Different weather patterns encourage different insects to become problems during each growing season, and soybean aphids are one of those insects that can quickly become a big problem or not reach levels of plant injury—all of it dependent on the weather. As one Extension entomologist described them, soybean aphids are kind of “wimpy” insects in being able to survive and reproduce in extreme weather conditions.

“I think you can prepare yourself for general weather and pest trends,” said Roy Boykin, Syngenta, insecticide technical brand manager. “Growers should probably be prepared for a warmer summer and a summer that might be drier than usual.”

Boykin referenced the La Niña weather pattern, which the National Weather Service reported peaked sometime in the December-January timeframe and has been weakening ever since. The national Climate Prediction Center’s Ed O’Lenic estimates the impact of La Niña will have “weakened to near neutral status by June.”

Boykin suggested that no matter the weather, some species of insect will appear in crop fields, but whether they reach economic impact levels is never known much in advance. Predictions have to be based on long-term weather forecasts, but the final truth comes down to finding the actual insects.

An early warm weather pattern or extended fluctuations with many days in the 70-degree range can encourage early hatches of insects. “You could start out with a higher insect population, and that means through the year there are more generations, and we end up with higher populations during the summer. This is not a population shift based on weather but a change in population dynamics in terms of how large the infestations are,” said Boykin.

 Evaluating Soybean Aphid Populations

APHIDS AND THE WEATHER

Warm weather early is what soybean aphids like. The longer or more often temperatures are above 65 degrees and below 80 degrees and the relative humidity is fairly low then soybean aphids are going to be rapidly reproducing.

Soybean aphid is a new invasive insect first discovered in Wisconsin in 2000. The insect has only one known overwinter host—buckthorn—which is predominately an unwanted upper Midwest shrub.

“If we have really nice temperatures like this (above 65 degrees in mid-April) and moderate moisture, they do very well, but if we have temperatures that fluctuate way up and down combined with heavy wind and rain, this is not good for them because they are kind of wimpy insects,” said Erin Hodgson, Ph.D., Iowa State University, Extension entomologist and assistant professor.

“Soybean aphids coordinate their egg hatch with the bud burst of buckthorn in the spring. They have several generations on buckthorn before they move to soybean. By the time they are ready to move from their winter host to their summer host there are usually some emerged soybean. So, definitely, early warmer temperatures accelerate everything as far as egg hatches because the hatches are all based on temperature,” Hodgson explained.

“If you start seeing fields colonized early and a few fields are heavily infested early, they are the source aphids that can infest whole surrounding areas,” said Hodgson. “I really think the spring migration sets the tone. If the aphids have successful spring colonization to soybean and the weather is conducive for them to be reproducing and moving, then that is when I think you can have widespread outbreaks.”

Hodgson does not abide by the theory that soybean aphid populations fluctuate from high to low in alternate years. She bases populations on the weather.

Hodgson explained that there are both winged and wingless soybean aphid in fields, and those winged ones can travel hundreds of miles, although discovery of them in Mid-South states normally doesn’t mean they will reach economic threshold levels, and they won’t overwinter without their buckthorn host.

INSECTICIDE USE

Companies such as DuPont have been countering soybean aphids ever since they were discovered with several effective insecticides such as the company’s version of a pyrethroid, Asana. The effectiveness of a multitude of other insecticides (i.e. Warrior, Lannate) have not been questioned, but
new insecticides such as Endigo ZC from Syngenta
are labeled for soybean aphid. A few candidates in
final field research are potentially going to be labeled for soybean aphid control. Use of different modes of action insecticides or rotation of insecticides alleviates resistance by aphids; limiting aphid control to one application per growing season helps, too.

“There are situations where the population of aphids accelerate, or they proliferate quickly,” said Dan Sherrod, DuPont, U.S. product development manager for insecticides. “It is the nature of the aphid lifecycle that they can reproduce so quickly that the population goes up very fast. You can have a population that is out of control, so to speak.”

