Getting to Know Popular Insecticides Used in Iowa Field Crops

Encyclopedia Article

Agriculture plays a major role in Iowa’s economy, with 72% of Iowa’s area (26 million acres) used for crops and rotational pastures. It is no surprise that crop protection is of the utmost importance. In 2018, Iowa farmers applied nearly 560,000 pounds of insecticide (USDA-NASS 2019). Most of the compounds used can be grouped into three insecticidal groups: pyrethroids, organophosphates, and neonicotinoids (USDA-NASS 2018, 2019; IRAC 2019). These three groups consist of a large number of compounds known to be toxic to insects by targeting their nervous system. These insecticides are widely used in agriculture, but how they work is not well understood by the public. Sure, they kill insects, but how? Here, we describe the modes of action by which these insecticidal groups function as well as outline the potential risks to humans in agricultural settings.

Pyrethroids

Pyrethroids (PYs) are the most common group of insecticides used to manage insect pests in Iowa field crops. This group of insecticides is designed to mimic an insecticidal compound originally found within chrysanthemum flowers (pyrethrum). Agricultural professionals may recognize the trade names Warrior II, Brigade®, and Hero®, which all contain pyrethroid active ingredients. The corresponding chemical names of their active ingredients (AIs) are lambda-cyhalothrin, bifenthrin, and zeta-cypermethrin. Bifenthrin was the most used pyrethroid for both corn and soybean in 2018 (USDA-NASS 2019).

PYs function by altering the state of the insect nervous system. Normal movements of the insect muscles control the flow of sodium in and out of the insect synapse (the junction between two nerve cells; Figure 1). Ions move through the sodium channel, often referred to as a “gate.” When the gate is opened, the muscle is firing. When the gate is closed, the muscle is allowed to relax. This gate is opening and closing continually, allowing the insect to move normally. A pyrethroid molecule fits in the sodium channel and directly interferes with the gate properly closing. Due to this inability to close, the insect remains in a state of constant muscle firing. This leads to an overstimulation of the insect muscles; the insect will eventually die if the exposure is great enough.

mode of action of pyrethroids
Figure 1. An illustration of the mechanism of action of pyrethroids. The images on the left depict the changes that occur when a pyrethroid interacts with the sodium channel (the “gate”). Illustration obtained from Hénault-Ethier et al. 2016.

Organophosphates

Organophosphates (OPs) are synthetic compounds that usually contain phosphorous in the chemical structure and are quite effective against insects. Some of the commonly used OPs are chlorpyrifos, malathion, and dimethoate. These AIs are found in products such as Lorsban, Fyfanon®, and Dimethoate 4E.

While OPs also function by acting on the insect nervous system, they function differently than pyrethroids. Acetylcholine, a common neurotransmitter found in both insect and human nervous systems, binds to a receptor and a reaction follows (Figure 2). Think of this as a lock and key system. While the key (acetylcholine) is in the lock (the receptor), an action is allowed to take place. Acetylcholinesterase is an enzyme that is responsible for breaking down acetylcholine, thus ending the action caused by the binding of acetylcholine to its receptor: this enzyme is required to ensure that the nervous system is not overstimulated and actions can cease properly. When OPs come in contact with the insect, they interfere with the activity of acetylcholinesterase (i.e., the enzyme is not able to properly break down acetylcholine). This leads to an overstimulation of the insect nervous system, causing hyperactivity that can lead to death.

mode of action of OPs
Figure 2. An illustration of the mechanism of action of organophosphate insecticides. The binding of the OP to the acetylcholinesterase (ACh Esterase) is responsible for the toxic action associated with this insecticidal group. Illustration obtained from https://depts.washington.edu/opchild/acute.html.

Neonicotinoids

Neonicotinoids (NNs) are a group of synthetic insecticides based on the naturally occurring compound nicotine. Neonicotinoid AIs such as thiamethoxam and imidacloprid can be applied during the growing season but are commonly used as seed treatments. Trade names for foliar products include Actara® and Admire®, while commonly used seed treatments include Cruiser® and Gaucho®.

This group of insecticides functions similarly to organophosphates. NNs bind to nicotinic acetylcholine receptors found in the insect synapses (Figure 3). This binding is irreversible and can lead to paralysis and then death of the insect.

mode of action of neonics
Figure 3. An illustration of the mechanism of action of neonicotinoids. This mode of action can ultimately lead to paralysis and death of the insect. Illustration obtained from https://ib.bioninja.com.au/standard-level/topic-6-human-physiology/65-neurons-and-synapses/neurotransmitters.html.

Human Safety Considerations

In order to compare the human safety of these common insecticidal groups, it is important to understand a frequently used value in toxicology studies, LD50. This number describes the toxicity of a compound in terms of the dose required to kill 50% of a tested population. These values are typically given in a weight:weight ratio such as mg of compound per kg of the tested organism (mg/kg). A lower number indicates that less of that compound is required to kill the test organism (i.e., lower LD50 = more toxic).

Table 1, organized by insecticide group, provides some toxicity information about commonly used insecticides in Iowa. It should be noted that the LD50 values were obtained through laboratory studies on rats to estimate the effect of oral exposure on mammals (i.e., humans). This is a common practice since poisoning humans is not a feasible option. It is unlikely that oral exposure (ingestion) is the primary route of exposure for agricultural professionals; dermal exposure (skin contact) is the more likely route. Dermal exposure tends to have much higher LD50 values which indicate lower levels of toxicity from this route of exposure.

As you can see from the LD50 values shown in Table 1, toxicity varies considerably within an insecticidal group. The lethal effects of these insecticides are not observed in humans, likely due to the much smaller size of insects and increased sensitivity of the target site in insects. Even so, safety measures should be taken to avoid direct contact with insecticidal products used in agricultural settings as these compounds target the nervous system.

Table 1. The LD50 values of some of the most used insecticides in the state of Iowa

Compound Identity

LD50 (mg/kg; oral exposure)

Insecticidal Group

IRAC Classification

Lambda-cyhalothrin

144

Pyrethroid

3A

Bifenthrin

54

Pyrethroid

3A

Zeta-cypermethrin

250 – 4,123

Pyrethroid

3A

Chlorpyrifos

82 – 270

Organophosphate

1B

Malathion

480 – 10,700

Organophosphate

1B

Dimethoate

60 - 387

Organophosphate

1B

Thiamethoxam

1,563

Neonicotinoid

4A

Imidacloprid

450

Neonicotinoid

4A

Additional Resources

References

USDA-NASS 2019: https://www.nass.usda.gov/Statistics_by_State/Iowa/Publications/Other_Surveys/2019/IA-Ag-Chem-Corn-Soybeans-2019.pdf

USDA-NASS 2018: https://www.nass.usda.gov/Statistics_by_State/Iowa/Publications/Other_Surveys/2018/IA-Ag-Chem-Soybeans-2018.pdf

IRAC 2019: https://www.irac-online.org/modes-of-action

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