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Tree Pest Management Philosophy
The City’s pest control philosophy is based on minimizing insects
and diseases that threaten the health of the trees. Generally, pest eradication
is not the goal because most pests can be present and not cause a serious
threat to the health of the tree. The City does respond to all the specific
pest control requests that are received.
Like most cities, Redwood City has experienced insect and disease problems
with many different tree species. The most severe infestations gain both
the public’s and the City’s attention. Redwood City has about
17,000 street trees consisting of twelve main species and up to 100 other,
less common species. Not every tree is susceptible to all insect or disease
pests, and few trees experience a serious infestation.
Property owners have different tolerance levels about the presence of insects,
diseases, and the resulting conditions they cause. Property owners also
have different tolerances and desires over the use of pesticides for the
control of insect or disease pests. Some residents object to the use of
any pesticides, while for others pesticide use is acceptable.
The City practices an Integrated Pest Management (IPM) approach which focuses
on the pest and the tolerance level of the pest . This practice reduces
the amount of chemicals used in a pest control response. For example, instead
of a calendar schedule approach (i.e., spray all trees on a regular schedule
regardless of the presence of pests), the City prefers a monitor-and-response
approach, using the least-toxic form of pest control. One example of an
effective use of non-chemical pest control was the City’s response
to the Ash Whitefly which attacked Ash and Pear trees. Several years ago,
the situation became intolerable as the insects caused “insect clouds” around
the host trees. After different control application efforts failed, the
City worked with the University of California to introduce the Ash Whitefly’s
natural predator, a parasitic wasp, which controlled the Whitefly in about
60 to 90 days. This pest and predator have remained in balance and no further
outbreaks have been experienced.
This illustrates a key benefit of the IPM approach: it considers the natural
predator of a pest. When a chemical spray is introduced, it can kill both
the target insect pest and that insect’s natural predator. If the
target insect pest reproduces faster than the predator, the infestation
will never be in control balance.
In other tree pest situations, the insect pest is so prevalent that the
only successful control is the use of pesticides. An example of this is
the Aphid infestation present on some Modesto Ash trees. Out of 1,350 Ash
trees approximately two-to-five percent (between 25 and 75 trees) have experienced
significant infestations. The insects secrete a sticky substance or “honeydew” that
can coat sidewalks and vehicles, causing a nuisance. Large-scale infestations
create such a great volume of honeydew that the area beneath the tree becomes
unusable. In these cases, the City will respond with pesticide treatments
to immediately reduce the insect population and decrease the subsequent
honeydew production.
Each pest control effort is based on the level of infestation present,
available control options effective on the pest, and the preference of the
adjacent property owner. Under normal circumstances, the City does not spray
the tree unless the adjacent property owner requests a spray. There are
a few circumstances where the City does spray for the control of insect
pests without the consent of the property owner. An example is the use of
Bacillus thuringiensis (Bt), a microbial insecticide, to control Elm Leaf
Beetle. Elm Leaf Beetle is present in annual infestations and causes severe
stress to the Elm Trees, which significantly increases the possibility of
the trees being infected with Dutch Elm Disease – a deadly, infectious
disease. Bt is one of the least toxic, host-specific treatments available.
During years of severe infestations, all Elm trees in a neighborhood may
receive a Bt spray treatment. Residents are advised of any pest control
activity.
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