Dr. Blake Layton is Extension Entomology Specialist at Mississippi State University.

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How Toxic is This Insecticide?
by Blake Layton    

Gardeners often have concerns about the toxicity of the pesticides they are using. Pesticide labels provide information on toxicity and how to use and apply products safely.

 

Few gardeners enjoy applying pesticides, but it is something we all need to do occasionally to protect our vegetables and ornamental plants from pest damage. One question that often comes to mind when planning or applying a pesticide treatment is: “How toxic is this product and how do I handle and apply it safely?” First let’s consider the question of pesticide toxicity, realizing we have limited space to devote to this very complex subject. Although we focus on insecticides for examples here, the concepts discussed also apply to other pesticides.

The subject of pesticide toxicity is quite complex because the toxicity of any given product depends on many different factors. Two of the most important are the species of animal being treated and the route of exposure (ingestion, inhalation, skin absorption, injection or other), but there are many other factors, such as sex, age, health and diet, that also affect the toxicity of a product to a particular animal. Fortunately, toxicologists have developed a standardized method of measuring toxicity, known as the LD50, that is useful for comparing relative toxicities of various products.

Handle Pesticides Safely

• Read label carefully before use.
• Follow label directions.
• Store out of reach of children.
• Keep only in original container.
• Wear all required personal protective equipment.
• Do not exceed maximum label rates.
• Observe re-entry intervals.
• Observe pre-harvest intervals for edible crops.
• Properly rinse and dispose of empty containers.

Simply defined, LD50 is the amount of test substance required to kill 50 percent of the test population. LD50s are usually expressed as mg of test product per kg of body weight, which is equivalent to parts per million. This means the lower the LD50, the more toxic the product.

Determining LD50 values using a standard test species and method of exposure provides a way to compare the toxicity of various products. When developing new insecticides, LD50s are routinely determined for a wide range of insect species to determine if a product has potential use as an insecticide and what insects it will be most effective against. Acute oral LD50 values are also determined for laboratory rats and mice to provide relative measures of acute mammalian toxicity. Ideally, an insecticide should have low LD50 values for the insect pests it is used to control and a high LD50 for rats or mice, indicating low toxicity to mammals.

Now that we have an understanding of LD50s and how they are determined, we can compare LD50s of some common insecticides to LD50s of other products that most people either use regularly or recognize from old murder mysteries (see following table). There are several interesting points to note here. First, caffeine, something most of us consume every day, is as toxic, or more toxic, than most of the listed insecticides. Some insecticides, such as spinosad and azadirachtin, have a lower acute oral toxicity than table salt. Also note that organic insecticides are not necessarily less toxic than nonorganic insecticides. Rotenone is an example of an organic insecticide with relatively high acute toxicity.

Seeing the signal word “Caution” on a pesticide container lets you know the pesticide is classified as having “Low Toxicity.”  Pesticides bearing the signal words “Warning” or “Danger-Poison” are classified as “Moderately Toxic” or “Highly Toxic.”

Before going further we should point out that acute oral toxicity alone does not fully represent the toxicity and hazards associated with a particular pesticide or product. There are many other factors to consider: How toxic is it if inhaled or absorbed through the skin? Can it cause eye damage? Is it caustic? Is it explosive? What is the flash point? Is it carcinogenic? What are the long-term effects of sub-lethal exposure? But, because of time and space constraints, we will continue to focus largely on acute oral toxicity.

The LD50 values in the accompanying table are for pure, active ingredients or technical-grade insecticides, but a cup of coffee is not pure caffeine, and the LD50 of coffee is much higher (lower toxicity) than that of pure caffeine. Likewise, the insecticide you buy at the lawn and garden center is not technical grade, and the LD50 of the formulated insecticide is higher than that of the pure, active ingredient. For example, Hi-Yield Bug Blaster Bifenthrin contains only 2.4 percent bifenthrin, and the rat oral LD50 of this insecticide formulation is 903 mg/kg, considerably higher than the LD50 of technical bifenthrin shown in the table.

Insecticide labels do not usually state the LD50s of the products they contain, but they are required by law to display standardized signal words that indicate their relative toxicity. The following table lists these signal words and what they mean. This table does not show the full range of criteria used to assign signal words because potential hazards from all possible routes of exposure are considered. For example, a product that has an oral LD50 of 3,200 mg/kg but can cause irreversible damage if you splash a drop in your eye will be in Toxicity Category 1 and required to display the signal word “Danger.”

