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Commercial Horticulture > Comm Hort Blog > Posts > Vegetable insecticide modes of action: An overview of chemical insecticides

Insecticide shelf.JPGHave you ever wondered how insecticides sold at various stores actually work? Why are there so many different types of products in the market?

The simplest answer is that insecticides have a variety of modes of action and not all insecticides are suitable for the multitude insect pest species. While there are many different types of insecticidal formulations and packaging, there are four basic modes of action of organic and conventional insecticides, plus one separate category for insecticides with non-specific action. Below is brief description of each category. For detailed information, we strongly recommend readers to refer to the Insecticide Resistance Action Committee (IRAC) website that has very good educational materials for download.

  • Most common insecticides sold and used in the world today target the insect nerves and muscles. As many 12 IRAC Chemical Groups have nerve action. Many of them are contact poisons meaning insects die quickly from directed sprays. The nerve toxins include many old chemistries such as the carbamates and organophosphates. Think about Sevin, Lannate, Malathion, and Orthene. The old and new generation synthetic pyrethroid insecticides are also nerve toxins. However, the relatively new generation synthetic pyrethroid insecticide like bifenthrin are quick-acting and more photostable. Within the nerve toxins, there are slight variation in the actual action. Interestingly, new scientific discoveries have led to the development of neonicotinoid insecticides that mimic insect nervous system enzymes and products have systemic action. Another relatively new product line include high selective insecticides including Beleaf and Coragen that arrest aphid and caterpillar feeding by affecting special muscle receptors. Organic insecticides like Pyganic contain natural pyrethrin and they also belong to this category. Since there are many popular insecticides in this category, long-term use has led to the development of insecticide resistance that is a cause of major concern. If you don't see a product working well in your farm or garden, make sure to contact an entomologist or extension system in your state to manage resistance issues.
  • Another emerging group of insecticides that are relatively selective to target pest include the insect growth regulators (IGRs) with contact action on immature insects. Nearly seven IRAC Chemical Groups affect insect growth and development. Some insecticides mimic the juvenile hormone to stop insects growing while other chemicals inhibit the synthesis of chitin (or new skin) and disrupt the molting process. Some insecticides that are indirectly growth regulators include ones that will stop lipid production or some other action. Some popular products that are direct disruptors of insect growth include Knack for whitefly control, Zeal for spider mite control, Rimon and Intrepid for caterpillar control. Best effectiveness of these products comes from their timely usage when insects and small (immature) and low in numbers. Products will take many days to act and usually have a good residue on foliage for long-term control. Growth regulator insecticides will not work on mature insects. Research in Alabama and elsewhere has shown IGRs to be very good rotation partner with synthetic pyrethroids. Interestingly, the neem (organic insecticide) also disrupts insect growth but is not classified as such in the IRAC system due to the complexity of the active ingredient azadirachtin.  
  • About six chemical groups in the IRAC system are inhibitors of mitochondrial or cellular respiration with primarily contact action. Some of the more popular miticides belonging to this category, such as Portal, Acramite and Kanemite. Old organic products like rotenone that have been long discontinued also belonged to this group of electron transport inhibitors.
  • Bacillus thuringiensis, or Bt, is one of the largest selling organic insecticide that has a stomach action. In simple words, Bt proteins disrupt insect feeding and receptors in the insect midgut with long term effects. Bt products can be specific to certain types of insects such as caterpillars, maggots, and grubs, although many Bt products are not readily available at all times. Most commonly sold Bt products like DiPel, Javelin, and Xentari are effective against caterpillars. They have to applied to the foliage when caterpillars are small and given the time to fully act.
  • Last, but not the least, there are a number of popular products that have unknown modes of action or have multiple targets. One such product is neem (containing azadirachtin) which has antifeedant and growth inhibitor properties on small, soft-bodied insects.

For more details about individual IRAC Chemical Groups, visit www.irac-online.org. For Alabama Vegetable IPM recommendations please refer to the IPM guides available on Alabama Vegetable IPM.

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Ayanava Majumdar, Extension Entomologist and

State SARE Program Coordinator, Auburn University


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