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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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