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.
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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.
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| 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.
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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.
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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.
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.
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