5.02 Pesticide Metabolism

5.02 Pesticide Metabolism
Learning objectives

  1. List three classes of insecticides that affect honey bees and explain their modes of action.
  2. Predict the chemical properties of an organic insecticide based on its chemical structure.
  3. Explain how pesticides are detoxified in bees.
  4. Make practical recommendations to beekeepers based on knowledge of pesticide residue accumulation, pesticide metabolism, and routes of environmental exposure.

The advent of modern insecticides revolutionized agriculture.  While in the past it was not unusual to lose an entire crop due to a pest, farmers now have easy ways to defend their crops against herbivorous insects.  Many synthetic pesticides are organic compounds that mimic neurotransmitters and work by paralyzing insects.
Types of insecticides
Organophosphate insecticides block acetylcholinesterase thereby preventing the breakdown of acetylcholine, causing neuronal sodium channels to remain open.  This ultimately results in paralysis and death. Organophosphates bind to acetylcholinesterase irreversibly and are very toxic. The organophosphate malathion is sold under the trade name Spectracide Malathion Insect Spray.

Image from [1]

Carbamates compete with acetylcholine for binding to acetylcholinesterase and, like organophosphate, causes sodium channels to remain open.  Unlike organophosphates,  carbamates dissociate from acetylcholinesterase. The carabamate carbaryl is sold under the trade name Sevin.
More recently, neonicatinoid pesticides have been introduced because of their low relative toxicity compared with organophosphates and carbamates. Neonicitinoids work more directly by functioning as acetylcholine receptor agonists, binding the receptors and keeping sodium channels open. Imidacloprid is a neonicatinoid sold under the trade name Advantage.
Pyrethroids bind to open sodium channels on neuronal axons, preventing signal transmission.  Permethrin is a pyrethroid sold under the trade name Nix.
Beekeepers regularly use pesticides in their beehives to control pests such as Varroa destructor.

Pesticide detoxification

Pesticides are broken down over time by sequences of enzymes.  If the compounds can be broken down before pesticides paralyze the insects, the insects can survive.  Increased detoxification by enzymes such as Cytochrome P450s is one of the major ways that insects become resistant to pesticides.  [2]
Clearly, dose matters when it comes to toxicity.  When exposed to low doses of pesticides, insects can detoxify them, but when certain other compounds are present, the lethal dose can be lowered.  This is known as a drug interaction.  Interactions have been documented between common compounds used to treat bees for mites and fungicides. [3]


  1. Gobo, Anagilda B., Kurz, Márcia H. S., Pizzutti, Ionara R., Adaime, Martha B., & Zanella, Renato. (2004). Development and validation of methodology for the determination of residues of organophosphorus pesticides in tomatoes. Journal of the Brazilian Chemical Society, 15(6), 945-95
  2. Scott, Jeffrey G. 1999. “Cytochromes P450 and Insecticide Resistance.” Insect Biochemistry and Molecular Biology 29(9):757–77.

  3. Johnson RM, Dahlgren L, Siegfried BD, Ellis MD (2013) Acaricide, Fungicide and Drug Interactions in Honey Bees (Apis mellifera). PLoS ONE 8(1)

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