As in the past two years, Arizona’s cotton growers shouldn’t face much of a threat from lygus this season, says entomologist Peter Ellsworth, University of Arizona IPM Coordinator, who also directs the Arizona Pest Management Center.
“Based on my findings thus far, lygus pressure this year appears to be quite light,” he says. “It would be generous even to say it was tending toward moderate.”
That assessment includes observations he made the third week of June on a trip that took him from Mexicali, Mexico, through Yuma and Parker, Ariz., to cotton areas around Buckeye, Goodyear and Phoenix, as well as Maricopa and Stanfield areas.
“Of all the fields I looked at, I saw only one with significant lygus counts,” he says. “They were all adults, so the population wasn’t yet resident in that field. It made sense to find them there — the cotton was adjacent to an alfalfa field that had just been cut following a heavy bloom. Still, it could be two or more weeks before the bugs might reach threshold levels for spraying cotton.”
The low lygus numbers, so far, reflect a combination of developments this year, Ellsworth notes.
For one thing, Arizona growers, attracted by record-high cotton prices, may have planted 30 percent more cotton than in 2010. At the same, many of these new cotton fields were in alfalfa last year.
“Taking that much alfalfa out of the system is reducing potential breeding areas for lygus, while increasing the distance between alfalfa and cotton fields,” Ellsworth explains. “When you spread fewer lygus from alfalfa over more cotton ground, you end up with lower numbers of the bugs in all the fields.”
What he has been seeing this year is consistent with computer models that show that increasing the proportion of cotton acreage in the landscape relative to alfalfa reduces the risk of lygus in cotton.
Strong prices for both cotton and alfalfa this year have helped to discourage planting of other crops that host lygus, such as safflower and seed alfalfa. The exceptionally dry weather in Arizona this past winter and spring has limited the number of uncultivated plants that also host lygus.
Ellsworth has also been finding plenty of natural enemies of lygus and other cotton pests, including Drapetis flies, bigeyed bugs, Collops beetles, spiders and lacewings.
The weather didn’t really get hot until the end of June. That’s later than usual. The cooler than normal conditions helped keep many predators in cotton for a longer period than usual, especially while the canopy was open. As canopies close, that helps create the sheltered conditions that some predators prefer.
At one location during tour of Arizona’s cotton fields, Ellsworth took three-sweep samples for a total of 75 sweeps. In all, he caught about 50 to 60 insect-eating predators, but just two insects that feed on plants. In other areas, he was catching 20 or more bigeyed bugs per 100 sweeps.
“Although the abundance of specific species varies from one location to the next, generally predator numbers are uniformly very high across all fields,” he says. “Right now, the number of predators far outweighs the number of pests in cotton — and that’s a good position for Arizona growers to be in.”
Still, there’s cause for possible concern later this season: the wide variation in cotton planting dates this year.
“It’s probably as broad a difference as I’ve ever seen,” Ellsworth says. “Early-planted cotton can sometimes host the earliest lygus, but escape the need for treatment. Then, as that early cotton moves into cut-out, blooming out the top, lygus begin to move in and concentrate in the much later planted, still actively blooming cotton. This could be a challenge for some growers in August, when late plantings are hitting peak bloom and populations of lygus are much higher.”
By late June, adult lygus were beginning to move from alfalfa stands into cotton fields in central Arizona. But, it was too early to get much pressure from the insect this season, says Tom Montoya, a PCA with Fertizona. Based in Sacaton, Ariz., he works with growers in Maricopa and Pinal counties.
“Normally, lygus don’t become a problem here until the end of July,” he says. “Right now, we’re about seven days behind normal on heat units.”
As in the past two years, lygus numbers appear to be light in his area, Montoya notes. “I’m not surprised. We haven’t had much rain and it’s pretty dry. There’s not a lot of vegetation for lygus to migrate from, and growers seem to be keeping weeds down.”
Typically, his growers spray lygus with one or two selective insecticides, flonicamid (Carbine) and clothianidin (Belay). If needed, they rotate between the products for a second spray, applying it 21 to 30 days after the first treatment.
In the southwestern corner of Arizona, lygus populations seem to be about 10 days later than usual, says independent PCA Bill Fox, Yuma, Ariz. In fact, he swept up his first lygus of the season in the Roll area, east of Yuma, on the last day of June. He netted, at the most, three nymphs per 100 sweeps.
“They outnumbered the adults,” says Fox, who planned to begin spraying in the area the first week of July.
Several days previously, he started to see the first nymphs in the Yuma valley, where cotton is planted earlier. In one 150-acre cotton field, he was catching anywhere from four to eight nymphs in every 100 sweeps. Although he sprayed that field the last day of June, he expects most of the spraying for lygus will begin around the middle of July, followed later by a second treatment, if needed.
When it comes to making the most profitable use of insecticides to control lygus, Ellsworth advises growers to spray only when the population in the field reaches the action threshold. That’s when the number of lygus in a 100-sweep sample totals 15 and the number of nymphs found in those sweeps totals four. This 15:4 level, as it’s called, is a well-established figure, based on research and validated on commercial farms in Arizona.
That threshold is as appropriate with today’s high-priced cotton as when the market was much lower, he points out. This year a number of growers have been asking Ellsworth if, in light of such valuable cotton, it makes sense to lower the threshold. His answer is an emphatic “no.”
This threshold value indicates when spraying for lygus offers the best opportunity to maximize both yields and revenues. For this insect, the existing threshold is valid regardless of the price of cotton, he says.
“We’ve tried to control lygus below the four nymphs level and were unable in increase yields, even with nearly double the number of sprays. In fact, research supports thresholds as high as 15:8.”
Growers have a choice of products representing five classes of chemistries for spraying lygus. “Control is fairly straightforward,” says Ellsworth.
His top choice, flonicamid, is fully selective for lygus and, therefore, safe on beneficial insects, he says. He recommends rotating it with another selective insecticide, clothianidin (Belay), a product that was registered last year.
“We’re in the final year of testing its selectivity,” he says. “It’s definitely more selective than the older broad spectrum products used on lygus.”
Other possible options are the broad spectrum standbys, acephate and oxamyl. Another, endosulfan, is being phased out and will no longer be registered for use in cotton after July 31, 2012.
“Because they’re harmful to beneficial insects, we recommend using these broad-spectrum products rotationally and, ideally, as a last resort for controlling lygus infestations very late in the season when the benefits of natural enemy conservation may be lessened,” Ellsworth says.
As he cautions growers, none of these products will kill a high percentage of the adult lygus. “If you’re expecting zero adults from using any of them, you’ll be disappointed,” he says.
None kill lygus eggs directly, either. Instead, he emphasizes, they’re designed to kill the nymphs, the most damaging stage of the lygus life cycle.
“The more growers and pest managers focus on this basic biology,” says Ellsworth, “the happier they’ll be with the results of their treatments.”