The first soybean aphids have appeared in Michigan fields. Though it is too early to think about spraying, Michigan State University (MSU) Extension entomologists say it’s not too early to think about making sure that spray equipment is correctly calibrated and ready to use should the need arise.
“The keys to successful aphid control are applying the insecticide at the right time, achieving good penetration of the crop canopy and providing good coverage of leaf surfaces,” says Mike Staton, MSU Extension agriculture and natural resources educator based in Van Buren County and Soybean 2010 coordinator. “It’s important to scout fields and apply insecticides when aphid populations have exceeded 250 per plant and are increasing.”
Whether a grower achieves good canopy penetration and leaf coverage depends on how the sprayer is equipped and operated. The volume of spray delivered per acre has the greatest impact on penetration and coverage. Increasing the volume improves penetration and coverage. When the plants are small (R2 growth stage), 15 gal./acre is adequate. However, 20 gal./acre may be needed for larger plants.
The size of the spray droplets is the second most important factor affecting canopy penetration and leaf coverage. Research has shown that fine to medium droplets with volume mean diameters (VMDs) between 200 and 350 microns are best. Choose a nozzle that produces 200- to 350-micron droplets at the desired ground speed, nozzle pressure and spray volume. When using droplet size classification charts, select nozzles that produce droplets near the fine end of the medium (yellow) category of operating pressure.
“We also suggest that farmers consider nozzle type,” Staton says. “Flat fan nozzles perform better than cone nozzles, and properly sized pre-orifice or low-drift flat fan nozzles may perform better than traditional flat fan nozzles because they can be operated at higher pressures and still produce the optimum droplet size.”
Venturi or air induction nozzles are not recommended because they require very high operating pressures to produce 200- to 350-micron droplets. Individual nozzles or combinations of a nozzle body and two nozzles that generate two flat fan patterns – one angled toward the front and one toward the rear of the boom – improve coverage on smaller plants (R2 growth stage). A recent study at Ohio State University, however, showed that a single flat fan pattern improved canopy penetration and leaf coverage in larger soybeans (R4 growth stage).
“Ground speed is also important to consider – it affects spray volume and the vertical velocity of the droplets entering the crop canopy,” Staton says. “We recommend going 10 mph or less.”
Operating pressure affects droplet size, spray volume and droplet velocity. Higher pressures generally provide better canopy penetration and leaf coverage as long as droplets remain in the medium category and not too many fine droplets are produced.
Correctly adjusting boom height will also improve spray pattern uniformity and droplet velocity. Ohio State University researchers recommend establishing the target area on tall soybean plants (R3 growth stage or later) as the midpoint between the lowest leaves and the top of the canopy. Determine the recommended boom height for nozzle type and nozzle spacing, and set the boom to operate this distance above the target area.
To learn more about pesticide application technology, download a free fact sheet from MSU and Soybean 2010 at http://web1.msue.msu.edu/soybean2010/sbf-application-tech-rev-071.pdf.