Toxicity of Insecticides and Determining the LD50  
      Kenneth J. Stein and F. William Ravlin
Department of Entomology
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061
      Insecticides and Labels
All insecticides and pesticides in general, purchased in stores today must by law, have labels with various bits-and-pieces of information. What are these labels?--and--What do they mean? Insecticide labels provide an extensive amount of information and indicate that the insecticide has been extensively tested, evaluated, and regulated by the Environmental Protection Agency. In fact, all insecticides cannot be legally registered, much less sold, without going through these procedures.

Among the information included on an insecticide label is the "directions for use", and perhaps more importantly, information with regard to the toxicity of the chemical. What then, is toxicity? Toxicity is the quality, state, or degree of being poisonous. Too often most people think of toxicity as poisoning that is caused by ingesting a small amount of some substance. However, almost any chemical is potentially toxic given enough of it and the right circumstances. In fact, every homeowner has cabinets in their bathroom, kitchen, or garage that contain bottles of substances, that if ingested, inhaled, or spread on the skin, may cause harm. Even something as seemingly innocuous as table salt, is potentially toxic, if sufficiently large quantities are ingested.

Determining the lethal dose (LD50)

How is toxicity measured? Toxicity is usually expressed as the lethal dose or LD50. The LD50 denotes the amount (single dosage of the substance by mouth) in milligrams per kilogram of body weight required to kill 50% of a group of test animals. As an example, let's conduct a hypothetical experiment: 100 rats are obtained, each of which weighs 1 kilogram (2.2 lbs.). (WOW!!! That's a big rat!!!) They are then fed 200 milligrams of chemical "X". You later observe that 50 rats (50%) die as a result of eating the chemical--the LD50 in this case is 200. In other words: if we took 100 rats, each weighing 1 kilogram and fed them 200 mg of chemical "X", we would expect to kill 50% of the population.

LD50 denotes the potency of a substance; the lower the number, the less of that substance is required to kill an animal. Conversely, the higher the number, the more of that substance is required. An example of the LD50's of two common household substances are table salt, LD50=3,300 (mg/kg), and aspirin LD50=750 (mg/kg). In comparison, the LD50 of acephate (an insecticide) is 1,494 (mg/kg). The LD50's of different substances can be easily compared and are represented on insecticide/pesticide labels by the signal words DANGER or POISON, WARNING, or CAUTION. These signal words are associated with different ranges of LD50s and hence different degrees of toxicity as listed in the table below.
Toxicity Category Signal Words
(required on pesticide label by EPA)
Oral LD50
Probable Lethal
Adult Human Dose
I--Highly Toxic DANGER and POISON,
plus skull and crossbones symbol
0 to 50 A few drops to 1 teaspoon
II--Moderately Toxic WARNING 50 to 500 1 teaspoon to 2 teaspoons
III--Slightly Toxic CAUTION 500 to 5,000 1 ounce to 1 pint (1 pound)
IV--Almost non-toxic CAUTION more than 5,000 1 ounce to 1 pint (1 pound)