Utah Pests News Spring 2008

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Preventing Pesticide Resistance in Insects

Problem: Utah farmer Joe Gardener has used malathion for the last 10 years to control western tarnished plant bug.  The last few years, it had not been working as well, so he applied the material at a higher rate and more often.  But in 2007, the insect problem was worse than it’s ever been.

Explanation: There is a strong likelihood that the plant bug population on Joe’s property has developed resistance.

Resistance is happening around the world; over 500 species have shown pesticide resistance. In Utah alone:

• Pear psylla, codling moth, white apple leafhopper, and McDaniel spider mite have all shown resistance to Guthion
• Alfalfa weevil is resistant to over five different pesticides


Pesticide resistance begins with over-applying one kind of chemical.  For example, a pesticide sprayed on a colony may leave two resistant survivors.  The survivors pass on their genetic predisposition to resistance to their offspring.  Each time that same pesticide is applied, the number of survivors increases.  As the use of the same pesticide continues over several seasons, a large population of insects becomes resistant, and the efficacy of the pesticide is reduced.  Higher rates and more frequent applications become necessary until eventually nothing works.

How did those first two individuals survive?  The two primary mechanisms of resistance in insects are decreased sensitivity, and detoxifying enzymes.  Like humans, not all insects of the same species are identical.  For example, some insects may contain a form of an enzyme that a pesticide targets that tolerates the pesticide, or have an enhanced metabolic ability to detoxify certain chemicals.

Some insects may develop cross-resistance.  This is when a specific mechanism that is causing resistance to a certain pesticide also causes resistance to other pesticides in the same chemical class.  An insect that is resistant to a certain pyrethroid and is also resistant to other pyrethroids to which it has never been exposed, has developed cross-resistance.

A scarier form of resistance is multiple resistance.  This occurs when insects have acquired two mechanisms, independent of each other, for surviving two different pesticides (one mechanism per pesticide).  The pesticides may be of the same or different classes.  This may occur through the overuse of two pesticides over many insect generations.


Lately, chemical companies have increased their testing of new products on a variety of insect populations, and often include resistance management strategies on their labels.  Both commercial and residential growers should manage pesticide resistance by taking proactive steps:

Practice IPM to lower pesticide use
Pest monitoring, proper timing of applications, accepting higher insect thresholds, and using non-chemical treatments will significantly reduce reliance on pesticides.

Rotate chemical classes
Pesticides are grouped into chemical classes, each with a specific mode of action on a pest.  Using a single or variety of pesticides with the same mode of action over several seasons will promote the build-up of a resistant population.  Tank mixing more than one chemical with the same mode of action will significantly speed up the resistance process.  Additionaly, continuous application of mixed chemicals from two different classes may result in multiple resistance.

Learn the chemical classes of pesticides and rotate from one class to another.  For insecticides, switch to a new class every six-eight weeks.  Switching at each application may increase the likelihood of the insects developing multiple resistance.  We are fortunate in that a whole new arsenal of insecticides of multiple classes have been developed in the last 10 years that provide excellent control using less toxic chemicals.

Using Metabolic Synergists—New Research
Scientists in the UK and Australia have recently developed a formula for combating pesticide resistance.  The synergist PBO (piperonyl butoxide), which is derived from sesame oil, inhibits the production of detoxifying enzymes that allow for resistance.  Surprisingly, mixing this chemical with pyrethrins (which has been done for many years) still results in resistance.

Scientists have discovered the problem: it takes 5 hours for the PBO to “shut down” the detoxifying enzymes in the insects.  Two sprays spaced 5 hours apart, is cost- and resource-prohibitive.  To solve this, the scientists developed a microencapsulated pyrethroid mixed with PBO.  The PBO attaches to the insects immediately, while the “insecticide crystals” take 5 hours to dissolve, thus releasing the pesticide at the right time.  The formulation was tested in the field on the highly resistant silverleaf whitefly, a serious pest of cotton, and resulted in close to 100% mortality.

-Marion Murray, IPM Project Leader