Selecting Pesticides

When choosing a pesticide, the landscaper should select a product that is specific to the pest to be controlled in order to spare beneficial arthropods. Systemic insecticides are less likely to affect predators and parasites on plant surfaces. Insecticidal soaps may spare hard bodied predaceous or parasitic arthropods. Microbial insecticides such as Bacillus thuringiensis products (Dipel®, Thuricide® and others) are target-specific for certain groups of insects.  Specific miticides generally spare other arthropods.

Care must also be taken when using pesticides other than insecticides and miticides. Certain fungicides such as Benlate® will affect insectivorous fungi and other non-target organisms. Broad-spectrum insecticides often are useful when more than one pest is present, but they may cause a rapid resurgence of the primary or secondary pest. Pyrethroid insecticides can be more harmful to parasitic wasps than to the pests listed on  their labels and may result in secondary pest outbreaks. They are most effective against chewing insects.

Interpreting acute LD50 values. Another important consideration in choosing a pesticide is its toxicity. The  LD50 value of an insecticide is commonly used as a measure of its toxicity. The LD50 is defined as the single dose of a chemical that  results in a 50 percent mortality to a population within a specified time. It is expressed in milligrams (mg) of pesticide per kilogram (mg/kg) of the test animal’s body weight. Generally, the lower the number the higher the acute toxicity.

Example: LD50 = 50 mg/kg

Sometimes  this relative measure of toxicity is qualified by the species and sex of the test animal.

Example: LD50 male rat = 39 mg/kg

LD50 values can be further qualified by route of absorption or the way in which the pesticide was administered to the test animal.

Example: Acute oral LD50 male rat = 39 mg/kg
Example: Acute dermal LD50 male rat = 98 mg/kg

Acute oral (or dermal) LD50s for an experimental animal are useful indications of the probable relative toxicities of compounds to man and other warm-blooded animals. However, they are not an absolute representation of how much pesticide would be required to kill a human (as compared to a rat), nor the only important measure of the potential toxic effects which might occur with exposure to a given compound.  A number of pitfalls must be considered when interpreting acute LD50 values. These include the following:

  • Individual  variation (genetic, age, sex,  nutrition, hypersensitivity, etc.)
  • Difference in route of exposure (oral, dermal, inhalation)
  • Formulated  versus  pure  chemicals  (effects  of  solvents,  types of formulations, etc.)
  • Impurities of technical material
  • Possible cumulative effects from multiple exposures to the same chemicals (chronic toxicity)
  • Interactions  with  other  chemicals  in  the  individual’s  environment (drugs, environmental pollutants, etc.)
  • Past or present disease status

LD50s tell us nothing of the possible chronic effects of these compounds nor possibilities of sublethal toxicity. A high acute LD50 (low acute toxicity) does not mean that a compound can be used carelessly, or that it is safe as long as exposure levels are below that of acute toxicity.

Since  toxicity tests cannot accurately predict a chemical’s effects on humans, the only way to reduce the hazard is to minimize exposure to any pesticide. The only indicator of toxicity of pesticide on a product label is the signal word (CAUTION, WARNING, DANGER).