Why Are Soybean Aphids So Bad This Year?

For supposedly being an "off year" for aphids, the pest has been doing surprisingly well in August.

Airplanes have been flying soybean fields in Iowa and ground-driven sprayers have also been busy, as many soybean fields have been sprayed for aphids the past couple of weeks. For supposedly being a soybean aphid "off year", the insect has been doing surprisingly well during August of 2008.

Typically, once you get to around August 20 or so, aphid numbers are declining at this time of year because the soybeans are approaching maturity and the temperatures are quite warm. However, this year is slightly different from recent years and you should not let down your guard. That warning comes from Iowa State University Extension entomologists Marlin Rice and Jon Tollefson.

Continue to scout your soybean fields

There are a couple of reasons to stay on the alert for soybean aphids, they say. One is the later planting dates for many soybeans in 2008. The other is the cooler than normal weather this summer. Tollefson and Rice are giving farmers the following recommendations.

The soybean aphid is not expected to cause any additional yield loss after soybeans reach R5.5 stage. The R5.5 stage is between the R5 stage, when the seed is 1/8 inch long in the pod at one of four uppermost nodes on the main stem, and the R6 stage, when the pod contains a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem.

At the R5.5 stage, the canopy should be closed in bean fields and there will not be any additional gain from spraying for aphid control. But with the cool, wet spring and summer this year some fields were planted quite late and the maturity of the beans has lagged behind. As a result, you need to continue to scout soybeans for aphids in fields where the canopy has not closed across the rows.

Cool weather is another reason to keep scouting

The second reason scouting should continue is the cool weather. As long as the daytime temperatures stay in the 70s and low 80s, the aphids will continue to reproduce and feed. They will not begin to move to overwintering sites until there are shorter days and night-time temperatures drop into the 40s.

Therefore, continue to scout aphids in any soybean fields that have not reached maturity stage R5.5 and while the temperatures stay cool. Don't forget to recheck fields that were sprayed two weeks or more ago. These sprays will have destroyed the predator populations and the chemical's residual activity will have ended by now; aphid numbers may be rebounding in those fields.

Normally not a problem in even-numbered years

Jim Fawcett, ISU Extension agronomist in the Cedar Rapids area in eastern Iowa, says all crops in his area are behind normal in development. "The one thing we're seeing that we thought we wouldn't see anymore this late in the season are soybean aphids," he says. "Normally we don't have aphids in the even-numbered years like 2008. But they aren't following that pattern this year. We still have populations increasing in a lot of fields. It's more like we're in early August than late August this year."

Is there a lot of spraying for aphids in eastern Iowa? "We have had a lot of spraying, but hopefully we're getting close to the end of it," says Fawcett. "Aphids didn't show up in our area until about two weeks ago. It's just been in the last week or so that a lot of spraying has occurred."

When should you quit spraying for aphids?

Are soybeans in eastern Iowa moving toward maturity now with the drier weather or are they sitting still? "They are not sitting still," Fawcett says. "But they are moving fairly slowly. Once beans get to about stage R5.5 they are halfway during seed fill, and we don't think it will pay to spray for the aphids anymore. We're just now, on August 22, approaching that in some of the bean fields that were planted earliest. So it's going to be several days to a week or so before we get to that stage in many of the fields."

Fawcett says a lot of corn in eastern Iowa is now starting to show signs of loss of nitrogen that has occurred due to wet weather earlier this season. Those fields that had extremely heavy rains, and eastern Iowa had them worse than other parts of the state in June this year, are showing signs of the worst nitrogen losses. The fields are now running out of nitrogen and corn is suffering from it.

TAGS: Extension
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