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Make No Soybean Aphid Assumptions

Even if your fields have low populations so far, keep scouting.

In many fields this year, soybean aphids have been a no-show. However, that’s not the case everywhere, says Bruce Potter, University of Minnesota (U of M) Extension integrated pest management specialist. He says fields at the U of M Southwest Research and Outreach Center near Lamberton have experienced aphid infestations.

Even if you’ve so far had no soybean aphid infestations, Potter says it’s a bad idea to make the following soybean aphid assumptions.

1. It's too late for soybean aphids to hurt yield.

Potter says aphid populations are late developing this year in some areas. Due to planting delays, soybean development also lags in some areas, such as southwestern Minnesota.

Last week, soybeans were just beginning to fill in parts of Minnesota, says Potter. Several more weeks exist in which soybean aphids can damage yield potential. Yield can be lost until the R6.5 stage (between full seed and beginning maturity).

“If populations continue to increase and persist late, we could still lose 10 to 15 bushels (per acre) in most plantings and 40% in late-planted soybeans,” Potter says.

Aphid control does require an insecticide and application investment, he says. Still, in states like Minnesota, soybean aphids are a consistent threat to yield loss, he adds.

“Particularly because of the tight farm economy, you should have budgeted for foliar soybean aphid control in 2016,” he says. “If you don’t need to treat – great. That goes on the black side of the balance sheet. If you do need to treat to avoid economic loss (remember the difference between economic injury level and economic threshold), you are likely to have a good return on your investment.”

Use this economic/action threshold for soybean aphid treatment until soybeans reach the R6 stage.

* 80% or more of the plants with aphids
* An average 250 aphids per plant
* Increasing aphid populations

2. I don’t have an aphid problem because I looked last week and populations were low.  

Did you look effectively? On more mature soybeans, aphids are now distributed throughout the soybean canopy. It makes scouting more difficult, says Potter.

“If you are seeing spots of honeydew on lower canopy leaves, you are (or were) likely to be over 250 aphids per plant,” he says.

In some fields, aphid populations now are mostly in the small white dwarves in the lower canopy. Count these aphids. They reproduce rapidly, and these populations will begin to produce normal aphids later in the season. These populations can build to many thousand aphids per plant late in the season.

Aphid populations rapidly can change in a week. In many fields, a large number of winged aphids were being produced on crowded plants earlier this month. Look for a large percentage of colonies of aphid nymphs with darker heads and wing pads as a clue that aphids may be preparing to leave the plant and perhaps the field, he says.

Conversely, plants (or leaves) with few aphids are now being colonized by winged aphids. Fields with high populations can rapidly decline. Winged aphids leave, and fields with low aphid populations can rapidly increase with winged aphid immigrants.

Often they arrive in fields that previously had few aphids and few predators. Younger soybean plants present a particularly attractive landing strip. The comings and goings of the winged ones make predicting population increases more difficult. This migration happens every year about this time, and soybean aphids can be transported long distances on weather systems.

Fortunately, you have some free helpers in the field.

“We are just now starting to see a small amount of aphid mortality from fungal pathogens,” says Potter. “Look for groups of dead, discolored aphids. High humidity and cool temperatures will favor an epidemic.”

Potter says population collapses from disease, predation, or emigration are the reason the soybean action threshold includes the caveat of “soybean aphid populations are increasing.”

3. I sprayed earlier, so I won't have a problem.

Keep watching. Rapid population increases and the winged aphids that are now free to move about the country are two reasons that you need to scout until R6.

“Earlier-sprayed fields may be reinfested since the insecticide removed predators and parasites. This can key explosive soybean aphid population increases,” Potter says.

“It is one reason we recommend waiting to spray until the 250-aphids-per-plant aphid threshold,” he says. “Fields that were sprayed early should be watched closely for recolonization by winged aphids.”

Pyrethroid insecticides are also not performing as well as they have previously in parts of Minnesota.

“Some of these have occurred outside the area where there were issues last year,” he says. The problem fields have not yet been documented with an assay, but they do not seem to be due to application errors.”

Check aphid control three to four days after an insecticide application, says Potter. Shaking the plant can dislodge alive but dying aphids. An insecticide-resistant aphid population is revealed by dead predator insects and pockets of healthy aphids with dead aphids in the remainder of the field.

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