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Soybean Aphids Again A
Threat
Migrating from Asia to the U.S. nine years ago, the
soybean aphid has become one of the most feared insects in the Corn Belt
and other bean-producing areas. This aphid has been confirmed by USDA in
Delaware, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan,
Minnesota, Mississippi, Missouri, Nebraska, New York, North Dakota,
Ohio, Pennsylvania, South Dakota, Tennessee, West Virginia and Wisconsin
and in the Canadian provinces of Quebec, Ontario and Manitoba.
This newsletter will hopefully help you better understand the threat
aphids present, how to spot them on time and how to control them before
the damage is done.
Aphids May Be Perennial Pest
Midwestern growers should be prepared for the third
straight year of soybean aphid invasions. It’s not a guarantee, but
the trend of aphids as perennial pests has certainly been set, says Iowa
State University Entomologist Matt O’Neal.
USDA says that in 2008 the soybean aphid “went west,” reaching the
western most cultivation of soybeans in the U. S. in Wyoming. More than
8,500 soybean aphids were caught in the Midwest suction trap network in
late September (to see the Midwest trap network, click here www.ncipmc.org/traps/index.cfm).
Soybean aphids caught in the traps during September and October were
migrating from soybeans to buckthorn where the overwintering eggs are
deposited. “This number exceeds any seen in previous years during the
same time period,” says USDA.
The tiny terrors saw the on-again, off-again cycle broken in 2008.
“In the fall of 2007, we collected very few aphids after the growing
season,” says O’Neal. “We thought 2008 would be a quiet year. It
wasn’t. I try not to make predictions, but now we may be in a period
where soybean aphid outbreaks may be more intense. It may not be a
cycle. It may be every year.”
Aphids were first seen in the U.S. in Wisconsin in 2000. “They come
from a region of China with a climate much like the Midwest,” says
O’Neal.
After emerging here, there was an apparent soybean aphid cycle
throughout the Corn Belt. Populations would be low one year, followed by
outbreaks well above the economic threshold of 250 aphids per plant the
next year. There were small outbreaks in 2006 followed by the large
invasion in 2007. So entomologists and growers expected a light year in
2008.
O’Neal says Iowa State research on 36 separate farms is helping
researchers and growers learn more about the aphid cycle. It has led to
several hypotheses from O’Neal on why 2008 was a bad year for aphids.
“First, the spring of last year was wet and cold and delayed
planting,” he says. “That may have disrupted the predator
relationship with aphids. We have also found that as the amount of corn
and soybean production increases around a soybean field, the amount of
predation decreases (as there are fewer predator insects, like lady
beetles, that eat aphids due to a lowered number of field borders).”
O’Neal adds that growers who spray insecticides to control other
insects may also have reduced predator numbers. He says a well-rounded
integrated pest management program is needed for best aphid control.
That program could include the use of new aphid-resistant soybean
varieties that are coming on line.
O’Neal and others in the North Central Soybean Research Program
authored a Soybean Aphid handbook sponsored partially by the Iowa
Soybean Association. For more on the publication, go to www.planthealth.info/pdf_docs/sba_update08.pdf.
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Identifying The
Insect
 The soybean aphid is a small, yellow aphid with
distinct black cornicles that resemble “tailpipes” on the tip of the
abdomen, according to the North Central Integrated Pest Management
Center (NCIPMC). The aphid can be found on stem apices and young leaves
of growing soybean plants and on the undersides of leaves of mature
plants.
Because no other aphid species develop colonies on soybean in North
America, NCIPMC says it is safe to assume that colonies of tiny yellow
aphids on soybean are indeed the soybean aphid. Several other aphid
species feed on soybean, but they are migratory and do not colonize
soybean.
The aphid has a complex life cycle with as many as 15-18 generations
annually. Two different types of host plants are required by the soybean
aphid to complete its life cycle. Overwintering eggs survive on species
of Rhamnus (buckthorn). Nymphs hatch in the spring and after two
generations of wingless females, a generation of winged females is
produced that migrates from buckthorn in search of soybean.
