Soybean aphid numbers are beginning to climb in South Dakota, says Kelley Tilmon, SDSU soybean entomologist.
"We're beginning to hear more reports of soybean fields that are at or near the treatment threshold," he says.
The threshold supported by research at SDSU and other Midwestern universities is 250 aphids per plant through at least 80% of the field. Some data suggests the threshold can be increased (i.e., to tolerate a higher number of aphids before treatment) as plants near maturity, but that there has not been enough research to supply different guidelines, since heavy aphid infestations usually occur earlier in the season.
"Until we know more about the relationship between late-season aphid feeding and the yield response of near-mature plants, we won't have different thresholds for this late stage. Using a threshold of 250 through R5 is a safe way to go. But as plants get close to maturity, there's probably a bit more wiggle-room."
The 250-aphid threshold is valid through the R5 growth stage. Once beans reach R6 (full seed), research has not shown a reliable yield gain from insecticide treatment. In cases of very heavy infestation in early R6 (1,000 or more aphids per plant), the likelihood of increasing yield by spraying is not clear, Tilmon says.
Aphid infestations can be spotty, Tilmon said, so it is important to make a treatment decision based on monitoring several locations in field, chosen randomly. When scouting for aphids, examine 20 to 30 plants per field, over 10 different locations within the field.
Count the total number of aphids on each plant. If 16 out of 20 (or 24 out of 30) plants have 250 aphids are more, treatment is recommended within seven days.
Aphid populations can climb rapidly. Continued scouting for soybean aphids at least until R6 is still well worth it, he says.
If you treat fields, watch for spider mite outbreaks.
"We often see mite flare-ups in recently treated fields because the insecticide tends to knock down the natural enemies that often help keep mite populations in check," Tilmon says. "Mite outbreaks can occur anyplace, but keep a particularly close eye on fields that have been treated with pyrethroids."