Soybean aphid populations could be high this summer in the Midwest. The situation poses an interesting question for growers concerned about the potential or likelihood of having to spray to manage Asian soybean rust in addition to soybean aphids.

Many experts say tankmixing herbicides with fungicides is not a good idea, but what about insecticides? Matt Montgomery, University of Illinois Extension educator in crop systems, shared a few thoughts with Agriculture Online.

Overall, he says, spraying for both rust and aphids at the same time is unlikely to be the best strategy, mainly because of the timing of when the problems will strike. Experts agree that the ideal time to apply fungicide for preventive rust treatment is just before first flowering (R1). The chances of aphids striking simultaneously are relatively low.

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"Timing is likely the greatest problem associated with combined control," Montgomery says.

"When will rust get here? When will aphids appear? When will critical thresholds be met for aphids? If (a pretty big if) the two appear in a roughly synchronized manner, there may be some options for combined aphid-rust management," he says.

If growers were to apply controls for both at the same time, are there chemical interactions that might be an issue depending on the chemicals they select? Montgomery says it is unclear, but advises growers to consult the label and their company contact.

Droplet size is another issue of concern in applying fungicide and insecticide at the same time.

"Fungicide and insecticide ideal droplet sizes are similar," Montgomery says. "There should not be a coverage issue associated with the two," he says. "If the label says that they can be applied together legally, droplet size should not be a problem."

Two reasons to consider options carefully

Montgomery says there are at least two other points worth thinking about in this discussion:

First, "We know critical aphid populations won't be in every field and we know building aphid populations may be deterred by the environment.

"Even though we believe soybean aphids problems are likely, due to high aphid captures in the fall, environment might reduce pressure from the pest via excessive heat, which reduces reproductive rates in the soy-aphid population, and/or excess moisture, which could reduce aphid populations via entomopathogenic fungi.

"Rust could increase production costs drastically and additional production costs should only be incurred with evidence of need," he says. His advice is this:

"Keep costs down, don't apply an insecticide unless the threshold population is present. Scout for soybean aphid on your acerage, but don't assume that pest will be in every field.

Second, you should consider the big picture when using combined controls on your soybean crop:

"One of the big problems that we might run into could be the elimination of entomopathogens that help keep aphid populations 'in check.'

"We know that soybean aphids are probably kept in check by many natural controls," he says. "For instance, Asian lady beetles, lacewings, parasitic wasp, etc. And we believe that these entomopathogens, which are evident in the field, likely play a major role.

"If rust appears before an aphid problem develops... we might eliminate or impact entomopathogens and thus reduce natural controls in the field. Reduced expression of entomopathogens via rust management could 'seed' the development of a soybean aphid problem," he says.

One of the critical natural controls could be knocked out when fungicides are used, for example, he says.

To see a picture of an aphid killed by a fungal entomopathogen visit: http://web.extension.uiuc.edu/Sangamon-menard/ag_articles/ag_photos/soyaphidi.jpg.

Parting thoughts

Montgomery offers two more words to the wise:

1. There could be some damage to plant tissue associated with some of these fungicides. If fungal tissue dies within the plant, the affected area may die, etc. Producers should not get too excited about some initial damage to the plant.

2. Tank mixing herbicides and fungicides is strongly discouraged by many agronomists, the University of Illinois, and many companies. A host of issues come into play here, but a big one would, again, be droplet size.

Fungicides are typically applied at small, more drift-prone, droplet sizes. Herbicides are applied at larger droplet sizes. You either risk good coverage with the fungicide or you risk herbicide drift.

StopSoybeanRust.com is a Web site with news, information and resources about Asian soybean rust, created by the Greenbook, Dealer & Applicator magazine and Successful Farming magazine. It is sponsored by Bayer CropScience US. Cooperating partners include the United Soybean Board and the Crop Advisor Institute."