Insecticide Resistance Management
Jude Boucher, University of Connecticut

Adult pests that survive an environmental hardship, such as the application of an insecticide, are likely to pass the trait that enabled them to survive on to their offspring.  Repeat applications with the same type of pesticide will eventually remove almost all the susceptible individuals from a pest population and leave only those with the resistant trait to reproduce.  Since insects and mites go through generations quickly, resistance to an overused insecticide can develop in as little as three years.

To preserve the useful life of a pesticide, it is necessary to practice resistance management.  The best way to extend the useful life of an effective insecticide is to use it on a single pest generation only, and then on the next generation, use a second insecticide with a different method of killing the pest (mode of action).  On some pests particularly prone to developing resistance (i.e. Colorado potato beetle), it is best to use a product with a particular mode of action on only one generation every other year. 

To help select pesticides with a different mode of action, scientists have begun to classify pesticides into different chemical groups (see insecticide chemical groupings in Table 1).  Insecticides that share a common chemical family and mode of action are placed in the same group (#1 through 28) and are prone to cross resistance.  Once a pest develops resistance to a group of insecticides with a particular mode of action, a higher rate of a similar chemical from the same group usually will not control the insect. 

Insecticides in different chemical families but with similar modes of action are placed in subgroups, such as 1A and 1B for carbamates and organophosphates, respectively, which both work by inhibiting the production of acetylcholinesterase.  Despite a similar mode of action, subgroups do not tend to have cross resistance problems.  When limited choices are available, it is possible to alternate between subgroups for different pest generations to help avoid resistance. 

There are similar systems of chemical group numbering for fungicides and herbicides.  These will be available in the updated 2006-2007 New England Management Guide due out next month.

There are many other techniques that can help delay the onset of resistance.  Most resistance management techniques help minimize the use of pesticides, so that a lower proportion of each pest generation is exposed to the toxin.

• Integrate chemical control with effective cultural, mechanical, physical, and genetic management options.
• Use biological/microbial control agents or other selective pesticides to preserve natural enemies and help minimize the number of repeat applications.
• Good spray coverage helps do the job right the first time and avoids unnecessary repeat applications: use the proper size nozzles, the correct angle or orientation, and an adequate amount of water per acre.
• Time the application so that the most vulnerable insect life stage is exposed to the spray.
• To help preserve susceptible individuals, use spot sprays, perimeter trap crop treatments, refuge plantings, and other methods that prevent the entire field or population from being treated.
• Avoid mixing insecticides to control a single pest.