However, they could be taking a standing eight count and don’t know it if they let the silverleaf whitefly spawn another year of sticky cotton. One more whitefly knockdown and Arizona cotton farmers could be declared the loser of the fight against the voracious pest that turns cotton lint into a sticky goo, according to a leading University of Arizona entomologist.
"If we let this pest derail us again this year it really could be the end of the cotton industry in Arizona," says Peter Ellsworth, University of Arizona IPM entomologist, at the Maricopa Ag Center in Pinal County, Ariz. "It’s a quality and reputation issue. Arizona has had disastrous whitefly problems in the past and has a reputation for sticky cotton."
The market, he says, needs to know that Arizona growers will not produce sticky cotton. Without such assurances the lint market in the region could potentially disappear.
It’s likely that no other insect has garnered as much attention in the region over the last 10 years, and Ellsworth says if whitefly populations are allowed to escalate in 2002 the damage done could be irreparable.
"We can’t afford to add 2002 to the list of disaster outbreak years that include 1992 and 1995, because the marketplace is still jittery. As compared to other areas of the Cotton Belt, we still don’t get as much per pound for our lint as we did in the late 1980s before the whitefly infestations began," he says.
Expected to be a bad silverleaf whitefly year because of its early start, the pest has so far remained at manageable levels in most areas. "Last year was one of the earliest invasions we’ve ever seen, and we reached threshold populations the first week of July," Ellsworth says. "In central Arizona, most growers this year haven’t reached threshold levels yet, but they expect to hit that mark within the next two weeks."
There are basically two species of whitefly making their home in Southwest cotton fields. The bemisia whitefly, also known as the silverleaf whitefly or the sweet potato whitefly, is noted for its creamy yellow hue. The banded wing whitefly, with small gray bands on its wings, is slightly larger in size and silvery gray in color
The banded wing whitefly, Ellsworth says, is more of an early season phenomenon in weedy areas of crop fields. "It doesn’t have the damage potential the other species of whitefly has, so you want to make sure it’s not included in your threshold counts."
Intensive management of the silverleaf or sweet potato whitefly, on the other hand, is critical.
The trick to controlling the sweet potato whitefly, he says, is to apply products only in those fields necessary because "you can’t kill what’s not there."
Stage one materials
Ellsworth recommends a "first strike" treatment using insect growth regulator (IGR) products. "Use stage one materials first when you reach the treatment threshold for the silverleaf whitefly. These products have a proven track record, and they do their magic best when they are used first. Used as a follow-up treatment, they provide substantially less bang for your buck."
If a sequential control treatment is needed, Ellsworth says cotton growers may then consider following-up with some of the newer non-pyrethroid materials, such as Intruder and Centric.
"It is reasonable to assume IGRs will outperform alternative products in most cases, when used first due to the biological residual you can obtain with an IGR regime," he says. "Intruder and Centric are great products, but growers still need to use IGRs as first-strike products. My bet is using the newer products as your first line of defense in mid-July won’t get you through to September."
According to the University of Arizona’s whitefly management recommendations, cotton growers should begin scouting for nymphs and adult sweet potato whiteflies at the fifth mainstem leaf below the terminal. If three or more adults or at least one large nymph is present, that leaf should be counted as "infested."
After sampling 30 leaves in at least two locations, if more than 57 percent (17 of 30) of sampled leaves have been found to be infested with whiteflies, a control treatment is recommended.
Less of a threat to the quality of this year’s cotton crop, but still important to maintaining yield is the control of lygus.
The good news is that while lygus pressure is usually high because of the large acreage of Bt cotton in the state, entomologists say infestations of the pest appear to be scattered this year. "The problem areas we’re seeing are those areas where lygus have been driven out of other hosts, such as in cotton crops adjacent to alfalfa and safflower fields," Ellsworth says.
Taking the brunt of the pressure is central Arizona, with an active monsoon and other pressures present, including expanded alfalfa acreage and fallow fields. Each of which presents increased opportunities for immature generations in alfalfa to mature into adulthood in the alfalfa, making the move to cotton that much easier.
This year’s cotton crop is off to a good start, which may help hold down the impact of the lygus pressure," he says. "In general, the lygus pressure is not as bad as it could be, but locally the problem may be more severe, depending on available hosts. Any late-planted cotton will be also very vulnerable to lygus attacks."
With populations of the pest building statewide, treatments started as early as two weeks ago in some areas. For most of the state, though, the peak time to treat for lygus will be within the next one to three weeks.
"We have a number of excellent tools available to manage lygus. It’s a matter of sticking to the thresholds and treatment recommendations," he says.
Ellsworth says he routinely recommends four options to cotton growers, which include treating with Orthene, Monitor, Vydate or endosulfan. "These products represent three classes of older chemistries, but they are the ones that work the best to control lygus in Arizona. The next best alternatives are so far down the list, they aren’t worth mentioning."
Despite the fact that there are other products marketed to the region’s cotton growers, and labeled for "plant bug" control, Ellsworth says they won’t do the job in Arizona. "Centric, Actera, Intruder, and Assail are all good products for the control of whiteflies and aphids, but they don’t work on lygus in Arizona. They cannot handle the lygus we have, so growers shouldn’t experiment with those products."
In general, the recommended treatment threshold for lygus is 15 total lygus, with four nymphs, per 100 sweeps. "This threshold routinely returns the most money to growers, protects yield, and saves money."
Researchers with the University of Arizona are currently researching when cotton growers should stop treating lygus infestations. Ellsworth explains, "Terminating treatments are now done a little bit by the seat of the pants. We want to nail that down and give growers some definitive information."