The Dose Makes the Poison
By John Garr and Jerry Green, GarrCo Products
A Swiss doctor named Paracelsus said, “All substances are poisons; there is none which is not a poison. The right dose differentiates a poison from a remedy.” His statement is usually shortened to the dose makes the poison. For the recognition of that principle over 500 years ago, Paracelsus is called the “father” of toxicology.
How much is too much?
Many pesticides are broadly tolerant to plants and most other non-target organisms. Drift can happen without any symptoms. For example, when fungicides and insecticides drift, they do not damage nearby crops and nobody complains if insects die unless they are bees or Monarch butterflies. The toxicity of a pesticide always depends on dose and factors such as species specificity, potency, ability to get to its site-of-action, persistence, and whether it is metabolically inactivated, stored or excreted.
We discuss pesticide dose in terms of application rates, unwanted exposure, how much remains on the crop or drifts off-target. Pesticide toxicity can be broadly grouped into short-term (acute) or long-term (chronic) categories. Short-term toxicity is usually easy to measure while long-term toxicity is more complex to determine with both direct and indirect effects often over a long period of time.
Of course, nobody wants any unwanted chemical exposure. No exposure or exposure only to a dose that will not cause any effect is always the goal. No exposure is a difficult goal for applicators, particularly when the ability to detect chemicals keeps getting dramatically lower. No exposure requires applicators use best spray management practices and application technologies to ensure what they apply stays on-target.
Sensitivities vary greatly
Chemists design pesticides to show different levels of activity on different species, differences can be 10,000-fold or more. Pesticides must be very safe to the crops they protect and very potent to the pests they control or customers would never buy them.
A very small amount of one herbicide can injure a non-target plant while a very high amount of another can be safe to the same plant. Many commonly used selective herbicides are tolerant to multiple crops and, of course, glyphosate (Roundup®) is going to be safe to all glyphosate-tolerant crops and injurious to other crops. Within a species, this variation leads to selection for the resistant individuals and the evolution of resistant pests.
The curious case of dicamba on soybeans
The issue of non-target sensitivity is often in the news. Currently, the headlines are about widespread damage to nearby fields from the illegal use of dicamba on the newly introduced genetically modified dicamba-tolerant soybeans. An unusual situation occurred when dicamba-tolerant soybeans were approved to plant but not the herbicide designed to be used on it. Many farmers were desperate to use dicamba to control Palmer amaranth, a ‘super weed’ resistant to glyphosate and other herbicides.
You can guess what happened. A large number of farmers could not wait and illegally applied currently available dicamba formulations and it drifted. Regular soybeans are very sensitive to dicamba so even a small amount of drift on them caused injury. The number of affected acres is very high and it is unclear how the issue will be resolved.
In contrast, if the new 2,4-D tolerant soybeans had been approved to plant first and farmers illegally used current 2,4-D formulations, the same amount of drift would not have been as noticeable because soybeans are inherently more tolerant to 2,4-D than dicamba. However, 2,4-D drift in that case might have appeared on cotton because cotton is more sensitive to 2,4-D than dicamba, the reverse of soybeans.
Are natural chemicals safer than synthetic?
Many people believe that chemicals produced by nature are safer than synthetic chemicals, chemicals made by people. Chemophobia is the fear of synthetic chemicals, some believe any exposure will cause cancer and other health problems.
The public generally thinks that biopesticides and other biologicals as safer than synthetic agrochemicals. Sometimes it feels like biologicals used in agriculture get a free pass. However, there is no clear correlation to support the interpretation that synthetic chemicals are worse than natural chemicals. Chemicals have identical toxicology whether derived from nature or synthetically. Chemicals from nature can be very toxic, just think of rattlesnakes and other poisonous animals.
Many chemicals we eat and drink can be poisonous when consumed in very high amounts. Most of us voluntarily drink caffeine multiple times every day and it is always in the news about possible health effects. Water, vitamins and many other chemicals are essential for us to live, but can kill when the dose is very high. The dose is the key for both synthetic and natural chemicals.
Play it safe
Scientists select new pesticides to have the best properties for efficacy on pests and safety to humans and the environment. They test millions of chemicals and study the ones they select extensively. On average, a new pesticide is studied for 11 years at a cost of over $286 million before it is ever sold to farmers.
The EPA and other government agencies with advice from experts carefully review all the studies companies do to determine which chemicals can be used safely. The safety of pesticides on non-target species can determine whether a product is approved or not. The greater the risk to non-target organisms, the greater the label restrictions that EPA requires to ensure the pesticide can be used safely. Everyone should be confident that the pesticides EPA approves are safe when we follow their rules.
Adjuvants play an important safety role by keeping pesticides on-target and to ensure their efficacy at lowest possible dose. Paracelsus would approve.
For more information, contact John Garr at (765) 395-3441 or firstname.lastname@example.org.