step. Resistant species such as paper birch and Whitespire birch should be the white-barked trees of choice for Midwestern landscapes. Trees that are planted in sites to which they are not adapted will be stressed, making them mores susceptible to borer attack. For example, since paper birch is an early successional species adapted to full sun, planting it in the shade is asking for trouble. Research has shown that shade decreases the photosynthetic rate of paper birch just as much as drought stress does. Since biotic stress can also predispose trees to attack, it is also important to protect trees from defoliating insects such as gypsy moth and birch leafminer. Irrigation during drought is probably the single most important thing that can be done to protect both newly planted and mature trees. The previously discussed experiment found that one inch of irrigation per week was sufficient to prevent bronze birch borer infestation of paper birch during an outbreak that killed adjacent drought-stressed trees.

Trees that have been severely stressed by drought, defoliation, or transplanting may need to be protected with insecticides until they regain their vigor. To be effective, insecticides must be applied preventatively, in advance of borer attack. Historically, chemical control of borers has emphasized protective bark sprays timed to kill newly hatched larvae before they enter the tree. Systemic insecticides offer an alternative to protective bark sprays. In OARDC trials, preventive soil and trunk injections of imidacloprid applied in late April and early May provided complete control of bronze birch borer. Fall soil injections are also being evaluated, which offer a bonus by providing effective control of birch leafminer during the following spring. Imidacloprid is not as effective for trees that are already infested by borers, as phloem damage from borer activity interferes with uptake and translocation of the insecticide.

Summary Conclusions
Research in Ohio has shown that white-barked birches native to North America can perform quite well in low-maintenance landscapes. Survival of paper birch (B. papyrifera) and Whitespire birch (B. populifolia ‘Whitespire’) was quite high over 20 years (74 and 76%, respectively). Their high survival is quite remarkable considering that it occurred without the benefit of irrigation or insecticide applications during an extreme bronze birch borer outbreak and several droughts that killed 800 European and Asian birch trees growing amongst them in the same plot. Of the two native white-barked species, paper birch exhibited superior horticultural traits, growing faster and having whiter bark than Whitespire birch.

Plant health care is the key to effective management of bronze birch borer. North American birches planted in sites to which they are at least reasonably adapted will be inherently resistant. Irrigation during severe drought is the single most important cultural practice for maintaining borer resistance. Trees stressed by defoliation, drought, or transplanting can be protected with insecticides until they regain their vigor. However, research has shown that paper birch does not become susceptible to bronze birch borer until it is severely stressed; even moderately-stressed trees are resistant. Native white-barked birches deserve a prominent place in Midwestern landscapes.