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- Two questions for growers at this point are when to terminate irrigation and when to apply final insecticides.
- Irrigation decisions, Martin says, should be based on agronomic, weather and entomological information.
- “Generally, considering all types of production scenarios, I’d recommend stopping lygus sprays anywhere from two weeks before the initiation of cut out to one week after the initiation of cut out.”
Final spray for lygus control
Making a good decision on when to stop insecticide use for late-season lygus control in cotton is also important, says Peter Ellsworth, UA integrated pest management specialist, also based at the MAC.
Thresholds for lygus bugs in cotton have long been established in Arizona. Insect sprays are recommended and usually cost-efficient when the minimum threshold density reaches 15 lygus including four nymphs (15/4) per 100 sweeps any time during the peak flowering period.
As crop growth slows and the blooming rate declines, Ellsworth says the amount of salvageable yield from lygus sprays is also reduced.
Factors which impact the best spray termination decision mirror Martin’s irrigation points: the variety maturity class (early to mid- to full-season); the planting date — April is optimal in central Arizona; production goals tied to irrigation termination timing linked to the primary fruiting cycle; and cotton development measured by NAWF counts.
Draft lygus control termination guidelines are available on the Arizona Crop Information Web site at http://ag.arizona.edu/crops/presentations/DRAFT_LT_guide2-pg.pdf.
“Generally, considering all types of production scenarios, I’d recommend stopping lygus sprays anywhere from two weeks before the initiation of cut out (i.e., before NAWF = 5) to one week after the initiation of cut out,” Ellsworth said. “Growers and pest control advisers should make the best decision based on their particular situation.”
The lygus bug, Lygus hesperus, reduces yield and quality. Adult lygus feed on the plant, but plant-bound nymphs create the bulk of the preventable damage. The insect changes the fruit’s spatial pattern and delays fruiting by feeding on and eliminating fruiting sites. This also changes how the plant allocates carbon and nitrogen.
“The energy captured from the sun enters the stems and leafy growth instead of the bolls,” Ellsworth said. “The results are taller plants that are more difficult to defoliate, plus more leaf trash and poorer lint quality.”
Ellsworth has conducted numerous cotton field trials including insecticide use on lygus. He recommends the use of Carbine as the first spray for lygus control.
“Carbine is the best game in town right now for lygus control; it’s very effective, selective and safe on beneficial insects,” Ellsworth said. “The product effectively targets aphids, a potential threat in cotton fields located along the Colorado River.”
He also urges growers to rotate Carbine with other products and chemistries including Orthene, Vydate, and Belay to reduce the chance of resistance over time.
“Belay might be the best rotational alternative to Carbine for lygus control, because the other options are very broad spectrum and less safe for beneficials,” Ellsworth said.
Surveys have revealed about 81 percent of central Arizona cotton growers used Carbine to control lygus in 2009, up from 66 percent in 2008.
Ellsworth’s studies suggest the insecticide Sulfoxaflor, which is several years from registration, has one of the most promising new chemistries for lygus control.
Insecticide use has decreased substantially in Arizona cotton, says Ellsworth; down to 1.5 sprays per season for all pests over the last four seasons.
Ellsworth’s work is supported by grants from the U.S. Department of Agriculture’s Risk Avoidance and Mitigation Program, Arizona Cotton Growers Association, and agrochemical companies.