Pesticides can be confusing. Companies create new ones every day, the rules are ever-changing, and to top it all off, every country has its own set of standards. How can one person possibly keep up?
Well, luckily for all of us, there are great resources online to help like Bryant Christie, a free online database with global MRLs and rules. But before we get too far down the rabbit hole with these more advanced concepts and tools, it’s crucial to get a firm understanding of the fundamentals.
The word pesticide is a general term for a substance used to destroy or control pests. A pest can be anything from a mouse, an insect, or a bacteria, to an unwanted plant or weed. Pesticides play a considerable role in the success of an agriculture company because they are involved in multiple steps of the process from the farm to the storage of the harvested product. When choosing which pesticide to use, it is essential to remember that each pesticide is designed to eradicate certain pests and has a specific way of doing so. If you don’t do your due diligence before application, you may accidentally eliminate something you didn’t want to.
Pesticides can be broken down into several groups:
- Insecticides – used in insect control: miticides, larvicides, ovicides, pheromones, termiticides.
- Herbicides – used in algae or plant control: algicides, brush killers, defoliants (a pesticide that kills leaves), desiccants (a pesticide that causes the plant to dry out), plant growth regulators (PGR), and weed killers.
- Fungicides and antimicrobials – used to control bacteria and fungus that infect plants: bactericides, disinfectants.
- Vertebrate poisons – used to kill higher level animals: piscicides, rodenticides.
- Fumigants – used to disinfect or purify an area. Fumigants are usually biocides.
Understanding the Types of Pesticides
As we discussed, there are many types of pesticides. The two examples that you will test for most often are insecticides and herbicides.
Insecticides affect the insect’s endocrine system, nervous system, and digestive system. They do this by “tricking” the insect to killing it or keep it off your crop, see Figure 1 below. These various methods of removing the pest or (modes of action) can be anything directly sprayed onto the insect, to something that is sprayed onto the leaves and eaten by the insect.
Insecticides in the Real World
One of the most modern examples of the ingestion aided mode of action is to spray the pesticide Bt on corn and cotton. Bt or Bacillus thuringiensis is a bacterium that produces a toxic crystal (Cry), with proteins that have been known to be insecticidal. In the past, farmers sprayed Bt on fields as an organic pesticide, but with difficulty and high expense. As a result, scientists took the genetic information for Bt Cry genes, extracted them from the bacteria, and inserted into the genome of the corn or cotton. Thereby allowing the plant to produce the effects itself.
When the plant produces the Cry in the tissue makes it more effective. The gene protects the plant from the sun instead of leaving it in the environment. Once the caterpillar ingests the Cry, it is solubilized and activated. The toxin binds to the gut cells, and through a series of biochemical reactions causes the cells to die and eventually the caterpillar.
Common Pesticides Used on Insects
Some of the most common insecticides which affect the nervous system of insects are DDT*, fipronil*, pyrethroids*, and neonicotinoids*. Neonicotinoids, such as Imidacloprid*, harm the insect by paralyzing the mouthparts of the insect. While Thiamethoxam*, another imidacloprid, paralyzes the muscles of the insect after direct application.
Herbicides affect the plant directly and are the most commonly used pesticide by homeowners. The mode of action for herbicides is dependent on the absorption (where it is applied to the plant) and translocation (how it moves through the plant) (Fig 3).
- Foliar-Translocated Pesticides: applied to leaves and are distributed in the plant via the phloem (a transport system in plants for sugars). This pesticide goes through the entire plant, starting at the leaves and goes down to the roots.
- Ex: Glyphosate*
- Foliar-Contact Pesticides: applied to the leaves and causes the leaves to die.
- Ex: Diquat
- (3 and 4) Foliar or Root Uptake Pesticides: applied to the roots or leaves and transferred to the rest of the plant. The pesticide is translocated via the phloem and xylem.
