Many people seem to have the attitude "if a little is good, more is better." This somewhat is understandable, when we consider that occasionally we hear individuals such as scientists recommend things like taking mega-doses of vitamins or minerals to ward off illness. While the scientific community debates the wisdom of the latter, we definitely can advise those involved with plant production that, when it comes to pesticide use, more is not better. In fact, more can be quite harmful to people, plants and the environment.
In recent years, I have witnessed on several occasions the unfortunate results when growers did not follow label directions and applied pesticides at concentrations far above recommended rates. In all cases, the result was near-to-total loss of the crop they were attempting to protect. The fact is that the plant injury and economic loss incurred most likely could have been avoided if the grower only had taken the time to read and follow pesticide label directions.
Pesticide labels serve an important purpose and are the way by which pesticide manufacturers communicate with growers. They are developed by the manufacturer and approved by the Environmental Protection Agency (EPA) as part of the pesticide's registration process. Pesticide labels contain detailed information on how to use the product in question correctly and legally. This includes the crops to which they may be applied and the rates (concentrations) at which they should be used. Following label instructions allows growers to minimize risks to plants while maximizing the benefits from using the pesticide.
Applying a pesticide at concentrations greater than dictated by the label can lead to a number of unfortunate results; the first being phytotoxicity. The latter might express itself as leaf burn, necrosis, chlorosis, distorted growth or stunting. A precaution that can be taken to help avoid phytotoxicity is to make one or more preliminary spray applications of a new pesticide to a few plants of the species being treated at the rate to be used when treating the entire crop.
Another consequence of applying pesticides excessively is the development of resistance by the target pest. Resistance occurs when pests (e.g. insects) develop the ability to withstand exposure to a pesticide because of a heritable genetic adaptation. The more often a pest is exposed to a particular pesticide, the more likely a resistant strain of the pest will develop.
Finally, applying pesticides at excessive rates wastes money. The newer pesticides, while effective, tend to be rather expensive. Applying them at a concentration greater than necessary uses more product and adds to the cost of treating for a particular pest. If the label for a pesticide recommends a range (e.g. two to four ounces per 100 gallons of water) start in the middle of the range and go from there.
In conclusion, the following recommendations concerning pesticide use were developed by the National Pesticide Information Center. The latter is a joint venture between Oregon State University and the EPA.
REVISED: June 26, 2019