Dr. Bugs: Insect growth regulators and insecticide resistance By Raymond A. Cloyd

Question: Can insect pests develop resistance to insect growth regulators?

Answer: Yes, several studies demonstrate that certain insect pests can develop resistance to insect growth regulators. Before addressing the issue of resistance, we need to discuss the different categories of insect growth regulators. 

There are three categories based on mode of action: 

  1. Juvenile hormone mimics, analogs or agonists 
  2. Ecdysone antagonists 
  3. Chitin synthesis inhibitors 

Juvenile hormone mimics arrest development, which results in insects remaining in the immature stage, thus preventing completion of the life cycle. Examples include: fenoxycarb (Preclude), kinoprene (Enstar II/AQ) and pyriproxyfen (Distance and Fulcrum). 

Ecdysone antagonists disrupt molting by inhibiting metabolism of the molting hormone ecdysone. An example is azadirachtin (Azatin, Ornazin, and Molt-X). 

Chitin synthesis inhibitors disrupt enzymes responsible for stimulating the production and formation of chitin, an essential component of the insect skin (exoskeleton). Examples include: buprofezin (Talus), cyromazine (Citation), diflubenzuron (Adept), etoxazole (Beethoven and TetraSan) and novaluron (Pedestal). 

Sweetpotato whiteflypopulations can develop resistance to insecticides.
Sweetpotato whitefly populations can develop resistance to insecticides.

Research has shown that pests, such as aphids and whiteflies, can develop resistance to certain insect growth regulators. For example, green peach aphid, Myzus persicae, populations can develop resistance to azadirachtin. Sweetpotato whitefly, Bemisia tabaci, populations exhibited resistance to buprofezin and sweetpotato whitefly Q-biotype populations developed resistance to pyriproxyfen. 

Whitefly populations resistant to pyriproxyfen may be associated with reduced penetration through the skin (cuticle) and/or enhanced degradation of the active ingredient by metabolic activity of detoxifying enzymes, such as esterases, glutathione-S-transferases and mixed function oxidases. The enhanced activity of mixed function oxidases is also the resistance mechanism associated with whitefly populations developing resistance to buprofezin. 

Selection pressure is caused by the frequency of applying insecticides where susceptible individuals are killed leaving resistant individuals to breed. Consequently, the selection pressure associated with the frequency of applying pyriproxyfen may contribute to whitefly populations developing resistance to the insect growth regulator. 

Green peach aphids can developresistance to azadirachtin. Photos courtesy of Raymond Cloyd.
Green peach aphids can develop resistance to azadirachtin. Photos courtesy of Raymond Cloyd.

Do not apply insect growth regulators with similar modes of action in succession. Instead, rotate with different modes of action to mitigate the potential for resistance developing in aphid and whitefly populations. In addition, rotating insect growth regulators with other insecticides may delay resistance developing in pest populations. 

You want to avoid selecting for insecticide resistance by rotating chemicals with different modes of action, which will reduce the selection pressure on insect pest populations. In general, resistance decreases as selection pressure is reduced, however, the increase in insect population susceptibility does not always return to the same level as before applying insecticides.

Raymond A. Cloyd is professor and extension specialist in horticultural entomology/plant protection at Kansas State University. He can be reached at rcloyd@ksu.edu.