Wallace's Farmer, IA
07-30-07
Soybean Aphids Reach Threshold in Many Fields
Rod Swoboda rswoboda@farmprogress.com
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."