Learning Modules for Pesticide Applicators:

Pesticide Residue, Tolerance, and Registration

The use of pesticides is strictly controlled in the United States. Every chemical, which has possible use as a pesticide, is closely tested and reviewed before it is marketed. The laws controlling the use of pesticides on food or feed crops are more strict. The amount of pesticide remaining on the crop at harvest is carefully regulated.

Key points on this page

• Distinguish between deposits and residues.
• Understand both the positive and negative features of long- lasting residues.
• Explain a tolerance and the criterion involved in setting a tolerance.
• Determine the importance of "Days to Slaughter" and
• "Days to Harvest."
• Learn what information is important in registering a pesticide.

Pesticide Residues

The pesticide which is on the leaves, skin, or other surface right after application is the deposit. Sometimes the deposit can be easily seen, as with many dusts or wettable powders. At other times it cannot be seen with the naked eye. If the pesticide deposit remains on the surface for a period of time, it is called a residue. Some pesticides leave little or no residue. Heat, light, moisture, soil organisms, and other chemical reactions in the environment quickly break them down. Other pesticides are not quickly broken down. They leave a residue on the crop or in the environment for weeks, months, or years. Depending on how and where it is used, each pesticide will vary in how long a residue remains on the crop or surface. Therefore, information on residues is required on each crop the pesticide is applied to. Unfortunately, a pesticide may drift over from a nearby field and leave a residue on a crop or surface.

A long-lasting residue may be desirable because the pesticide is effective for a longer period of time. It need not be applied as often and thus may be cheaper to use. However, long-lasting residues are not always desirable. The chemicals may remain on food or feed and be hazardous to those eating them. The residues may remain in the soil to interfere with crops that are planted at a later date. Or they may remain on the surface and injure workers or others who reenter the treated area. Clearly it is important to know what residue, if any, remains after a period of time. Food, in fact, may have no residue because it may never have been treated, or it was treated at less than the maximum dose and the residues may have degraded.

Pesticide Tolerances

Many times residues remain on food or feed crops at harvest time. Since these crops are to be eaten, safe amounts of residues must be established. The maximum amount of residue, which may legally remain, on a harvested crop is called a tolerance. Federal law requires that a tolerance be set for every food or feed use of each pesticide before it is registered. The tolerances vary from crop to crop depending on the many safety factors involved. If the residue exceeds the set tolerance, the crop may not be marketed or sold. It is subject to condemnation and seizure by federal or state regulatory agencies.

How Tolerances Are Set. Much information is gathered and studied before a tolerance is set. Studies on test animals are done to determine the acute and chronic toxicity of the chemical. Toxicity to fish, birds, and mammals is also determined. The length of time the pesticide remains in the environment is measured. Possible long-term effects such as buildup in animals or in the environment are studied. All these factors (and others) are taken into account before setting a tolerance. The tolerance is usually set at least 100 times smaller than the highest dose, which has no effect in test animals. For example, 200 parts per million of pesticide A have no effect on test animals. Then the tolerance for pesticide A on any food or feed crop could be no higher than 2 parts per million (ppm). The "safety factor" is 100 times. The tolerances for pesticide A may not be as high as 2 ppm. Field tests may show that acceptable pest control is achieved using doses and methods that result in a residue much less than 2 ppm. Then the tolerance would be set at 1 ppm. [The tolerance is stated in parts per million (ppm) by weight. That is, one part of pesticide to one million parts of crop or meat.]

Negligible Residue Tolerances. A residue may be on a food or feed crop even though no pesticide was ever directly applied to it. These residues result from indirect contact with the chemical. The residue may be found, for example, in livestock which have eaten sprayed forage and grass. Edible meat of livestock containing residues must have a negligible residue tolerance. Or when a herbicide is applied before a crop emerges, a residue may be left in the soil. As the crop grows it may pick up a small amount of the herbicide. If the residue is still in the crop at harvest, a negligible residue tolerance must be set. A negligible residue tolerance (usually just "negligible residue") is set when a very small residue is likely to be on food or feed at harvest. Negligible means small or minor. The negligible residue is usually one tenth (0.1) of a part per million or less. It is far below any toxic level.

