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IGRs Bring Relief in the War against the Whitefly Cotton growers were a lot happier this season as the cotton harvest got underway. The reason for their smiles? Two novel insecticides helped them to come out ahead this season in the battle against the silverleaf whitefly. Many researchers believe that IGRs have the potential to revolutionize integrated management of cotton in the state. But, according to Tim Dennehy, group leader of the UA Extension Arthropod Resistance Management Laboratory, growers must be careful not to overuse the IGRs lest they find themselves again facing a resistance treadmill. “At this time last year (1995), many growers in central Arizona faced high resistance to insecticides from the silverleaf whitefly,” Dennehy explains. “Some growers were making eight to twelve insecticide applications, spending in excess of two hundred dollars per acre.” Growing resistance of the whitefly to insecticides made it necessary to apply products more often, at higher rates and in mixtures of products. This only further disrupted biological control of the whitefly and accelerated the loss of chemicals to resistance, Dennehy comments. “A large project conducted at the Maricopa Agricultural Center (MAC) by the cooperating agencies, revealed the need for additional tools to manage the growing whitefly problem,” says Peter Ellsworth, an Integrated Pest Management specialist in the Department of Entomology who is located at MAC. “That is why we supported the section eighteen petition this year, presented before the EPA to get approval of Applaud and Knack, with a use pattern of one spray each per season.” Applaud is a chitin synthesis inhibitor which interferes with molting. Knack is a juvenile hormone mimic which impedes insect development; disrupting egg fertility, egg hatching and metamorphosis. “This year, whitefly treatments in central Arizona have been reduced remarkably to two to four sprayings, in many of the same fields that were requiring eight to twelve treatments in 1995,” Dennehy stated. “What’s more, many Cooperative Extension agents and members of the Arizona Cotton Growers Association found that natural enemy levels in Arizona cotton fields are the highest they have observed in many years.” Dennehy says that much of the success of the IGRs and reduced whitefly sprayings can be attributed to the cooperative efforts of the many agencies that worked together to solve this problem. “These IGRs were most effective when incorporated into an integrated pest management strategy,” says Jon Diehl, an assistant in Extension. “The proper use of IGRs required growers to sample both whitefly nymphs and adults and spray only levels exceeding thresholds. This practice can delay the need for other, more disruptive insecticides while conserving beneficial insects. All of this together with some timely local rains lowered the number of whiteflies throughout central Arizona.” “What we saw this year with the IGRs could have not been scripted better,” Ellsworth says. “The IGRs worked better than expected. We told everybody that if we could get through July with six weeks of whitefly control using both IGRs, we’d be okay. As it was, we got up to eight weeks of control in many cases. They performed better than what we had hoped for or predicted.” Both Dennehy and Ellsworth caution against overconfidence. While the two IGRs worked effectively at combating the whitefly, growers must stick to the plan for using IGRs in order to maintain the effectiveness of the products. The prescribed practice of spraying once per season will help preserve the effectiveness of the insecticides and limit the possibilities of the whiteflies building resistance. What was seen in cotton fields around the state, say Dennehy and Ellsworth, showed that the IGRs are extremely effective in managing whitefly populations. “Our ultimate resistance management objective is to identify and implement sustainable use patterns for whitefly insecticides,” Dennehy says. “It is for this reason we are suggesting once-per-season use of Applaud and Knack, and why we wish to limit and diversify our total insecticide regime in Arizona cotton. Doing so avoids putting too much pressure on any given group of chemicals.” Dennehy states that the once-per-season recommendation is based on experience with the IGRs elsewhere in the world. “If the IGRs are approved for more applications than once per season, resistance problems will be incredibly hard to combat and that’s a risk we shouldn’t take,” he says. If the IGRs are limited to a once-per-season use then these products could last five to 10 years longer with the same efficacy, Dennehy adds. And that would be bring even more smiles to Arizona cotton growers. Article Written by Crystal Renfrow, ECAT, College
of Agriculture Researcher:
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In
the 1995 season many growers endured several replantings of their
crop, bad weather and a high level of whitefly infestations. Yet in
1996, growers around the state experienced a dramatic decrease in
the number of insecticide sprays due to the cooperative efforts of
The University of Arizona College of Agriculture, USDA, the Arizona
Department of Agriculture and the Arizona Cotton Growers Association
to obtain emergency registration for the use of two new products known
as insect growth regulators (IGRs). IGRs are innovative insecticides
that disrupt growth and development of insects but are relatively
safe to humans and the environment.