Iowa Farmer Today
02/25/06
Winter favorable to insects
By Tim Hoskins, Iowa Farmer Today
Marlin Rice, Iowa State University Extension entomologist, stands behind
this bold statement:
“They will be back.”
The upcoming growing season will likely be about the same or worse than 2005
for insect damage to crops, he says.
“Overall, it has been a very favorable winter for insects,” Rice notes.
Looking over the past 2½ months of winter, he says, while December was the
32nd coldest December in the 134 years of winter history, much of Iowa had 1015 inches of snow at the time.
“That acts as a superb insulation,” Rice explains.
During a recent winter meeting, he told farmers the ground was soft in December
because it never froze before the snow arrived. Farmers in the audience nodded
in agreement.
“The frost line was not very deep.”
Rice says most of the corn insects in Iowa stay at the soil level or just below the
surface.
Moving forward, January was the third warmest recorded for the month.
Into February, the warm trend continued to Feb. 16, when there were reports of
thunder snow in some parts of the state.
“I can’t give exact numbers of how bad insects might be,” he says.
“But, the conditions are good. I would expect at least the same amount (of
insects) as last year, if not more.”
That prediction is mostly for corn insects including corn borers, rootworms, grubs,
and wireworms.
Rice says the black cutworm moves south for the winter and currently is in Texas
and surrounding area. Because of its south migration, Iowa winter conditions are
not a good tool to predict their numbers in the upcoming growing season.
While he mostly focused on how temperature affects insects, Rice says moisture
also has some effect.
Generally, cold and dry is harder on insect survival than cold, wet conditions.
However, he does not expect the dry winter soil conditions to affect insect
survival.
The mild winter also has been favorable for the main soybean insect — the
aphid.
However, that is not surprising. Rice says University of Minnesota data shows
the air temperature typically is not cold enough for a long enough time to freeze
aphid eggs.
Matt O’Neal, an Iowa State University entomologist who studies aphids, says the
temperature would have to get “very, very cold” — about -40 degrees — to kill
aphid eggs.
O’Neal says one of things that might happen with warm temperatures is it could
change when the pest flies from buckthorn this spring. This may cause soybean
aphid eggs to hatch earlier than normal.
Since Iowa does not have as dense of areas of buckthorn as other surrounding
states, he says, most of the aphids might migrate to to Iowa.
It is not clear what the aphid population will be like this year.
However, O’Neal says farmers might want to think of it a different way.
“Prepare for the worst but hope for the best,” he says.
O’Neal says the high/low cycle trend is only based on six years of research. It
also is based on suction trap monitoring in Illinois.
This past year, suction traps were set up in Iowa, Minnesota, Wisconsin and
Michigan.
While Illinois found low numbers of aphids returning to buckthorn this fall,
Minnesota and Iowa reported high numbers.
In the past, if high numbers of aphids returned to buckthorn, there were higher
populations of the insect the next growing season.
However, aphid numbers appeared lower in 2005 than in the outbreak of 2003.
“We estimate there were 2 million acres sprayed for soybean aphids (in 2005),”
O’Neal says.
However, that is only about half of the acres sprayed for the pest in 2003.
While some producers might tempted to spray for aphids, O’Neal says farmers
are being judicious with using insecticide due to the concern about drought.
If aphid populations are above threshold with drought conditions, this also might
lead to a spider mite infestation.
O’Neal says insecticides, such as pyrethroids, can control aphids, but it might
make the spider mite problem worse.
Some pyrethroids include Warrior, Mustang Max, Asana and Baythroid.