Mode of Action: Insect and mite growth regulators

Columns - Plant Health

The first part of this column will discuss the importance of understanding insecticide and fungicide mode of action (MOA) and why this information is important for growers to understand.

May 28, 2010
Scott Ludwig
Scott Ludwig

The first part of this column will discuss the importance of understanding insecticide and fungicide mode of action (MOA) and why this information is important for growers to understand. Over the next few columns I will discuss the MOA groups and how they fit into pest management programs. This month I will discuss the MOA of insect growth regulators (IGRs).
I have written previously about the importance of developing an integrated pest management program that includes a resistance-management program. To do so, you need to understand the mode of action of the products available and the biology of your target pest. The mode of action is the mechanism by which the pesticide kills the pest. In the past there were a limited number of insecticide classes. Times have changed, and the Insecticide Resistance Action Committee (IRAC) currently recognizes 26 primary sites of action. Within these groups there can be subgroups. For example, the carbamates (1A) and organophosphates (1B) are both acetylcholinesterase inhibitors (MOA Group 1).

Determining MOA
Not all insecticides are classified. In some cases the company has been unable to determine the mode of action for the product. This is the case with Overture (pyridalyl), Azatin (azadirachtin), and Floramite (bifenazate). Insect pathogens, soaps and oils fall under a miscellaneous category of products that are not classified.
Determining the mode of action of an insecticide has become easier during the last few years. Newer insecticides have the mode of action group listed on the top of the labels’ first page. You can also find a complete listing of the mode of action groups in a number of locations. These include OHP’s Chemical Class Chart and BASF’s Pest Management Guide. Insecticide Resistance Action Committee (IRAC) also has a great Web site for information on resistance management in different cropping systems around the world.

During the last 20 years there have been a number of pests where resistance has been documented, including spider mites (left) and whiteflies.Resistance management
There is still a debate over how many times a product can be used before you need to rotate to another product, and how many different products should be in the rotation program. Your first step should be to read the label and see if there are any label restrictions regarding resistance management. A general rule of thumb is that you want to rotate to a different mode of action for each insect generation. Insect pathogens and insecticides that kill by desiccation or smothering, such as soaps and oils, can be used anytime in a pesticide rotation scheme without negatively impacting resistance management programs.
It is critical to remember that resistance is genetically based. It takes a number of generations for an insect species to develop resistance to an insecticide. Generally, the shorter the generation time the faster the species can develop resistance. The first evidence of resistance is usually reduced efficacy against the target pest even when the pesticide was properly applied at the recommended rate. If you suspect a pest population is developing tolerance to a particular chemical, continued use or increasing the rate of the product will only accelerate the rate of resistance selection, eventually leading to complete control failure.
During the last 20 years there have been a number of pests where resistance has been documented, including spider mites, western flower thrips, whiteflies and leafminers. Recently, a national effort was undertaken to develop management programs for whiteflies (biotypes B and Q) and thrips (including western flower thrips and chilli thrips). These can be found on my Web site. We all need to ensure the effective insecticides we have today continue to be effective.

Insect growth regulators
Insect and mite growth regulators are compounds that prevent insects or mites from developing to the adult stage. Some of the IGRs also cause the adult female to lay sterile eggs. When using IGRs it is essential to target the immature stage of the pest. This requires scouting to determine when the immature stage is present. You need to remember that unlike contact insecticides, there is usually a delay period until the pests die with IGRs.

If you have a large number of adults present, you may want to target them first to reduce their numbers and limit egg laying. After you reduce the adult pressure, come in with an IGR to target the immature stages. In addition, some of the compounds require repeat applications, while with others you may be limited to one use.
Rotating IGRs is possible since the term insect growth regulator refers to products that comprise a number of different MOA groups. However, make sure you are rotating between different MOA groups.

  • Juvenile hormone analogs/mimics (MOA Group 7) mimic the juvenile hormones of insects. The resulting hormone imbalance results in abnormal development and death. These IGRs also causes sterility of the female and interfere with embryonic development of the eggs. Examples include, fenoxycarb (Precision), kinoprene (Enstar II) and pyriproxyfen (Distance).
  • Chitin synthesis inhibitors prevent chitin from forming after molting. Chitin is a major constituent of the body wall of insects. As a result, the insect is unable to molt properly and dies. Examples include diflubenzuron (Adept) MOA Group 15; novaluron (Pedestal) MOA Group 15; and buprofezin (Talus) MOA Group 16.
  • Anti-ecdysone compounds (MOA Group 18) interfere with production of ecdysone. As a result immature insects do not molt and die in three to 10 days. Examples include azadirachtin (Azatin XL and Ornazin.)
  • Mite growth inhibitors (MOA Group 10) target mites, not insects. The target site of activity for these products is unknown. Examples: clofentezine (Ovation)  MOA Group 10a; hexythiazox (Hexygon) MOA Group 10a; and etoxazole (TetraSan)  MOA Group 10b.
  • Moulting disruptors, Dipteran (MOA Group 17) target the moulting of immature flies. Cyromazine (Citation) is the only product in this class.

Insect and mite growth regulators can be a valuable tool in your pest management program. They have low mammalian toxicity and most are very selective in the insects and mites they control. Some biological control agents may be used together with IGRs. Unfortunately, IGRs need to be applied more than once to achieve control. If the adult stage is causing damage, another management tactic needs to be used.