Since the introduction of the Bt trait, soybeans experience more pest pressure than corn. The most predictable problem is soybean aphid.

Producers in the northern Corn Belt are accustomed to scouting for aphids and spraying to control them. Early in the past decade, a pattern emerged where the aphids built up in large numbers one year, farmers sprayed, and then the following year aphid numbers were low. Outbreaks occurred in 2003, 2005 and 2007; in 2002, 2004 and 2006 there were few fields that needed treatment.

In more recent seasons, a new pattern emerged. Aphid populations annually surpassed thresholds across much of the aphid's northern range. Farmers sprayed each year, sometimes twice each season.

"Our on-and-off-again theory is probably gone," said Matthew O'Neal, soybean aphid entomologist at Iowa State University. O'Neal suspects aphids are becoming an annual pest. He also believes their annual populations are building up to threshold levels earlier in the season.

"By late July and August, populations increase rapidly and now stay around until September," he said.

"We have a pest who has the capacity to break out early and stay late in the summer. For Iowa, aphids are flying in mid to late summer from other states and colonizing fields that were already sprayed with an insecticide."

O'Neal said growers need to scout throughout the season and use an insecticide when population reaches a threshold of 250 aphids per plant. Once the crop is past R5, it isn't economical to spray, he added.

PREDATOR INSECTS' ROLE

In the past, predator insects played a role in keeping aphid numbers down, and were a part of the on-year, off-year phenomenon. During years when aphid numbers built up, predator numbers also increased up over that summer so that fewer aphids migrated to buckthorn plants and laid eggs.

Predators are still there, but their numbers are down.

"The last couple of winters, it was not cold enough to kill aphid eggs, but would kill (Asian lady) beetles and other predators," O'Neal said. Aphid eggs are very hardy and can tolerate temperatures down to -40 F. The more routine use of insecticide on soybeans also kills predators and keeps their numbers down.

2010 PREDICTIONS

How soon aphids begin to build this season depends on how many overwintered.

"Huge numbers of aphids went back to buckthorn last fall, but we don't know how many laid eggs," O'Neal said. Large numbers of aphids migrating to buckthorn plants had been an indication of an outbreak the following year. With annual outbreaks more common, the large fall population now more likely indicates an earlier outbreak, he said.

He added scouts are seeing aphids on buckthorn as eggs begin to hatch in April. "I expect to start seeing aphids in May and June."

"Aphids are always going to be threat, some year less and isolated, and other years very widespread," concluded O'Neal.

The threshold for spraying aphids is 250 aphids per plant. That gives farmers a few days to spray before populations build to the 1,000-aphid level, which can reduce yields. O'Neal said new aphid-resistant soybean varieties could raise the threshold in the future. These new varieties aren't aphid-free, but don't support the reproduction and population buildup at the rate of older varieties.

NUMBER OF SPRAYS

Farmers should spray once aphids reach threshold. However, once the soybeans reach growth stage R 5.5, an insecticide application is no longer justified.

In 2008, some growers sprayed aphids twice, because an early outbreak was followed by a second wave in late summer.

"This happens when you spray (insecticide) early and kill off predators," O'Neal said." To save application costs, many growers want to apply aphid insecticides during their early post-emergence application of glyphosate. "When growers are too fast to pull the trigger and spray too early," it creates the perfect opportunity for a later outbreak, he said.

Daniel Davidson can be reached at daniel.davidson@telventdtn.com