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Managing Insect Pesticide Resistance

A resistant pest is one that suffers no ill effects after exposure to a pesticide that formerly killed it. This scenario has repeated itself over and over with all types of pests. Colorado potato beetle, numerous species of whiteflies, leaf miners and spider mites share a demonstrated ability to overcome the effects of pesticides.
Pests on Plants
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How pesticide resistance occurs

Insect, weed and disease pests have the ability to "learn" and pass their "schooling" down to their offspring. The learning takes place as a change in the inherited genetic material within the pest population. 

For example, year in and year out your neighbor has been spraying pesticide "A" to suppress aphids, his number one pest. Each time he sprays, 99 percent of the aphids are killed. 

The aphids are never entirely eradicated, and over the course of a season millions of new aphids are born. A small portion of those aphids have slight, accidental changes, or "mutations" in their genetic makeup. Most of the defective, mutant aphids don't live long enough to pass on the mutation to their offspring. 

However, one summer day, an accidental change in the genetic makeup of a single aphid turns the pest into a "super bug." The aphid is now no longer susceptible to a formerly toxic pesticide. Due to this highly beneficial mutation, this aphid leads a long and fruitful life, passing the genetic change onto thousands of offspring. 

Your neighbor notices more and more aphids, and increases his spraying frequency to every five days instead of every 10. By the end of the season, he's succeeded in killing off all of the susceptible aphids, leaving only the mutant "super bugs." If he doesn't make a change to a different, still effective insecticide, he's in for a huge pesticide bill and a poor return on an aphid-infested crop.
What can be done?

By the time a pesticide begins to fail, it's too late to do much, other than switch to a new pesticide with a different mode of action (if one is available). Fewer and fewer new pesticides are coming on the market, and they invariably cost a lot more than older materials. What is the answer?

Here are some ideas to help delay the development of resistance.
  • Rotate pesticides — Don't use the same material or class of material time after time. Choosing alternates that don't kill pests using the same mechanism prevents a mutant, resistant pest from surviving for more than a couple of treatments in a row. 
    Example: Organophosphate insecticides share the same mode of action. Alternate any one organophosphate with a pesticide from a different class, such as a Bacillus Thuringiensis, pyrethroid, soap, oil or insect growth regulator.

  • Use mixtures — When permitted by label instructions and manufacturers' recommendations, use a tank mix of two or more materials with different modes of action. 

  • Create "refugia" — Refugia are portions of a planting that are not treated with pesticides. These areas allow susceptible pests to escape a pesticide's toxic effects. These pests will reproduce and preserve susceptible genetic profiles in the pest population, ensuring that resistant pests won't take over. 

  • IPM — By treating only when absolutely necessary instead of on a regular schedule, you limit pest exposure to pesticides and the development of resistance.

  • Spot spraying — By treating border rows only (for pests that migrate into your planting from outside) and localized "hot spots" where pest numbers are over the action threshold. You leave large areas unsprayed for susceptible pests to survive. These areas can be treated at a later date, if thresholds are reached.

  • Evaluate results — Always check treated areas to verify the results of your treatment. Keep detailed records on which materials have been applied and where, and what percent kill was achieved. 
Only by careful post-treatment monitoring and conscientious record keeping can you be sure to avoid an unpleasant surprise from
resistant pests.
Pesticide Resistance Mechanisms

Pests formerly killed by pesticides can develop defenses in a number of different ways: 

Action Description
Reducing Penetration Genetic (inherited) changes in the makeup of pest's "skin" or outer layer can prevent pesticides from entering the insect, disease or weed pest's system.
Behavioral Modification Insects may inherit the tendency to stay on the undersides of leaves, and not venture to the upper leaf surfaces where pesticide deposition may be much greater.
Detoxification Just like humans, insects come in contact with a lot of materials in their environment, as well as food sources, which could be harmful. Mutation can add new "metabolic" or digestive chemical methods to make harmful materials harmless.
Altered Target Some pesticides act by entering pest systems and "binding" to a specific chemical within the pest system and interfering with the normal functions at the binding site. Mutations may alter the binding site such that the pesticide can no longer lodge itself there, or if it does, interference with normal functions is lessened or eliminated.

Resistance to Pests

Resistance is a double-edged sword: pests become resistant to pesticides, and plants can develop resistance to pests. 

In general, however, plants need our help. For centuries, humans have "bred" or consciously selected individual plants with superior qualities for propagation. This has been accomplished through slower, traditional breeding programs, whereby seeds or other propagation materials are selected from superior plants, or by rapid, new genetic engineering technology which allows desirable traits to be "inserted" into plants. 

Examples include traditionally bred disease-resistant varieties of apples, including Liberty, Prima, Priscilla and Sir Prize. Genetically engineered plants including Bt cotton and corn are also products of genetic engineering.
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