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Consider All Costs Before Insecticide Applications
Broad-spectrum insecticides work well for controlling soybean aphids. Still, this benefit is accompanied by costs extending beyond product and application expenses.
“One of the costs that often gets overlooked with the application of a broad-spectrum product is the natural biocontrol in the field,” says John Gavloski, an entomologist with Manitoba Agriculture, Food and Rural Initiatives. Nonselective insecticides blitz both soybean aphids and insects that prey on them.
Soybean Aphid Thresholds
Soybean aphids quickly multiply under favorable conditions. A 2004 Iowa State University (ISU) analysis showed that soybean aphid populations can double every 1.5 to 1.9 days.
Large infestations can decrease soybean yields up to 40%, according to Pennsylvania State University entomologists. In these cases, insecticides are a valuable management tool.
There’s good reason, though, why university entomologists established a soybean aphid economic threshold of 250 aphids per plant with 80% of the plants infested and increasing populations.
“This is really a flaky pest,” explains Erin Hodgson, ISU Extension entomologist. “Aphid populations can quickly crash and burn.”
Temperatures of 90°F. and above can scorch rapidly rising aphid populations. Natural predators can also quickly devour aphids. Meanwhile, friendly fungi can infect aphids and dent an infestation.
Better yet, these control agents are free. Of course, soybean farmers can’t just order up a heat wave to control aphids. They do, though, have some control over predator levels via judicious use of broad-spectrum insecticides.
“When you apply a broad-spectrum insecticide, it just doesn’t take out the aphids,” Gavloski says. “You take out the predators, too.”
A University of Guelph study shows multicolored lady beetle females devoured an average of 95 aphids in 24 hours. Meanwhile, seven spotted lady beetle females consumed 115 soybean aphids in the same time frame. A broad-spectrum insecticide that kills both soybean aphids and predators leaves few natural enemies if soybean aphids emerge later in the summer.
“Aphid populations can then explode,” he says.
Costs may rise in other seemingly innocuous ways.
Teaming a fungicide with an insecticide at R1 (beginning bloom) in soybeans seems like it kills the proverbial two birds with one stone. Success, though, hinges on whether both applications are justified.
If soybean aphids are below threshold levels, insecticide payback is questionable. That’s not just due to product and application cost. Each time a pesticide is applied, it selects for resistant biotypes. Over time, these biotypes increase and eventually render the pesticide ineffective.
That’s why “insurance” applications meant to control soybean aphids when they are below threshold levels can backfire. That’s what some farmers in Minnesota and Iowa are experiencing with soybean aphids resisting pyrethroid insecticides.
“Resistance happens because people find products they like to use, and they use them repeatedly,” says Gavloski.
Hodgson says soybean aphid resistance potential increases if the same chemical is used multiple times within a single growing season.
Weigh all Costs
The fungicide part of the double-shot strategy has merit in some cases. That’s often true if seed quality is the focus, says Heather Young-Kelly, University of Tennessee Extension plant pathologist.
Fungicides can help manage fungal diseases like frogeye leaf spot. Left unchecked, this fungal disease reduces photosynthesis and defoliates soybeans to the point where soybean yield reductions tally 10% to 60%, she says. Some fungicides also can boost yields through physiological benefits like increased chlorophyll production. Still, a well-timed single-shot application has protected the same yield amounti in many of her trials, she says.
A four-year study funded by the Iowa Soybean Association and soybean checkoff involving hundreds of treatments on soybeans across more than 20 locations found strobilurin fungicides have a greater response with no disease than other fungicides.
Still, these benefits need to be weighed against costs other than product ones.
Fungicides that kill fungal pathogens may also kill soilborne entomopathogenic – or friendly – fungi. “Beneficial fungi can do a really good job of reducing aphid populations, and this is something totally overlooked,” says Gavloski.
Cornell University entomologists found that a 2003 soybean aphid population crashed following an 84% infection rate by a pathogenic fungus. Still, entomopathogenic fungi need the right environmental conditions to activate and to attack insects like soybean aphids.
“I see infected insects every summer, but rarely do I see epizootics (disease) where a pest population is wiped out,” says Hodgson.
What to Do?
“Farmers need to be looking out for their crop, and do what they need to do to protect it,” says Gavloski.
In many cases, pesticide applications are needed. Still, think about all costs associated with the application.
“You have to consider all hidden costs – not just the price tag of the product,” says Gavloski. “Whether it is a fungicide or an insecticide, we advocate spraying only when needed. If you build up to a strong base level of biocontrol and keep it there, it can save you a lot of money in the long run.”
Scout, Scout, Scout
Scouting is still the key for successful soybean aphid management.
A 2005 to 2007 land-grant university study in six locations in Minnesota, Iowa, and Michigan compared the following four soybean-aphid management systems:
- An untreated control.
- A prophylactic mix of an insecticide and fungicide applied at R1 (beginning bloom).
- A thiamethoxam (Cruiser) insecticidal seed treatment.
- An IPM strategy of field scouting followed by a foliar insecticide application if the economic threshold of 250 aphids per plant on 80% of the plants and increasing population was reached. If it wasn’t reached, no application occurred.
The IPM approach showed the best results across the board.
“The odds of making a profit increase with economic threshold-based spray applications are highest,” says Erin Hodgson, Iowa State University Extension entomologist.
Scouting is the key, she says. “You aren’t likely to get a yield bump from spraying if you don’t have insects in the field.”
Consider the impact on pollinators when making a pesticide application, says John Gavloski, a Manitoba Agriculture, Food and Rural Initiatives entomologist.
In Manitoba, canola farmers consider using an insecticide when they apply a fungicide for sclerotinia stem rot. However, off-target movement of insecticide may kill pollinators.
“With canola, you can get a decent yield without pollinators, but you can get a 10% to 15% better yield with a good level of them in the field,” says Gavloski. “If insects don’t merit control, mixing in the insecticide can inadvertently decrease yields.”