- Ex: Tebuthiuron, Picloram, Metsulfuron
Interested in learning more about pesticides like glyphosate? Read Testing Your Product For Glyphosate
What’s Happening Inside the Plant
Once inside the plant, the pesticide works on various cellular functions, affecting the light systems, cell metabolism, or growth/cell division of the plant and eventually breaking it down. Below are some examples of each:
- Acetyl CoA carboxylase Inhibitors: Inhibit a protein that works in the generation of fatty acid synthesis inside of the cells. Downstream it blocks the building blocks for the membranes of the cell walls. (Ex: cyclohexanedione)
- ALS Inhibitors: Inhibits the production of a key enzyme that is used to build branched-chain amino acids. Amino acids are essential in the formation of many components, including DNA. (Ex: Imidazolinones, pyrimidinylthiobenzoates)
- Photosystem II Inhibitors: This group of pesticides affects everything from the CO2 fixation and stopping the production of ATP (cellular energy) to inhibiting carotenoid biosynthesis or anthocyanin and RNA synthesis. (Ex: nitriles, and phenylpyridazines, Phenylcarbamates, pyridazinones, triazines, triazinones)
- Photosystem I Inhibitors: PSI inhibitors accept electrons from the photosystem and are reduced (electron is removed) to form hydrogen peroxide and are further combined with lipid radicals which form a compound that destroys the integrity of the cell membrane and allows the cells to leak. (Ex: Bipyridyliums)
- PPO Inhibitors: Protoporphyinogen oxidase (PPG Oxidase) is an enzyme of chlorophyll. As downstream actions are taken, lipids (fats) and proteins are attacked and result in loss of chlorophyll and carotenoids. This leads to leaky membranes and the cells and organelles drying and disintegrating rapidly. (Ex:oxadiazoles, oxazolidinediones, phenylpyrazoles, pyrimidindiones, thiadiazoles, and triazolinones)
- EPSP Synthase Inhibitors: This group of herbicides inhibits 5-enolpyruvylshikimate-3-phosphate synthase (EPSP), an enzyme that is important in the production of aromatic amino acids needed for further protein synthesis or biosynthetic pathways for growth. (Ex: glyphosate*)
- Microtubule Inhibitors: Pesticides found in this category inhibit the formation or polymerization of microtubules. Microtubules are fundamental structures in the creation of cell walls and chromosome separation in mitosis (cell division). Without microtubules, the cells cannot divide nor elongate. This is also important in seed germination. (Ex: Benzamide, benzoic acid (DCPA), dinitroaniline, phosphoramidate, and pyridine)
- Synthetic Auxins: This group is closely associated with plant growth regulators. At low levels, once inside the plant, this pesticide stimulates RNA polymerase, an enzyme that makes RNA, resulting in increases in RNA, DNA, and protein biosynthesis. As these cellular products increase, it leads to uncontrolled cell division and growth, which results in vascular tissue destruction. On the other hand, at high concentrations, auxins inhibit cell division and growth, usually in meristematic regions (growth regions). (Ex: Benzoic acids, phenoxy carboxylic acids, pyridine carboxylic acids)
How Each Herbicide Affects the Plant
Why is this important?
Many different outcomes can happen when pesticides are applied. It is essential to know the effects, as well as the targets when using pesticides. You want to be confident the pesticides you apply are going to give you the intended outcome you are looking for (control of a pest, regulation of plant growth, defoliation of a plant). Pesticides must also be applied according to the label and done according to state and local laws. An excellent resource for pesticide application is an Agricultural Pest Control Adviser (PCA). A PCA will be able to give you recommendations based on your commodity and needs.
After you complete pesticide application and need to test your product for pesticide residue, send your samples to Safe Food Alliance for analysis. We can also assist you with drift validation and documentation for FDA detention or exports.
How We Can Help You
At Safe Food Alliance, we analyze a wide variety of pesticides with technical expertise and certification from the state of California. Our lab is ISO 17025 accredited utilizing the most cutting-edge methods and instrumentation to deliver you speedy results to help you make the important decisions. To make things simpler, the pesticides with an asterisk (*) denote pesticides we can test or have the ability to analyze an analyte in the group.
If you would like to get started with pesticide testing or have questions about the article, I can be reached at firstname.lastname@example.org.