Agriculture Online
08-14-07
Don't stop scouting for soybean aphids
Some populations still rising; know what to look for, specialists say
Jeff Caldwell
Agriculture Online News and Features Editor
Soybean aphids have been at treatment threshold levels in fields around the
Corn Belt for up to a month now, and numbers still on the rise in parts of Iowa
and Illinois. But, the later in the season it gets, the sharper eye it takes to
recognize which variants of the pest will cause most damage in your fields,
specialists say.
Aphid populations, the growth of which is typically slowed once temperatures
exceed 86 degrees Fahrenheit (and stops completely above 95 degrees), vary
widely in Illinois, according to University of Illinois Extension entomologist Kevin
Steffey. This makes field-by-field scouting even more important than earlier in the
summer.
"Many fields of soybeans have been and are being treated to control soybean
aphids in 'hot spots' throughout northern Illinois. However, I reiterate that
densities of soybean aphids vary widely in affected areas, so scouting of
individual fields is essential," Steffey says. "And despite the dramatic increases in
numbers of soybean aphids in the two examples just provided, densities have
reached plateaus in other fields, the growth of the populations having been
suppressed by high temperatures."
Some fields, at least in east-central Iowa, are near or right at economic threshold
or the point at which it's economically viable to treat the pest. With this being the
case, farmers should be actively scouting each of their fields, taking care to note
whether what they find are actually damage-causing aphids, says Iowa State
University Extension field agronomist Virgil Schmitt.
Schmitt says growers in his area may be finding aphids of different sizes and
colors. Knowing which of these varieties cause damage is key to determining
whether the aphids are causing enough crop damage to justify spraying.
"Many are seeing the small white aphids again this August. These are soybean
aphids and are also causing damage, so should not be ignored. Don't confuse
the shed white skins with the living aphids," Schmitt says. "And look closely to
see if the majority of aphids are winged or have broad shoulders, which means
they have wing pads and are going to form wings. If the majority are winged or
are forming wings, they will leave the plant, and perhaps the field, and an
insecticide application may not be necessary. Also, brown colored aphids are
either dead or will soon die, so do not count them."
How much longer should growers scout soybean aphids? With more rainfall in
recent weeks compared to earlier in the summer in many parts of the Corn Belt,
the window for scouting is likely wider than under drier conditions, Schmitt says.
"Because of the unusual rainfall this late summer, soybeans are continuing to
flower later into the season than normal, so ISU is suggesting that you manage
soybean aphids until growth stage R 6.0," he says. "To determine if your field has
reached growth stage R6.0, examine several plants. One each, find the main
stem and then find the uppermost node that has a fully expanded trifoliolate leaf.
Examine the pods at that node and at the next three nodes down the stem. If any
of the pods contain a green seed that fills the pod cavity, it has reached R6.0."
The economic threshold for treating aphids is 250 insects per plant, with 80% of
a given field infested. "It is best to count all aphids on a few plants to get a feel
for what 100 and 250 look like and then estimate from that point on," Schmitt
adds.