Here a professional contractor sprays a pine tree with carbaryl. Large jobs such as this are best left to the pros.

Now we have a really simple way to answer the question posed in the title, “How toxic is this insecticide?” Just look at the signal word on the container. If it says “Danger-Poison” or “Warning,” you know you are dealing with something that is highly or moderately toxic (or the product has been placed in one of these higher toxicity categories for reasons other than acute oral toxicity). If the signal word says “Caution, Keep Out of Reach of Children” then you know the product is classified as “Low Toxicity.” Most of the insecticide products used by home gardeners today are in this low-toxicity category. Let’s not make too much of this point, but if table salt were labeled and sold as a pesticide, it would also be placed in this category.

Before you can spray an insecticide in your garden, you usually have to mix it with water, and this further reduces the toxicity of the spray. The label for Hi-Yield Bug Blaster Bifenthrin says to use 0.5 fl. oz. per gallon of water. This means you are taking a product with a rat oral LD50 of 903 mg/kg that is classified as “Low Toxicity,” and then diluting it more than 250-fold. Some insecticides are sold as “ready-to-use” sprays, and this usually means they have been pre-diluted. For example, the ready-to-use spray Ortho Home Defense Max Ant & Roach Killer only contains 0.05% bifenthrin.

Notice how the toxicity of a product declines as insecticide concentration goes from 100 percent technical product to formulated product as sold in the store to diluted spray as applied to the plant. The LD50 of pure bifenthrin is 53 mg/kg, but the LD50 of the 2.4 percent concentrate sold at the garden center is only 903 mg/kg, and this is further diluted by mixing with water before it is sprayed.

In addition to the signal words, insecticide labels also provide instructions on how to mix and apply the insecticide safely. Read the label at least twice, once before you buy it and again before you mix and apply the product. One section of the label will tell you what clothing and protective equipment you need to wear when mixing and applying the product. Be sure you wear all of the personal protective equipment required by the label and follow all other directions for safe application. Insecticides are useful and necessary gardening tools, but they must be handled and applied safely.


 

Acute Oral LD50 values of selected products and insecticides

Compound

Strychnine
Caffeine
Arsenic
Aspirin
Acetaminophen
Vitamin A
Sodium chloride
Ethyl alcohol

               
Bifenthrin
Rotenone
Carbaryl
Imidacloprid
Permethrin
Acephate
Malathion
Spinosad
Azadirachtin

Identifying Information

Organic poison from Strychnos trees
In coffee
One of the basic chemical elements
Common pain medicine
Active ingredient in Tylenol

An essential vitamin
Table salt
In alcoholic drinks

                  Insecticides                      
A pyrethroid insecticide
Organic insecticide, discontinued
Active ingredient in Sevin
Active ingredient in Bayer Tree and Shrub Insecticide
A pyrethroid insecticide
Active ingredient in Ortho Fire Ant Killer
Older insecticide, sold since 1950s
A microbially produced insecticide
Organic insecticide from neem seed

LD50in mg/kg*

2.3
127
145
200
338
1510
3000
3450

             
53
60
264
425
430
1030
1375
3738
>5000

*Acute oral LD50 values, rat or mouse, from MSDS sheets for technical grade (near 100%) product.

Signal Words on Pesticide Containers and What They Tell You*

Signal Word
 

Danger-Poison
Warning
Caution
None required
(may say Caution)

Category
 

I
II
III
IV

 

Toxicity
Description

Highly Toxic
Moderately Toxic
Low Toxicity
Very Low Toxicity

 

Acute Oral
LD50 range

0-50 mg/kg
50-500 mg/kg
500-5000 mg/kg
Over 5000 mg/kg

 

Acute Dermal  
LD50 range 

0-200 mg/kg
200-2000 mg/kg
2000-5000 mg/kg
Over 5000 mg/kg

 

*Other criteria considered when assigning a product to a toxicity category include: acute inhalation toxicity, degree of eye irritation and degree of skin irritation.

 

A version of this article appeared in a November/December 2013 print edition of State-by-State Gardening.
Photography courtesy of Blake Layton and US Forest Service – Northern Region.

 

Posted: 11/29/18   RSS | Print

 

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