Finite Tolerances. When a pesticide will be applied directly on a food or feed crop and animal, a finite tolerance is set. A finite tolerance (often just "tolerance") is usually larger than a negligible residue. However, it is still well below possible toxic levels. To be eaten, a tolerance must have been set, unless it is exempt from tolerance.

Days to Harvest

Most pesticides break down in the environment. As they break down, the residue on the crop or animal becomes smaller. Therefore, the residues remaining at harvest depend on how long before harvest the pesticide is applied. "Days to Harvest" is the least number of days between the last pesticide application and the harvest day. ("Days to Slaughter" is used with livestock.) Both are listed on the label. For example, when pesticide A is applied on the day of harvest, it leaves a residue of 10 ppm. However, when it is applied 7 days before harvest, it leaves a residue of only 2 ppm. If acceptable pest control is possible by applying 7 days before harvest, EPA will often set the tolerance at 2 ppm and the "Days of Harvest" at 7 days. If days to harvest, recommended dosages and other label instructions are followed, the residue on the crop should be under the set tolerance.

Registration

Even though a tolerance is set for a pesticide on a specific crop, it still cannot be legally used until registered. Every pesticide and every use must be registered federally by the Environmental Protection Agency (EPA). EPA reviews all the required information on the pesticide. This includes toxicity studies, wildlife and environmental studies, breakdown and residue studies, chemical studies, etc. Registration will be granted only if the Administrator finds that the benefits of its use outweigh the risks. EPA also reviews and registers all statements which appear on the pesticide label. (See Chapter XIV, The Label.) No pesticide may be bought, sold, or used in the United States until it has federal registration for the product, the use, and the label.

It is up to you, the applicator, to help make sure that all food complies with established tolerances. Only you can be sure that no illegal residues remain on food crops. Follow label directions carefully. Do not be responsible for seizure of your customer's or his neighbor's crop!

Residue, Tolerance, and Registration – Self Study Questions

1. What is the difference between a deposit and a residue ?
2. How can long-lasting residues be desirable ? Undesirable ?
3. What is a tolerance ? When must a tolerance be set?
4. Can a food or feed crop have more than the set tolerance of a pesticide on it and still be legally marketed  ?
5. What information determines the setting of a tolerance?
6. At least what margin of safety (“safety factor”) is used in setting tolerances?
7. Does the marketed food sometimes have less pesticide residue than the set tolerance? When?
8. Can food crops and animals contain pesticide residues even when they are not directly sprayed by the pesticide?
9. When are Negligible Residue Tolerances set?
10. When are Days to Slaughter or Days to Harvest important? What are they for?
11. Is it necessary for every pesticide to be registered before it is sold or used?
12. What are some of the types of information that the Environmental Protection Agency reviews before registering a pesticide?

Answers to Self Study Questions:

1. The pesticide which remains on the treated surface is called a deposit. A residue is a deposit which remains on the surface for an extended period of time.
2. Long-lasting residues are desirable because of long-lasting protection, less frequent application, and are economical. Undesirable effects of residues are hazards to consumers, workers, or they may interfere with future crop production.
3. Tolerance is the established maximum amount of residue which may remain on a food or feed crop. A tolerance must be set before the pesticide is registered for use on food or feed.
4. No. It is subject to condemnation and seizure by federal and state regulatory agencies.
5. The results of toxicity studies on animals and the environment.
6. 100 times.
7. Yes. Food may have no residues, may never have been treated, was at less than the maximum dose, and the residues may have degraded.
8. Yes. Due to indirect contact with a chemical.
9. When a very small residue is likely to be on food or feed at harvest.
10. It is the least number of days between the last pesticide application and the harvest or slaughter day. To assure that the residue on the commodities are under the set tolerances.
11. Yes. Every pesticide is registered federally by EPA.
12. EPA reviews toxicity studies, wildlife and environmental studies, breakdown and residue studies, chemical studies, etc.

(RDG1302008)

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Disclaimer: Please read the pesticide label prior to use. The information contained at this web site is not a substitute for a pesticide label. Trade names used herein are for convenience only; no endorsement of products is intended, nor is criticism of unnamed products implied. Most of this information is historical in nature and may no longer be applicable.

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