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Soybean Aphids Reach Threshold in Many Fields

Now is the time to scout fields for aphids and for bean leaf beetle.

A couple of yield-robbing insect pests are showing up in soybean fields in many areas of Iowa. "It's very important now at the end of July for farmers to scout fields, especially for soybean aphids and bean leaf beetles," says Palle Pedersen, Iowa State University Extension soybean agronomist.

In central Iowa fields, soybean aphid populations are now getting close to the 2003 levels - which was a year when aphids caused a lot of damage. The economic threshold is 250 beetles per plant. Once the infestation reaches that level, it pays to spray an insecticide.

"However, you also need to keep in mind that aphid infestations are extremely variable across the state this summer," adds Pedersen. "Don't just assume you have aphids and spray. You have to walk out into the soybeans and scout several places in each field before making that decision."

Aphids are in many fields

It doesn't pay to spray fields that don't need the insecticide. On the other hand, there may be aphids in parts of the field you don't see by just scouting the entrance to the field, he adds.

The level of aphid infestation depends a lot on the soybean variety you planted and on the planting date and maturity group. Beans that were planted late and the full-season maturity bean varieties seem to have the highest populations of aphids.

"Currently, in central Iowa we have fields that have been above the economic threshold for treatment of aphids since July 15," says Pedersen. "In northeast Iowa the soybean population densities are now at the same level as in 2003 and the majority of the fields are now getting sprayed."

Aphid populations are also increasing rapidly in southern Iowa. "But it seems that the 250 aphids-per-plant threshold level for treatment in that part of the state may first be reached in a week or so—if it happens," notes Pedersen. "That would be sometime during the week of July 30 to August 4. The main point I want to emphasize is that there is a lot of variation in aphid infestations in fields throughout the state. It's critical to scout your fields for aphids."

Bean leaf beetles also present

Bean leaf beetles are another insect that's showing up in a number of soybean fields now. "Pretty much from Carroll in western Iowa to Marshalltown in central Iowa, we are seeing very high numbers of the first generation of bean leaf beetle," says Pedersen.

ISU Extension entomologist Marlin Rice has been doing a lot of work on this pest. His threshold is that if you have more than 16 bean leaf beetles of the first generation per 20 sweeps of a sweepnet, you've reached the economic threshold at current soybean prices.

"This summer in some fields we are seeing infestations as high as 40 beetles per 20 sweeps," says Pedersen.

"If you don't have aphids it doesn't mean you don't have any issues out there in your field," he adds. "We are seeing both bean leaf beetles and aphids in some fields. In addition to that we are seeing Japanese beetles starting to come into soybean fields in some areas. We have seen Japanese beetles south of Nevada in central Iowa."

Bean prices make scouting pay

It is critical to get out and scout. "We don't have the highest yield potential for soybeans this year," notes Pedersen. "But we have a high price for soybeans, so there is a lot to lose. That's why scouting is so important right now."

For the majority of the state, farmers are dealing with two pests—bean leaf beetles and soybean aphids. Aphids are very spotty and they are easy to see. They tend to attract ants, so that's a clue. Also, if there is a high level of aphids you can see the honeydew.

What if you are going to spray glyphosate herbicide anyway to kill weeds in a soybean field? Should you just go ahead and mix insecticide in with the glyphosate as a so-called "insurance" application to control any aphids that may be in the field?

"Don't do that," says Pedersen. "We don't recommend mixing insecticide with glyphosate as a prophylactic treatment because if the timing isn't right, you may have to go back and spray the aphids a second time. Scouting is critical and if you have to apply insecticide, be sure to apply it at the proper time to get the best return for your dollars invested."

TAGS: USDA Extension
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