Science in Action: Controlling Borers in Landscape Trees

Options available for controlling borers have actually increased over the past few years. While the traditional approach of spraying an entire plant with a chemical such as Lindane, a chlorinated hydrocarbon, has become less available to pest control operators other types of IPM (Integrated Pest Management) control strategies have blossomed on the commercial market. The problem for most applicators is that the "security blanket" of a traditional pesticide approach is disappearing and the research to support the newer products is not available to give applicators confidence that they will work.

The IPM model for borer control includes strategies such as cultural, chemical and biological methods. The most effective long-term control strategies are usually cultural in nature and should be our first consideration. Often times they are not used by commercial operators for a variety of reasons but usually get back to either economic reasons or lack of education on how to use them effectively.

 

Flat headed borer in small limb of peach tree. Sanitation would be removal of the small limb or cutting out
borers from larger limbs with a knife

When evaluating which method to use, we have to balance the efficacy of the control measure, it's economic benefit to the customer and the operator and impact of the control measure on the environment. Lets take a look at each component in the IPM scheme.

Cultural control options in traditional agricultural IPM include sanitation, crop rotation, tillage, host plant resistance and tolerance, mechanical or physical destruction of the pest and quarantine. Do any of these apply to controlling borers in landscapes?

Sanitation refers to the removal of infested plants or plant parts which helps reduce the level of plant pest in the urban landscape. This may mean removal of entire trees heavily infested or removal of limbs to reduce the infestation level. This can be done on an individual yard basis or community wide if there is a community "epidemic" of a particular pest.

Borer infested limb removed to save the tree from removal. A form of sanitation.

We have seen communities, or entire sections of a community, devastated by certain types of borers. Clear winged borers, like the ash or lilac borer, can be serious pests in this regard since these types of borers are known to attack healthy trees and the number of these insects in a community may dictate the degree of infestation and damage. Community-wide borer control efforts need the strategy of a community forester or city arborist to make these efforts work.

Crop rotation is the substitution of a crop (plant) with low pest susceptibility (host resistance) for a plant with high pest susceptibility. As an example, if we lose a tree to borers do we put the same type of tree back in that community or do we look for a reasonable alternative? As we begin planning a community or expanding a community are we checking to see what borers are problematic to the community or are we selecting plants only because they have aesthetic appeal? An example of this is the planting of weeping willows in climates like Las Vegas where they stress and become highly susceptible to attack by borers. These trees then become a source of infestation for other susceptible trees in the community. A proactive approach toward sound horticultural growth, balanced by future maintenance, requires public education and community coordination.

Weeping willow planted in Las Vegas with borer damage from clear-winged moth. For this reason
we never see weeping willows over about ten years old.

Quarantine is a legal restriction or exclusion of infested plants being brought into a community. On a state-wide basis, inspection stations are established that control the entry of infested plant material or documentation is required by the buyer that plant materials have been inspected, and found clean of, certain pest problems. Sometimes plant materials are simply not allowed into certain communities due to the highly virulent or infectious nature of certain pests. This has happened with elm varieties, and some of its relatives, known to be susceptible to Dutch elm disease.

Some arborists combine physical or mechanical control measures with their borer protection programs. Physical control can be as simple as noting borer activity in some young trees and carefully using a small knife to remove or kill the borer. This will work with borers such as some of the flatheaded types that that can be easily tracked and that feed near the surface of the trunk. A great deal of care has to be exercised so that the tree is not further damaged with the knife but with some experience it can be learned. Mechanical control includes barriers such as wraps that may help prevent sunscald and the physical entry of borers inside the tree. Some care has to be exercised that certain wraps don’t actually contribute to the problem by providing egg-laying sites for borers.

We are all most comfortable with cover spray applications of chemicals for borer control. This strategy was either to spray the plant periodically through the entire growing season or time the spray with some sort of insect trapping device. Cover sprays applied a poisonous, pesticide barrier to susceptible plants. The traditional approach was to apply a pesticide to the trunk and major limbs of trees either infested with borers or "threatened" with borer activity. This cover spray was a prophylactic treatment aimed at preventing the entry of the borer inside the tree. Once the borer entered the tree, cover sprays were ineffective.

Emerald ash borer damage to green ash which we do not have
 in southern Nevada

Our business practices, equipment and past education focused on chemicals for borer control. The problem with traditional cover spray applications of chemicals is that the chemicals traditionally used for this purpose pose a threat to the environment and human health, both by the nature of the chemical and how it was applied. This technique is still probably the most widely used one but is becoming more restricted with time. We have already seen Lindane become more restricted in its use and availability. Admittedly, using chemicals such as Lindane, Dursban, Thiodan and Sevin as cover sprays on large trees has its community environment and health drawbacks. The effectiveness of cover sprays always depended on a narrow window of time when the spray would be effective.

Applications of chemicals such as Metasystox R as a soil injection for borer control pose similar environmental problems. Applications of pesticides into the rootzones of plants pose an immediate threat to shallow groundwater supplies, wells and public health concerns.

 Application methods of pesticides such as trunk injection certainly minimize some of the environmental and community health concerns. Active injection systems exist, like Mauget, which rely on a pressurized system to inject pesticides such as Bidrin and Merit, into the tree's vascular system. Once inside the tree these chemicals become mixed with the vascular fluids and thus become systemically distributed. There are passive injection systems, such as trunk implants, that have no pressurized system but rely on the vascular fluids to carry the pesticide throughout the tree. This mixture of vascular fluid and pesticide poisons the immature borer feeding on the vascular tissue.
 

 How to Use Mauget trunk injection

 

In my opinion, never use systemic insecticides or inject insecticides systemically into fruit trees or other edible crops.

 

Entomologists have been concerned that trunk injections, made late in the protected borer's life cycle, would not be effective. They reason that borer feeding activity disrupts the flow of vascular fluids and prevent the pesticide from coming in contact with the borer. In the case of borers in an advanced stage of its life cycle, this may be the case. However in the very early stages of an infestation, injection may provide some measure of control.

The injection of pesticides into trees poses other problems such as the damage created when holes are drilled into the trunk. Research many years ago in the United States and Canada has both come to the same conclusions. If holes have to be drilled into trees then the holes should be as small as possible. The other piece of research conducted with injection equipment is that on larger trees, injection should be made as low on the trunk as possible. Injection holes drilled in the root flares provides better mixing and a larger trunk circumference to absorb the damage created from drilling the holes.