Arizona cotton growers hailed the arrival of insect growth regulators when Applaud and Knack became available for whitefly control in 1996. Today they are more important than ever. Growers say IGRs are saving the Arizona cotton industry because without clean cotton, no one would want to buy Arizona cotton at any price.
“With the price of cotton where it is now, we wouldn't be in business today if we didn't have the IGRs,” said Shannon Schulz, who farms 5,100 acres of cotton in Arizona Harquahala Valley west of Phoenix. “IGRs saved the cotton industry in Arizona”
“Before the IGRs came along, our reputation had gone sour due to sticky cotton,” said Bruce Heiden of H Four Farms in Buckeye, Ariz. “The whitefly was about to put us out of business.”
Prior to the Section 18 registrations of Applaud and Knack in 1996, resistance was eroding the effectiveness of conventional pesticides, forcing growers to increase their number of treatments and providing only limited control. According to University of Arizona Extension entomologist Peter Ellsworth, growers treated for whitefly an average of 6.6 times in 1995 at an average cost of $145 an acre. Since then, Arizona cotton growers are averaging 1.18 treatments per year at an average cost of $35 an acre. And problems with sticky cotton have dropped substantially.
“We're talking about a tremendous change in chemical control of whitefly since the IGRs came on board,” Ellsworth said.
Applaud, manufactured by Nichino America, controls whitefly primarily by interrupting the molting process of nymphs, and to a lesser degree by suppressing oviposition by adults and reducing egg hatch. The active ingredient in Applaud, buprofezin, kills nymphs within three to seven days following application. Control typically lasts for one month.
Applaud is delivered to whiteflies in three different ways: contact, ingestion and vaporization. The vapor activity of Applaud enables the product to reach both the underside of leaves and new growth. Knack, manufactured by Valent, is a translaminar material that works by sterilizing adults and eggs. Both Applaud and Knack have excellent environmental profiles and are relatively harmless to beneficial insects.
“Half of the residual control growers enjoy from these chemicals is due to their effectiveness,” Ellsworth said, “and half is due to the preservation of natural predators.”
Because Applaud and Knack have different modes of action and are not chemically related, the products can be applied to the same field in the same year without threat of resistance build-up. By label, Applaud and Knack can each be applied only once a year. The products can be rotated, however, enabling growers to treat twice a year with an IGR if necessary.
“In general,” Ellsworth said, “it's great that they're not chemically related because it helps in resistance management. Both products are very effective, but they act differently. Applaud behaves more like a conventional insecticide than Knack in that it works more quickly on the nymphs in residence.”
In side-by-side comparison trials sponsored by Nichino and facilitated by university researchers, nymph counts were substantially lower where Applaud was used than where Knack was used three weeks following treatment; nymph counts were relatively similar five weeks out.
Whitefly nymphs damage cotton plants by feeding on leaves and excreting honeydew, which causes sticky cotton. At high populations, whitefly nymphs can dramatically reduce yield and quality. Treatment with Applaud or Knack is recommended when large nymph counts reach one per leaf disk and adult counts are three to five per leaf. Delaying treatment can be costly.
‘Wait too long’
“When whiteflies arrive earlier than usual, a lot of growers wait too long before pulling the trigger,” Ellsworth said. “That can minimize the effectiveness of these products.”
Whitefly struck early this season, causing many growers to apply an IGR three to four weeks earlier than normal. “By my studies, 2001 was the second earliest onset of threshold densities of whitefly in history,” Ellsworth said. Heiden said he began applying Applaud July 10 this past summer, nearly a month earlier than his usual treatment date.
“It's been our experience that when you start fooling with whitefly, you better put down an IGR,” said Heiden. Normally he treats for whitefly with an IGR in early August and receives control through harvest. Last summer he made a second IGR application in late August, seven weeks following his initial treatment, when whitefly populations “came on like gangbusters.”
Under proper usage, Ellsworth said IGRs can provide effective whitefly control for years to come and help preserve the effectiveness of conventional insecticides by reducing insect exposure and subsequent development of resistance. “Through the use of IGRs, we're trying to delay the period in the season when we use pyrethroids,” Ellsworth said, “and we recommend that growers don't use a pyrethroid more than twice in a season.” Ellsworth also recommends usage of IGRs, “because they work.”
“IGRs provide cotton growers three main advantages over conventional insecticides,” Ellsworth said: “No. 1, they're very effective at controlling the target pest; No. 2, they last longer; and No. 3, they are very selective, which conserves beneficials and helps growers realize control periods of four to six weeks.”
“If we utilize IGRs as they should be used,” Heiden said, “we shouldn't have any problems with whitefly or sticky cotton. I don't think we could function without them. Our opinion is that IGRs really kind of saved our industry in this state.”
Here are sampling and treatment recommendations
When sampling for silverleaf whitefly, University of Arizona Extension entomologists recommend sampling at least 30 plants weekly. Start by choosing a plant at random and sampling the fifth main-stem node leaf. Turn the leaf over by the tip of the leaf blade or the petiole and tally the leaf as infested with adults if it contains three or more whitefly adults. Tally the quarter-sized leaf disk as infected with nymphs if it contains one large nymph visible to the naked eye.
Continue sampling along a diagonal or zigzag line moving over several rows and taking five to 10 steps before selecting a new plant. After sampling 15 plants, move to a new site within the field and sample 15 more.
Samples can be taken any time of day, but samples taken within 24 hours after a rain may not accurately reflect adult densities.
To schedule the use of IGRs, treat when 40 percent of the leaves are infested with three or more adults and if 40 percent of the leaf disks are infested with one or more large nymph.
For conventional (largely adulticidal) insecticides, treat if more than 57 percent of the leaves are infested with three or more adults.