April 19, 2006 | By: Laura Skillman

With proper management, farmers can help reduce the likelihood that pests will become resistant to certain chemical controls. Resistance can result in damaged crops, higher control costs and lower profits.

“We have had a history of insects developing a resistance to insecticides,” said Ric Bessin, entomologist with the University of Kentucky College of Agriculture“More than 1,500 instances of resistance have been documented. So it is a problem and is going to continue to be a problem. Pests are pests because they can adapt.”

Insects can develop a resistance to any type of management tactic that causes selection in their population. Even resistance to crop rotation has been found in some areas of the country. But chemical resistance is one area where a farmer’s vigilance can play a role in reducing the potential.

The key to combating resistance is to fight human nature, Bessin said. It is human nature, for example, to use a control method over and over again until it doesn’t work at all anymore. That philosophy actually helps insects in their ability to adapt.

“We should not overuse a single tactic,” he said. “We need to keep switching tactics on them. That’s how we maintain our products. At times, you may need to use something that’s a little more expensive just to break up the pattern. But that’s better stewardship of those products on your farm.”

A mixture of cultural, biological and chemical tactics should be used, Bessin said. These include crop rotation and using chemicals that are not harmful to beneficial insects, which are the natural enemies to insect pests.

“I had an opportunity to see what would happen if we didn’t have natural enemies and I’ve seen some incredible insect populations,” Bessin said. “This gave me an appreciation for the natural enemies and what they do 24 hours a day, seven days a week. When you remove them, we see some huge pest problems.”

Scouting and threshold levels also need to be used. Scouting makes the farmer more aware about what’s in his field. The thresholds are the level of pest infestation where chemical controls are economically viable.

Rule No.1, Bessin said, is to not treat successive generations of a pest with a chemical that has the same mode of action. For this to work, farmers have to know which of the two dozen chemical classifications they are using. Farmers should also know something about the pest and its life cycle.

While there are a number of chemical classes, the bulk of those used in grain production come from only a couple classes, making the need to be vigilant against resistance critical.  

Timing of applications and rate of usage is also important to limiting resistance. If less than the proper dose is used, this may favor insects that are partially resistant, allowing them to reproduce while the more susceptible pests will be killed.

In some instances, refuges are important. These are areas in a field where controls are not used. This can help keep susceptible insects in the population.

Insects aren’t the only pests that develop resistance. Some weeds have been found that are no longer killed by pesticides, and Kenny Seebold, UK plant pathologist, noted that disease resistance to fungicides is widespread. In Kentucky, this has been documented with Ridomil-resistant blue mold on tobacco.

“Fungicide resistance is a major concern,” he said. “You are talking about organisms which, like insects, are very adaptable and have many generations over a short period of time. Once pathogens become resistant to a particular fungicide, they generally become resistant to all fungicides in that class. Something we don’t see very often is resistance to multiple classes of chemicals.”

As with insect controls, farmers can do a lot to reduce resistance by using proper doses and frequency of application. The more applications, the more times that disease sees it, selectively eliminating the organisms that are susceptible.

“Alternate modes of action,” Seebold said. “Multi-site inhibitors (fungicides that affect more than one biochemical process) are important rotation partners for fungicides that are at risk for the development of resistance.”

The most important point is not to rely on any one disease-control practice but to have a multipronged approach, he said. Use cultural practices such as crop rotation, use resistant varieties and use fungicides only when needed.

“We are going into a time when we are seeing pesticides that are much safer for the applicator, they are much safer for the environment and they are softer on the natural enemies,” Bessin said. “So they may be more inclined for resistance. I think this is going to play a bigger and bigger role in the future.”

Vigilance by farmers can help ensure they will have the tools needed in the future to combat pests and keep resistance at bay, according to the UK specialists.



Ric Bessin, (859) 257-7456; Kenny Seebold, (859) 257- 7445, ext. 80721