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Corn+Soybean Digest


Soybean aphids, smaller than an “I” on this page, showed up in staggering numbers in 2003, causing yield losses of up to 9 bu./acre if left untreated.

The soybean aphid is a native pest of China, discovered in Wisconsin in 2000. Since then, scientists have focused on thresholds, natural enemies, chemical treatments and resistance to fight the tiny bug.

David Voegtlin, a taxonomist with the Illinois Natural History Survey, estimates that one heavily aphid-infested 80-acre field could produce 400 million winged soybean aphids a day — that's more than 100 million more aphids than there are people in the entire U.S.

Rob Venette, an entomologist at the University of Minnesota, estimates that, assisted by wind, aphids can move 3-6 miles per day and, over the course of the summer, up to 600 miles.

Until this year, different states recommended different thresholds, says Ken Ostlie, University of Minnesota. Research has confirmed that the threshold of 250 aphids/plant — one that several states were using — has proved to be correct.

The threshold, for plants up to soybean growth stage R3, is designed to give the grower ample lead time — 5-7 days — to spray before the insect reaches 1,000 aphids/plant. That's where the amount of damage equals the cost of treatment, says David Ragsdale, entomologist, University of Minnesota.

But once the soybean forms pods, the decision to spray gets a little dicey.

“If there was adequate moisture, there was no damage when aphids increased late in the cycle (at R4 and R5),” says Ragsdale. “But where we had significant drought, and aphids began their increase in R4 and R5, the aphids caused significant yield loss.”

Ostlie's insecticide trials showed a 9-bu./acre yield advantage on treated fields over untreated fields.

“That 9-bu./acre return is probably an underestimate of what would take place if we had timely scouting and application in every field,” Ostlie says. “In some areas it took five days for applicators to respond to growers' spraying requests. With a pest that's doubling every two to three days, that means a field could have a four-fold increase between the time a farmer decided to treat and when the field was sprayed.”

Ostlie tested nine insecticides labeled for use on soybean aphids in replicated field trials — five pyrethroid chemistries, three organophosphates and one carbamate (see chart below.)

Most products did really well against the soybean aphid when the insect's spread, or colonization, had stopped. Growers had problems if an applicator cut corners by using a low water volume, if there were high temperatures or if there was extensive recolonization, he says.

Temperatures also affect aphids.

Research shows that aphids reproduce best in temperatures of 82° F or lower. High temperatures significantly slow their rate of reproduction.

“When we hold aphids at a constant 95° in the lab, they never survive to adulthood,” says Ragsdale. Since fields don't stay at constant temperatures, he's studying how high temperatures will affect the populations of aphids that survive in fields.

It's unknown if aphids affect soybeans between spraying and full kill, but kill rates vary among insecticides.

“We found out that Lorsban has an extremely quick kill — about 24 hours,” says Ostlie. “Warrior achieves its maximum kill between 2-4 days. Asana, Baythroid and Mustang Max each take between 4-7 days to achieve full kill.”

He stresses that growers may underestimate their products' effectiveness because they may check for results too soon after spraying. Even though re-entry intervals (see chart at right) may be as short as 12 hours, Ostlie says it may take up to a week for a product to show its maximum effectiveness.

Some growers' fields were infested early, sprayed and then reinfested with aphids that have blown in.

“Retreatment doesn't mean you made a mistake the first time,” says Ragsdale. “It just means aphid pressure was that heavy.” He estimates that 15-20% of Minnesota's acres needed to be retreated within a couple of weeks in 2003.

“Pyrethroid insecticides are known to repel aphids, and that may prevent fields from being recolonized as quickly,” Ostlie says. “In one study, Lorsban-treated fields were recolonized four days after spraying, whereas the pyrethroid insecticide held the pests off between four and seven days.”

Growers should also keep a close eye on their preharvest intervals (PHIs) before spraying. Ostlie notes that Warrior and Baythroid have 45-day PHIs. In August, farmers need to shift to products with PHIs shorter than that, he adds.

For earlier infestations, farmers can tankmix glyphosate and an insecticide in mid-vegetative stages says Ostlie. He plans further experiments this summer to test if a glyphosate-insecticide tankmix works well in later-season sprays.

Scouting is the best way to determine whether a field needs to be treated for aphids. The insects are so small they're hard to pick out on a soybean leaf, let alone from the cab of a pickup, says Kevin Steffey, entomologist, University of Illinois.

He stresses that aphids must be at threshold levels (250 aphids/plant) before treating.

“People think they waited too long in 2003 to spray. So the minute they see their first aphid in 2004, they may want to treat the field right away,” Steffey says. “That could prove to be a mistake. If they spray too early they may have to spray again, which gets expensive.”

It's possible that aphid populations will crash on their own due to natural predators, heavy rain showers or other factors, he adds.

Spraying isn't the only answer, adds Curt Hill, plant scientist, University of Illinois. He's been finding resistant germplasm in USDA's soybean germplasm collection and working with seed companies to introduce the resistance into commercially available germplasm. Those varieties could be available in as little as five years, he says.

“This kind of resistance is usually overcome by the insect over time,” Hill says. “I think resistant germplasm should be integrated with chemicals and cultural methods to increase its effectiveness over a longer period of time.”

Researchers say it's unlikely that aphid numbers will be as high in 2004 as they were in 2003. But since the aphid is unpredictable, they're planning more research and waiting to see what the wind blows in.

Labeled Soybean Aphid Insecticides

Insecticide Rate (/acre) REI PHI Chemistry
Asana XL 5.8-9.6 fl. oz. 12 hours 21 days Pyrethroid
Baythroid 2E 2.8 fl. Oz 12 hours 45 days Pyrethroid
dimethoate (see label) 48 hours 21 days Organophosphate
Furadan 4F 0.5 pt. 48 hours 21 days Carbamate
Lorsban 4E 1.0-2.0 pt. 24 hours 28 days Organophosphate
Mustang Max 3.4-4.3 oz. 12 hours 21 days Pyrethroid
Penncap-M 2.0-3.0 pt. 4 days 20 days Organophosphate
Pounce 3.2 EC 4.0-8.0 fl. oz. 12 hours 60 days Pyrethroid
Warrior T 1.92-3.84 fl. oz. 24 hours 45 days Pyrethroid
REI - Re-Entry Level
PHI - PreHarvest Interval
All pesticides listed are restricted use except dimethoate.
Source: Ken Ostlie, University of Minnesota
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