Sap-sucking aphid populations can thrive in summer conditions
For a while last summer, the skies above parts of Missouri and Iowa resembled an aerial dogfight, as Air Tractor spray planes banked and dived across aphid-ridden soybean fields. The soybean aphid had arrived in damaging numbers and growers declared all-out war on the pest, including air attacks.
“They hit in east-central Missouri in early August, when most soybeans were in the R-4 stage [early reproductive stage],” said Andy Schmidt, MFA staff agronomist. “They can really explode within a few days, especially when temperatures are in the 70s and 80s and that’s what happened in the area around Audrain and Montgomery counties.”
“They showed up here about the first of August and built up quickly,” said Richard Hunt, regional MFA agronomist based at Laddonia, Mo. “You could find a few one day, and a day or two later, they’d be all over the field. We fought them from early August virtually through the rest of the month, with both ground rigs and air sprayers. Some days, we ran more than 5,000 acres. This was the worst aphid infestation I’ve seen in more than 35 years.”
Hunt coordinated the aphid battle; setting up a sort of task force with men and equipment from MFA outlets at Laddonia, Martinsburg, Mexico, Vandalia and Montgomery City.
“It was a team effort all the way,” he added. “We also called in spray planes from central Illinois. Things went fast and furious for several days. We were making two round trips a day to the MFA farm supply warehouse at Sedalia to pick up chemicals. When it was over, we had treated more than 35,000 acres, and I think we were timely enough in most cases to head off much yield damage.”
Some growers, who waited awhile to pull the trigger, did take a yield hit.
“I didn’t start spraying until about mid-August,” said Jim Curtis, a soybean producer in Audrain County. “I knew the aphids were here; I just underestimated how fast they were multiplying. They seem to spread faster in relatively cool, damp weather and we had that last August. I think they cost me a few bushels.”
Aphids typically overwinter in states north of Missouri, where buckthorn shrubs are prevalent. Buckthorn is the main alternative host for the pest.
“They can withstand a lot of cold, if they have buckthorn for a host; fortunately, we don’t have a lot of buckthorn in Missouri,” said Schmidt, who noted that soybean aphids readily change from winged to wingless states. “Aphids typically swarm into Missouri fields from areas north and east of here and they can build up numbers in a hurry. We watch surrounding states to see what’s going on and scout fields often. The threshold for treatment is about 250 aphids per plant.”
Unchecked, soybean aphids can build up to thousands per plant, and can multiply quickly under favorable conditions. They suck sap from soybean plants, reducing yields and even killing the plants.
“I’d recommend that growers start checking for aphids in early July and keep at it every two to three days during the aphid season,” he added. “If you find an average of 250 or more aphids per plant, take action right away. Products like Lorsban, Warrior II or pyrethroid-based insecticides are effective against the aphid when they are applied at the right stage. One drawback, though: all insecticides that control the aphid are hard on beneficial insects [predators of aphids, such as ladybugs and lacewings].”
But Richard Hunt points out those beneficial predators of aphids cannot keep up with the pest once it gets a foothold in an area.
Hunt agreed with Schmidt that a main key in fighting soybean aphids is to scout early and scout often.
“Start looking for aphids in early- to mid-July and keep at it every week or so until the threat is passed,” he said. “They’re hard to see—small and about the same color as soybean plants. But if you find 250 to 300 aphids per plant, hit them with insecticide.”