Winter Annuals and Insects 05/23 14:56
Delayed control of winter annuals in the spring can lead to outbreaks of
secondary insects in corn and soybeans.
By Daniel Davidson
DTN Staff Agronomist
OMAHA (DTN) -- Seeing stands of winter annuals in late April or early May
even after corn has been planted is becoming much more common; however,
delaying control of winter annuals could increase insect pressure on newly
Outbreaks of winter annuals are most common in soybean stubble that is being
planted to corn. Alex Martin, a weed scientist at the University of Nebraska
said increased adoption of no-till practices along with a decline in use of
residual herbicides in favor of glyphosate has contributed to the increase in
Keith Jarvi, an integrated pest management specialist with the University of
Nebraska, said that since winter annual outbreaks are a fairly recent
development, there is not a lot of information on what possible insect problems
may occur with delayed control.
The first insect that comes to mind, however, is black cutworm, he said.
They migrate into the Corn Belt from Southern states and are attracted to any
early green vegetation.
The eggs hatch and the cutworm larvae feed on the annuals for a while then
move on to emerging corn after the winter annuals have been killed.
Winter annuals can also serve as an attractive egg-laying site for white
grubs, Jarvi said. "The (white grub) adults are May or June beetles that emerge
fairly early in the spring. After mating they may choose winter-annual infested
fields to lay eggs in, which causes a potential problem in those fields a year
or two down the road."
White grubs, similar to cutworms, attack corn from emergence to early whorl
(VE to V3 leaf stage) in May. True white grubs, which cause injury by chewing
off root hairs and small roots, have a three-year life cycle and can be a
problem for several years.
Kevin Steffey, an extension entomology specialist at the University of
Illinois said some of the two-spotted spider mite soybean infestations in 2005
can be traced back to winter annual weeds that were not killed until Roundup
was applied later in the spring.
The overwintering two-spotted spider mite females shelter in many species of
winter annual weeds, and they get off to a good start when these weeds grow in
the spring. "By the time Roundup is applied, colonies have established. If
drought conditions persist as they did in 2005, mites simply move from the
dying weeds (e.g., chickweed, henbit) to the seedling soybeans, giving the
mites an early foothold in the crop," he said.
"It's also possible that stalk borers, which overwinter in weedy areas,
could become established in the fields with winter annuals in the spring, then
move to corn when the weeds die," Steffey said.
Daniel Davidson can be reached at email@example.com.
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