How Herbicides and Pesticides Work

Herbicides and pesticides are essential tools in modern agriculture and gardening, designed to manage and control unwanted plants and pests. Their effectiveness relies on various mechanisms tailored to target specific problems.

Herbicides are chemicals used to eliminate or control weeds that compete with crops for nutrients, water, and sunlight. They can be broadly classified based on their mode of action. Selective herbicides are designed to kill specific types of plants while leaving the desired crops unharmed. These herbicides often target specific biochemical pathways or structures that are present in weeds but not in crops. For instance, some herbicides inhibit the synthesis of essential amino acids crucial for weed growth. On the other hand, non-selective herbicides, also known as total herbicides, kill all vegetation they come into contact with. These are typically used for clearing land or controlling weeds in non-crop areas and disrupt fundamental plant processes like photosynthesis or cell membrane integrity, affecting all plants indiscriminately.

Pre-emergent herbicides are applied to the soil before weeds germinate, preventing seeds from sprouting or growing by interfering with the early stages of seedling development. Post-emergent herbicides, however, are applied after weeds have emerged and target actively growing plants by affecting processes such as protein synthesis or hormone regulation, which are critical for their growth.

Pesticides encompass a range of chemicals designed to control insects, fungi, rodents, and other pests that can damage crops and spread diseases. Insecticides, for instance, target insect pests by disrupting their nervous system, affecting essential physiological functions, or repelling them. Some insecticides, like organophosphates, inhibit enzymes necessary for nerve signal transmission, causing paralysis and death, while others, like pyrethroids, interfere with nerve impulse movement. Fungicides, used to control fungal diseases, work by inhibiting fungal cell growth or disrupting their reproductive processes. They might interfere with cell wall synthesis, enzyme activity, or the production of essential fungal metabolites. Rodenticides, designed to control rodents, operate in various ways, such as anticoagulants that prevent blood clotting, leading to internal bleeding and death, or neurotoxins that affect the rodent’s nervous system.

Some products combine herbicides and pesticides to address both weed and pest problems simultaneously, offering a comprehensive solution to agricultural challenges. The application of herbicides and pesticides requires careful consideration to minimize potential environmental and health risks. Following recommended dosages, application methods, and safety guidelines ensures their effectiveness while protecting non-target species, including humans and wildlife. Integrated Pest Management (IPM) strategies, which combine chemical control with other methods like crop rotation and biological control, help manage pests and weeds sustainably.

Herbicides

Herbicides usually interfere with plant growth and often imitate plant hormones.

• • ACCase Inhibitors kill grasses and inhibit the first step in lipid synthesis, acetyl CoA carboxylase, thus affecting cell membrane production in the meristem. They do not affect dicots plants.

  • ALS Inhibitors affect grasses and dicots by inhibiting the first step in some amino acid synthesis, acetolactate synthesis. The plants are slowly starved of theses amino acids and eventually DNA synthesis stops.

  • ESPS Inhibitors affect grasses and dicots by inhibiting the first step in the synthesis of tryptophan, phenylalanine and tyrosine, enolpyruvylshikimate 3-phosphate synthase enzyme.

  • Photosystem II Inhibitors reduce the electron flow from water to NADPH2+ causing electrons to accumulate on chlorophyll molecules and excess oxidation to occur. The plant will eventually die.

  • Synthetic Auxin mimics plant hormones and can affect the plant cell membrane.

Neonicotinoids and the decline of bees

Neonicotinoids are a class of neuro-active insecticides chemically similar to nicotine. In the 1980s Shell and in the 1990s Bayer started work on their development. The neonicotinoid family includes acetamiprid, clothianidin, imidacloprid, nitenpyram, nithiazine, thiacloprid and thiamethoxam. Imidacloprid is the most widely used insecticide in the world. Compared to organophosphate and carbamate insecticides, neonicotinoids cause less toxicity in birds and mammals than insects. 

Some breakdown products are also toxic to insects. Neonicotinoid use has been linked in a range of studies to adverse ecological effects, including honey-bee colony collapse disorder (CCD) and loss of birds due to a reduction in insect populations; the findings used to be conflicting and thus controversial , but recent studies by the EFSA have confirmed the risk to bees.  In 2013, the European Union and a few non EU countries restricted the use of certain neonicotinoids. In 2018, the EU banned the three main neonicotinoids (clothianidin, imidacloprid and thiamethoxam) for all outdoor uses. Several states in the United States have also restricted usage of neonicotinoids out of concern for pollinators and bees.