On soybean, aphids spend the summer in a repeated series of wingless
generations followed by a winged generation that disperses from its
crowded hosts in search of other soybean plants. In the fall, they
migrate back to buckthorn by winged females that produce a generation of
egg-laying wingless females.
Males develop on soybean and search for buckthorn and mate with the
wingless females, which lay the overwintering eggs alongside the buds on
buckthorn twigs.
For more on the soybean aphid life cycle
click
here.
Damage Will Hurt
Yields
The economic threshold for soybeans aphids is about
250 insects per plant. Aphid infestations that peak at the R1–R2
growth stage of the host may cause stunted plants with reduced pod and
seed counts, resulting in lower yields. Damage is worst when 80% of the
plants are infested, and natural predators are not controlling aphid
population.
The threshold is designed to give about seven days of lead time between
scouting and applying insecticides. When that level is reached plants
can see stark damage. Leaves may express symptoms of distortion. Later
in the growing season, heavily infested plants may have distorted and
yellowed leaves. Charcoal-colored residue on stems, leaves and pods is
sooty mold that grows on honeydew, a by-product excreted by aphids.
Some of the most severe damage from aphid populations occurred in 2003
when aphid populations exceeded 3,000 per plant in many fields. In parts
of Iowa, soybean yields averaged 32 bu./acre, a 16 bu. or a 32%
reduction from 2002. This yield reduction was partly due to the soybean
aphid coupled with drought conditions.
In low numbers, soybean aphids can feed on soybeans but cause little or
no damage. Large populations can result in visual evidence of damage.
Honeydew accumulates on the top surface of leaves.
Excessive honeydew permits the growth of sooty mold, turning the leaves
dark and interfering with photosynthesis in the plant. Heavily infested
plants may be stunted. Plants that become stunted during the early
reproductive growth stages of soybean
may have reduced pod set and seed counts, resulting in lower yields.
Aphid-transmitted viruses can be an important part of the ecology of the
pests. They can transmit numerous viruses to plants, including a number
of viruses present in the U.S. that naturally infect soybean. Alfalfa
mosaic, soybean mosaic, bean yellow mosaic, peanut mottle, peanut stunt
and peanut stripe are among them. (Transmission of these viruses by the
soybean aphid has not been documented in the U.S., but is certainly a
threat.)
Scouting For
Aphids
Tom Hunt, University of Nebraska entomologist, was among
those who worked with growers to get soybeans properly scouted for
aphids last year. He says growers should begin scouting soybean fields
once or twice a week in late June to early July.
He says to check 20-30 plants per field. Aphids are most likely to
concentrate at the very top of the plant early in the season, and will
move onto stems and within the canopy as populations grow and/or the
plant reaches mid to late reproductive stages. As the season progresses,
aphid numbers can change rapidly and populations can double in two to
three days.
“Soybean aphid populations can grow to extremely high levels under
favorable environmental conditions,” says Hunt. “Reproduction and
development is fastest when temperatures are in the 70s through the mid
80s (F.). The aphids do not appear to do well when temperatures are in
the 90s and are reported to begin to die when temperatures reach 95
degrees.
“When populations reach high levels during the summer, winged females
are produced and migrate to other soybean fields. Like a number of other
insect species (e.g., potato leafhoppers), these migrants can be caught
up in weather patterns, moved great distances, and end up infesting
fields far from their origin.”
Brian Kemp, a grower from near Sibley, IA, sees aphids in his soybean
fields every year. “I use the speed scouting method to monitor my
fields, starting the third week of July,” says Kemp. “My advice,
however, is to keep scouting. I think I lost 6-7 bu./acre last year in
fields that weren’t sprayed because I didn’t scout later in the
season. I have learned that every year is different. I think the fields
that I scouted and decided not to spray reached threshold later than
normal. The pest was more challenging to manage in 2008.”
