After the late, wet spring delayed planting in many areas, soybean stands are finally established and approaching critical reproductive stages. Now is the time when insect pressure is likely to build. Bean yield potential hangs in the balance.
"The reproductive period, from beginning bloom to seed set, is the time to intensify your soybean scouting efforts," says Erin Hodgson, Extension entomologist at Iowa State University. "You don't want any clipping of the pods, or abortion of the flowers or the seeds. Now is when scouting is most critical to protect yield from damaging insects."
Since its introduction the United States in 2000, the soybean aphid has become one of the most threatening pests for many soybean growers. Although the cool, wet spring may delay aphid onset this year, Hodgson cautions growers not to rule out pressure from this devastating pest. "Soybean aphid populations can explode almost overnight. If left untreated, soybean yields can take a significant hit, reducing output per acre by 15% to 40%."
Aphid populations in Iowa have been highly variable the past few years
Hodgson reports that aphid populations in Iowa have been highly variable the past few years. "In 2011, it's likely that not every field will need to be treated with an insecticide, but some might need to be sprayed to protect yield. That's why scouting and use of integrated pest management tactics will pay off financially."
Soybean growers who scout regularly and track insect numbers will be well positioned to make timely treatments when insect populations exceed the economic threshold, she notes.
Paul Compton, who grows1,000 acres of soybeans near Homer, Ill., in the northeast part of the state, says soybean aphids are joined by Japanese beetles, bean leaf beetles and stink bugs as key pests on his scouting list this year. "Piercing and sucking insects are a serious threat to soybean acres in our area. Sometimes, you don't realize the damage they are causing until you take steps to control them. Compared to untreated fields, my neighbors and I have seen yield increases of at least 3 to 5 bushels per acre in our treated fields. Spread across 2,000 acres, the return on investment from a properly timed insecticide spraying is significant, especially with favorable soybean prices these days."
Timing: when should you scout soybean fields for aphids?
Dr. Dan Sherrod, product development manager for DuPont Crop Protection, recommends that scouting efforts be in full swing by mid to late June, and intensify when weather conditions are dry and between 78 and 84 degrees Fahrenheit. That's the optimum environment for soybean aphid populations to flourish. Scouting should continue through the reproductive growth stages of soybean plants, as soybean plants fill their pods.
Scouting is particularly important in late-developing fields, says Sherrod. Under normal conditions, soybean plants have already filled their pods and are starting to dry down when insect pressure increases in August, "The trouble with late planting," he says, "is you'll have green growing plants that are attractive to pests later in the season."
How to scout fields for soybean aphids, be sure to do it right
For accurate scouting, Sherrod recommends examining 20 to 30 plants at multiple points throughout the field and recording pest counts and foliar damage. As pest populations are identified, scouting efforts should intensify, particularly with soybean aphids. "Once aphids are detected in the field, check the crop every two to three days, as populations can double in that brief time," he says.
Soybean aphids are just one of many pests growers should watch for as they walk their fields. "When you're out there making aphid counts, look for cloverworms, bean leaf beetles, Japanese beetles and other pests common to your area," he advises. Foliage damage is best way to spot a worm infestation.
Familiarize yourself with the treatment thresholds for insects recommended by your university Extension service. For soybean aphids the typical threshold is an average of 250 aphids per plant over 80% of the plants in the field. When populations reach this critical number, particularly during the flowering to early pod stages, it's time to take action, before yield potential is compromised.
Spotting the sap-like secretion called honeydew that soybean aphids leave behind, which encourages mold growth, is an indication that the infestation is out of control. "If the field is sticky and you see black, sooty mold, it's critical to make aphid counts and move quickly with a rescue treatment if thresholds are exceeded," he warns.
Tips for choosing and using insecticides to control soybean aphid
Sherrod recommends a broad-spectrum insecticide treatment with good residual activity, such as DuPont Asana XL. It will effectively control aphids and clean up other incidental pests present in the field, such as bean leaf beetles, grasshoppers, green clover worm, Japanese beetles (adults) and green and southern green stink bugs; and guard against secondary pest outbreaks. For severe and rapidly expanding populations, an insecticide with fast knockdown, such as DuPont LannateLV can halt the most aggressive outbreaks, and can be tank mixed with Asana XL or another pyrethroid for long-lasting control.
Sherrod points to recent university studies on soybean aphids in high-infestation areas that revealed that on average, applications of Asana XL helped prevent yield losses by more than 11 bushels per acre compared to the untreated check.
The bottom line is being attentive to the progress of your crop, intensify scouting prior to the critical reproductive growth stages and familiarize yourself with treatment thresholds and insecticide options. "When insect pressure mounts," he says, "growers who take steps ahead of time won't be caught off guard. And that's the key to a profitable harvest."
Even when factoring in the input costs of making an insecticide application, the returns make it one of the most profitable investments a farmer can make in his fields, says Compton. "Some growers enhance their insecticide applications with nitrogen or a fungicide, and those could add value, but I believe it's the insecticide treatment that offers the greatest return on your investment. Insects are what really rob your yield, especially piercing, sucking insects, which also can introduce diseases. Left untreated, your crop just won't yield to its full potential."