Aphids starting to appear in area fields

Counting soybean aphids is strenuous, tedious work. Jim Donnelly, crops specialist with Ag View FS, said farmers need to check plants in several areas in their fields, and look for a threshold of 250 aphids per plant. Donnelly found an average of five aphids per plant in this field south of Princeton, but warned the population could expand quickly. (BCR photo/Barb Kromphardt)

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It’s time for area farmers to drop to their knees in their soybean fields.

No, not to pray for rain, although that might not be a bad idea.

It’s time to begin scouting for soybean aphids.

“Anytime after emergence we start scouting for them,” said Jim Donnelly, crops specialist with Ag View FS. “Soon after emergence all the way through the rest of the year.”

Donnelly said he’s been finding some soybean aphids in the fields, not enough to worry about, but enough to start paying attention.

In a heavy aphid year, such as 2003, aphids started appearing in the V1, V2 and V3 stages, whereas in a light year, the aphids might not appear until the V4 or V5 stages.

Donnelly said scouting for aphids requires meticulous work.

“They’ve very small insects, so it takes some pretty tedious work to actually be going through and counting them,” he said.

What the farmer should look for is a threshold number of aphids per plant, which is about 250 aphids.

Donnelly said farmers shouldn’t wait until there are symptoms before getting into their fields.

“Oftentimes there will be no physical sign of threshold numbers,” he said. “Pretty much when you only have 250 aphids per plant, you do not have any visual symptoms. Only when you get to 500, 1,000 aphids plus do you start to get sooty mold growing on leaves, and the plants get kind of a sappy, sticky appearance.”

When scouting, farmers should check out plants in several different areas of the field, and look for pockets of aphids.

Donnelly has found aphids in several different fields. He said the numbers are relatively light, but it’s also very early in the season.

“It’s a little bit on the early side to see some of the densities I’ve been seeing, but not at all uncommon,” he said.

In a field south of Princeton, Donnelly found about five aphids per plant in stage V4 soybeans.

“That’s relatively low, but consider how fast they double,” he said. “Generally double time is once every two days under optimum conditions.”

Soybean aphids are usually all female, and they give birth to nymphs, or live young, that are clones of their mothers. When temperatures are in the upper 70s and lower 80s, the population can double every two days. When temperatures are hotter, such as they were earlier this month, Donnelly said the aphids’ reproduction rates  drop drastically and their mortality is increased.

Soybean aphids were discovered in North America in 2000, and their worst year to date was 2003. Fields literally had thousands of aphids per plant on average. In Illinois, between 750,000 and 1 million acres were treated, at a cost of $9 to $12 million in treatment costs, and an additional $45 million in yield loss.

Donnelly said the aphids were also heavy in 2005, but not as bad as in 2003. It helped that 2005 was hot and dry, which reduced populations in some areas.

Donnelly said the alternate year pattern is pretty common.

“When you start to bring in some natural enemies, you always get that cycle,” he said. “You get a heavy aphid influx and here come the natural enemies to work on those. Once the aphid populations reduce, then so do the natural enemies.”

While farmers should be out scouting, Donnelly said they don’t need to be concerned about treatment.

“You have more of a danger by treating early and eliminating what few aphids are out there, but also natural enemies,” he said. “You run a risk of actually inflating your aphid populations more than you normally would if you left it alone.”

If a field reaches the 250 aphids per plant threshold, the farmer has a week to schedule an application to kill off the aphids because there is no yield loss until the figure reaches 750 to 1,000 aphids per plant.

Donnelly said that since 2003, most farmers are aware of the need to scout because they realize what aphids can do to their fields.

“In a couple weeks, guys need to be focusing on what soybean fields they have because if we have some cooler temperatures over the next couple of weeks, these things are going to explode pretty fast,” he said.

Scouting should continue to about the R6 stage, which is reached at the beginning of September.

“They’ve got a long ways to go,” he said.

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