The Dilemma Regarding Birch Trees

Bronze birch borer adult female laying an egg on the bark surface of a birch tree. (Photo Credit: Ohio State Coop. Ext.)

Bronze Birch Borer (Agrilus anxius) adults (440-800 GDD) are now actively searching landscapes for various species of Betula (Birch) to lay eggs upon. The adult beetles will be attracted to susceptible trees that are genetically defenseless or are compromised from stress (e.g., especially drought stress).

It is well known that the native River Birch (Betula nigra) has a strong resistance against the BBB. As a result, this species has been extensively planted in NJ landscapes during the past few decades to the extent of arguably being over-planted. And it is also widely understood that the non-native Asian & European birch species have virtually no or little resistance to the BBB. Therefore, the planting of these non-natives has rightfully been reduced over the decades. However, what about some of our other native species such as the paper, gray, & yellow birches. Is it best to avoid planting these species as well?       

River birch (Betula nigra) is a native birch that is strongly resistant to the bronze birch borer. (Photo Credit: Steven K. Rettke, Rutgers Coop.Ext.)

Is it best to avoid planting other native birch trees in our landscapes, such as the gray birch shown above? (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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Definition

The Plant Stress Hypothesis generally states that plants subjected to stressful conditions become more susceptible to attack by insects and diseases. This is often thought to be due to the plant’s increased suitability as a food or due to the reduced ability of the stressed plant to defend itself. It is commonly believed that when a plant is stressed it becomes better food for “bugs.” Is it correct to state that plant stress leads to pest outbreaks, or is the statement too simplistic?

The Experiments

A publication written in 1998 (Koricheva et. al.) summarized approximately 70 controlled experiments that were performed to study how distinct types of plant stresses affected insect/mite attacks. Diverse types of environmental stresses were included in these studies (e.g., drought stress, waterlogged roots, ozone exposure, excess shade, as well as many others). Plant nutrient stress & insect impacts has been researched but was not performed within these studies.

Plant roots stressed from waterlogged soils were included in these studies. Research also included other types of environmental stresses such as excess shade & ozone exposure. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

Dogwood leaves scorched from environmental drought stress. How does this plant stress impact insect herbivores? (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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The old saying that “everyone talks about the weather” is certainly true when it involves landscape plant managers. Professional landscapers/arborists and their clients are always discussing how various weather factors are affecting plants. The New Jersey weather trends can often be variable from one growing season to the next. How do rain, wind and temperature changes impact plants and their corresponding insect/mite pests?

The chart below lists four weather conditions & categorizes how these conditions positively or negatively influence some of the key landscape insect/mite pests.

TABLE: PESTS AFFECTED BY SPECIFIC CONDITIONS (Source: David Shetlar, Ohio State Cooperative Extension)

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Clearwing moth borers comprise one of the most damaging groups of insect pests that attack shade trees and shrubs. Controls must be accurately timed, since larvae tunnel under the bark most of the year, where they are unreachable by most insecticides. Artificial pheromones of many of the clearwing moths found in the landscape are available. These pheromones are a synthetic of the sex attractant emitted by many insects & are commercially available and recommended for the IPM manager.

Plant manager viewing contents of clearwing moths captured in pheromone-baited wing-trap. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

This green ash tree is showing severe symptoms from banded ash clearwing moths. Too late for pheromone traps to be of value. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

Adult male clearwing moths are attracted to the pheromone-baited sticky trap that helps determine: 1) if the borers are present in an area; and 2) when to apply insecticide sprays. Pheromone traps enable landscape managers to accurately determine when the first emergence of the egg laying borers are expected so that insecticides can be timed & applied effectively to susceptible tree/shrub hosts. Spraying the bark of the tree to be protected with a long residual insecticide (i.e., such as pyrethroids) should effectively kill the young borer larvae as they hatch from eggs & attempt to tunnel into the stems or branches.

Hypothesized flight path of an adult male clearwing moth towards the pheromone trap. (Diagram: Steven K. Rettke, Rutgers Coop. Ext.)

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Removing the Tree-of-Heaven (TOH):

During the winter, arborist companies will often take advantage of the slow season and perform needed tree removal services for clients. However, these professionals know when dealing with the tree-of-heaven (Ailanthus altissima) the off-season is only a suitable time for removal if a previous herbicide treatment was applied during the late summer or early fall seasons (i.e., mid-July to mid-October). Then during the off-season, the trees can be cut & removed from mid-November thru April (Figure 1).

(Fig. 1) The cutting & removal of this cluster of tree-of-heaven trees should only be done in the winter if herbicide treatments were applied during the late summer or fall seasons. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

If a tree-of-heaven (TOH) is cut & removed without first using an herbicide & waiting at least 30 days, then the tree will respond with potentially dozens of trunk sprouts & root suckers that could emerge 50-feet from a large parent tree. Applying the herbicide during mid-July to mid-October when the tree is most actively translocating photosynthates to the root system will allow the herbicide to move further distances most effectively through the phloem. This will help reduce problems with trunk sprouts & root suckers which this tree species is notoriously known to produce (Figure 2).

(Fig. 2) Herbicides to TOH should be applied from mid-July to mid-October. The photo shows the onset of fall coloration & indicates the end of effective herbicide treatment for the season. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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