Stewardship way to stop resistance

Cotton producers now have at their disposal an impressive array of extremely selective insecticides. But the long-term efficacy of these products may depend greatly on how growers use and manage them.

Aubrey Harris, entomologist at Mississippi State University's Delta Research and Extension Center in Stoneville, Miss., says producers should always practice good stewardship when using the newer insect control technologies.

Many of the newer selective insecticides are needed by growers for pest control, and every time each of these chemistries is used there is going to be some selection pressure, Harris says.

“The less a specific pesticide is used, the less pressure there is to change a plant's gene frequency from susceptible to resistant,” Harris says. “It's just good stewardship to be conservative in the number of insecticide treatments you make to a cotton crop.”

Harris advises growers to avoid making any unnecessary insecticide treatments. “Try not to base your insecticide treatments on an automatic programs of spraying every five days or so. Growers should also avoid treating when insect numbers are relatively low, or spraying too late in the season.

Resistance management strategies should also include rotating classes of chemicals, entomologists recommend. In a population of resistant insects, resistance levels to a particular class of insecticide increases each time that class of insecticide is used. Once an insecticide is used, its level of effectiveness likely will be reduced against subsequent generations within the season.

Therefore, alternating the use of insecticide classes on different generations of insects during the season is a recommended resistance management tactic.

Since most cotton insect pests are highly mobile such a strategy will be most effective if adopted by all producers in a large geographic area, says Ron Smith, Extension entomologist at Auburn University in Auburn, Ala.

Smith says the newer selective and insect specific insecticides available to growers today may help ward off resistance. “Most of these are different classes of insecticides, meaning they work with different modes of action. This should be helpful in preserving this chemistry for the long term.”

Some of the newer insecticides on the market are so selective, he says, that they can take out caterpillar pests and leave lady beetles. Or, he says, it may control only one species of caterpillar.

“New cotton insect chemistry must be applied precisely and in a timely manner. Many of these insecticides work either on caterpillar pests or on bugs or sucking pests, with no crossover activity,” he says.

“Some of the newer insecticides available for the control of caterpillars include Tracer, Steward, Intrepid and Denim (registration for Denim is expected in 2003). These products represent four unique modes of action, so we can swap them around and preserve them for the future.”

In addition, the imminent release of Monsanto's Bollgard II, with a second Bt gene that works at a different site in the insect's gut, should make resistance development less likely, he says. Other companies are also developing their own transgenic varieties, including Dow, which is working with a variety that also has two Bt genes, and Syngenta, which is working on a different type of protein in its new Vip Cotton that it hopes to have in the market in 2004.

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