Scouting in order to immediately apply an insecticide when the population reaches 250 aphids per plant on 80 percent of the plants in a field is the standard recommendation. Making an Asana application immediately at threshold has shown on average to save 11 bushels per acre compared to untreated soybeans that reach threshold, according to Sherrod.

In the upper Midwest habitat where buckthorn extensively grows, scouting for soybean aphids in mid to late June into early July is necessary. “Scouting needs to be done on at least 20 to 30 plants to see if aphids are present and in what numbers if they are found. And the best times to spray are late vegetative and early reproductive stages of the plant. What you are trying to do is preserve yield and only have to spray one time,” Sherrod said.

Again, the weather and field location can cause exceptions, noted ISU’s Hodgson. “It is very rare that aphids reach threshold before soybean bloom. It has happened where a field was colonized really early, maybe right next to a border area of buckthorn, and this resulted in the soybean needing sprayed,” she said.

NEW SOYBEAN GENETICS

The need for insecticide application is being reduced more each year, not because of weather patterns, but because of improved soybean genetics and new soybean varieties on the market. This spring, three multinational companies owning soybean genetics promoted their soybean aphid varieties.

Syngenta took the lead in 2009 by advertising its soybean aphid-resistant varieties for planting in 2010 under the banner of its Aphid Management System (AMS); the program was highly regional because there were initially only two NK brand aphid resistant varieties (a 2.1 maturity and a 2.5 maturity) on the market. This year two more resistant varieties (a 1.2 maturity and a 1.7 maturity) were added to the NK lineup. Each of these varieties contains the Rag1 native gene specific to reducing survival and reproductive capacity of aphids feeding on the soybean plant.  

The AMS program requires the resistant seeds be treated with CruiserMaxx Beans insecticide/fungicide. If the gene and seed treatment don’t keep aphids from reaching threshold, then farmers who purchased the AMS varieties are eligible for a $5 per acre rebate on an application of Warrior or Endigo insecticides.

“Using CruiserMaxx treatment with non-resistant varieties, we have seen a 15 to 20 percent reduction in aphids just by treating seeds alone, especially early-season aphid infestations. And with the Rag1 seed technology used alone, we’re seeing 50 to 60 percent levels of control. This is not bad, but it is not a silver bullet to take care of the aphid problems. When you put those two control methods together, you get a combined effect that we’re seeing of 85 percent or greater control in aphids per plant as compared to doing nothing,” explained Bruce Battles, Syngenta Seeds, agronomy research manager.

As Battles noted, an insecticide spray at threshold doesn’t really result in 100 percent control, nor does it last over an extended period of time. He also said breeding even better resistance into plants is continuing, but the expectation is that it will be several years before additional technology is offered to growers.

For 2011 planting, Monsanto introduced a soybean aphid resistance program with limited varieties of group I and II maturing Genuity Roundup Ready 2 Yield soybeans containing the Rag1 gene. Best protection from aphids is also linked with using an Acceleron insecticide/fungicide seed treatment product.

At Pioneer Hi-Bred, a DuPont business, Paul Stephens, Ph.D., senior research director, crop genetics research and development, noted that the company is looking at novel antibiosis genes (those reducing survival and reproductive capacity of aphids) and antixenosis genes (those causing aphids to avoid the plants).

Currently, Pioneer does not claim Rag1 genetic resistance in any of its varieties, but the company has bred native gene resistance into some varieties. The company gives soybean variety ratings of resistance as exceptional (E), above average (AA), average (A) and below average (BA), which the company says, “allows growers to prioritize field scouting and insecticide application.”

Stephens said, “In the future, we will be incorporating genes such as Rag1 and Rag2 into elite varieties for release in 2012 and beyond. Another focus area is identifying novel genes to impart resistance to biotypes that may overcome these first genes. Researchers will use molecular marker technology to tap into these novel sources of resistance.”