The Iowa Soybean Association says a recent survey conducted by
researchers at the University of Minnesota measured farmers’ awareness
and use of IPM for soybean aphid control. According to this survey, more
than 70% of the respondents say the frequency with which aphids should
be treated for profitable control depends on aphid counts, weather
conditions and plant growth stage.
In addition, more than half of the farmers surveyed consider the lowest
aphid density for profitable aphid control to be 250 aphids per plant.
Sheila Hebenstreit, a crop consultant with West Central Cooperative near
Jefferson, IA, says that a great deal of progress has been made in
educating growers about the importance of proper scouting and adherence
to economic thresholds.
“Applying insecticides over large areas and indiscriminately killing
insects works, but is a problem,” notes Hebenstreit. Ideally, she
would like to see multiple modes of action to fight the pests, including
biological controls and resistant varieties.
For tips on speed scouting, go to click here
(then scroll down to the Soybean Aphid manual)
Sources: University of Nebraska, Iowa Soybean Association
Beneficials Attack
Aphids
 The biggest enemies of soybeans aphids are their natural ones
– predators that feed on aphids and other damaging bugs. The main
predators of soybean aphids are lady beetles, green lacewings, brown
lacewings, syrphid flies, aphid midges, pirate bugs and damsel bugs.
Entomologists, through funding from USDA and the North Central Soybean
Research Program (NCSRP), have been steadily monitoring the soybean
aphid and its natural enemies since the pest invaded U.S. soybean
fields. Part of its program involves importation biological control,
also called classical biological control, of the soybean aphid.
The process identifies, locates, evaluates and releases a pest's natural
enemies that evolved with it in its native home, with the intent of
significant long-term reduction of pest numbers. Insect predators are
generally free-living, very mobile and consume many prey during their
lives. Insect predators are usually as large as, or larger than their
prey. Predators can be predacious in the larval stage, in the adult
stage, or both. Many predators are generalist natural enemies although a
few types are specialized.
Spraying Tips
If the soybean aphid economic threshold is apparent, Ron
Hammond, Ohio State University entomologist, says the greatest benefit
from an insecticide application will likely come in July during
flowering or immediately thereafter.
The later in the summer an insecticide application is made, the less the
benefit from that spray, he says. Spraying late in the season after pods
have filled or following dispersal of winged aphids in mid-to-late
summer will be too late to prevent economic yield loss.
Hammond stresses to use care in spraying near apiaries. Most insecticide
label directions warn not to apply to blooming or flowering crops if
bees are actively working in the target area. Thus, growers should apply
most materials for soybean aphid control in early morning or late day
when bees would not be active in fields.
Good insecticide coverage and penetration is required for optimal
control of soybean aphid, as many aphids feed on the undersides of the
leaves and within the canopy. Use high water volume and pressure. Aerial
application works well when high water volume is used (5 gal. of
water/acre recommended).
Tom Hunt, University of Nebraska entomologist, says that when the
current threshold of 250 aphids per plant was developed, it was set so
growers had a week to apply insecticide before aphids were likely to
increase above the economic injury level (which is much higher than 250
aphids per plant). Midwestern soybean entomologists are recommending
that even with higher soybean market values, the economic threshold of
250 aphids per plant should still be used, he says.
Entomologists note that if soybean prices approach the high levels seen
in 2008, growers should consider a four- to five-day window to treat
rather than a seven-day window. Growers are encouraged to contact their
local or regional extension entomologists to determine the best soybean
aphid techniques for their region.
For more on soybean aphid control, click here .
Thanks For
Viewing This Aphid Alert Newsletter
Thanks For Viewing This Aphid Alert Newsletter
Again, thanks for taking time to read this enewsletter. Hopefully it has
helped answer some of your questions on soybean aphids. If you have a
particular question you’d like to see answered in our next issue
scheduled for early April, contact your Aphid Alert editor at beef2lar@suddenlink.net.
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