EUROPEAN CORN BORER UPDATE
Some consultants have reported low levels of European corn borers in southern
counties of our area (Madison, Cuming) While not very high, they are near
or at treatable levels. Make your 1st generation ECB decisions
this week.
THE
SOYBEAN APHID IN NEBRASKA
The soybean
aphid (Aphis glycines Matsumura) was found in two southeastern
Nebraska counties earlier this month. Only four aphids were found, but they serve
as a reminder that farmers should begin scouting for this insect.
The soybean
aphid is new to North America. It is an Asian soybean pest that was first
discovered in the United States in the summer of 2000 in Wisconsin. Since then it has spread throughout the north
central U.S. and parts of Canada. We
expect the aphid to spread to all soybean-producing areas of Nebraska.
Heavy infestations
of this insect can cause significant damage and yield loss. Yield losses
exceeding 25 percent were observed in Minnesota and Iowa in 2000.
In addition, soybean aphids can transmit viral diseases, such
as alfalfa mosaic, soybean mosaic, bean yellow mosaic, peanut mottle,
peanut smut, and peanut stripe.
Soybean
Aphid Description:
The aphid is light green to pale yellow, less than 1/16th inch
long, and has two black-tipped cornicles (cornicles look like tailpipes)
on the rear of the abdomen. It
has piercing-sucking mouthparts and typically feeds on new tissue near
the top of soybean plants or on the undersides of mature leaves. Later in the season the aphids can be found on all parts of the
plant. It is the only aphid in
North America that forms colonies on soybean.
Soybean Aphid Picture
Soybean Aphid Picture
Life Cycle and Injury to
Soybean: The seasonal life cycle of the soybean aphid is complex with
up to 18 generations a year. It
requires two different species of host plant to complete its life cycle,
buckthorn and soybean. Buckthorn
is a woody shrub or tree and is the overwintering host plant of the aphid. Soybean aphids lay eggs on buckthorn in the
fall. These eggs overwinter and
hatch in the spring, giving rise to wingless females. These females reproduce without mating, producing more females.
After two or three generations on buckthorn, winged females are
produced that migrate to soybean. Multiple generations of wingless female aphids
are produced on soybeans until late summer/fall, when winged females and
males are produced that migrate back to buckthorn, where they mate. The females then lay eggs on buckthorn, which
overwinter, thus completing the seasonal cycle.
Soybean
aphid populations can grow to extremely high levels under favorable environmental
conditions. Reproduction is fastest
when temperatures are between 72-77 degrees F.
Developmental time slows when temperatures exceed 81 degrees F.
When populations reach high levels during the summer (there were
reports of up to 13,000 aphids per plant in Michigan), winged females
are produced that 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.
Soybean aphids
injure soybeans by removing plant sap with their needle-like mouthparts.
Symptoms of soybeans infested by soybean aphid may include yellowed,
distorted leaves and stunted plants.
A charcoal-colored residue also may be present on the plants.
This is sooty mold that grows on the honeydew that aphids excrete. Soybean plants appear to be most vulnerable
to aphid injury during the early reproductive stages. Heavy aphid infestations during these stages can cause reduced pod
and seed counts. Large number
of lady beetles may sometimes be associated with the aphids, which are
the lady beetles favorite prey. The
lady beetles are often noticed before the aphids because of their bright
colors.
Soybean Aphid
Management: The aphid is very new to North America
and experience with the insect is limited.
As we gain more experience with the aphid, recommendations will
be refined and developed to manage the aphid under Nebraska conditions.
1.
Begin weekly scouting of soybean fields in late June. If possible, scout five locations per 20 acres.
At a minimum scout 10 locations per field.
At each location, select five plants and estimate the aphid density
per plant. Aphids are most likely to concentrate at the top of the plant, although
in later reproductive stages they will move onto stems and within the
canopy. As the season progresses,
aphid numbers can change rapidly (populations can double in 2-3 days),
so in July increase scouting to twice a week.
2.
Look for the presence of aphid natural enemies such as lady beetles,
green lacewings, and other insect predators. These predators may keep low or moderate aphid populations in check.
One can often find soybean aphids by examining plants where lady
beetles are observed. The presence of “fuzzy” aphid carcasses indicates fungal pathogens are present,
which can lead to dramatic reductions of aphid populations.
3.
Take note of winged aphids or “broad-shouldered “ nymphs. Nymphs with broad or squared- off shoulders
will become winged adults. These
aphids will leave the plant, so if the majority of aphids are winged
or about to become winged adults, the field may not have to be treated
because the population will rapidly decline.
A magnifying glass is helpful to see the “broad-shouldered” nymphs,
but the winged adults are easy to see with the naked eye.
4.
Take note of plant condition. Plants
under moisture stress are more vulnerable to economic damage.
5.
Closely examine late emerging soybean fields. It appears aphids damage late-planted soybeans
more than early-planted soybeans.
6.
Consider insecticide treatment when soybeans are in the R1 to R2 stage
(first bloom to full bloom) and there are over 200 aphids per plant,
most of which do not have wings or “shoulder-pads”.
7.
If the plants are covered with honeydew or sooty mold, or stunted, an
insecticide treatment may still be of value but the optimum time of
treatment is past.
8.
Initial research has indicated that the highest yield responses were
obtained when insecticide treatment occurred between mid-July and the
first few days of August (R1-R2 stages). Treatment after this period resulted in less yield response, however,
2002 field trials in South Dakota indicated treatment as late as stage
R5 (beginning seed) resulted in significant yield protection.
9.
If fields are treated, leave an untreated test strip to compare against
sprayed sections. This also
provides a refuge for beneficial insects.
10.
Good insecticide coverage and penetration is required for optimal control
of soybean aphid, as aphids feed on the undersides of the leaves and
within the canopy. Use high
water volume and pressure.
11.
Several insecticides are labeled for the soybean aphid (Chinese aphid
on some labels). A list of registered insecticides, rates, preharvest
intervals, etc. can be found at http://entomology.unl.edu/instabls/soyaphid.htm,
and we will list them in a future newsletter.
12.
Remember to follow label directions.
Current Status of
the Soybean Aphid in Nebraska:
Although the aphid has only been found in about fifteen Nebraska counties,
it is likely established throughout eastern Nebraska.
Common buckthorn, the aphid’s overwintering host, has been found
in much of eastern Nebraska. It
is particularly prevalent along creek and river bottoms and can also be
found as an under-story shrub/small tree in wooded areas.
Because the
soybean aphid is so new to North America, researchers are still determining
soybean aphid biology, impact, and management options in North America.
It is likely that the aphid will behave differently in different
parts of the Midwest, so it is important that aphid infestations be
reported to Nebraska researchers so Nebraska-specific studies can be
initiated. If you find aphids
in your soybeans this season, please contact Tom Hunt, UNL Extension
Entomology Specialist, Haskell Ag Lab, Concord, NE, (402) 584-2863,
thunt2@unl.edu .
More information
on the soybean aphid can be found through the UNL Entomology web site,
http://entomology.unl.edu ,
and at the Soybean Aphid Watch web site at http://www.pmcenters.org/Northcentral/Saphid/Aphidindex.htm
.
MANAGEMENT
TIPS
“Do
you check soil moisture? The Internet version of the NebGuide “Estimating
moisture by appearance and feel,” G84-690-A, has excellent photos of
four soil textures at three moisture contents (http://ianrpubs.unl.edu/irrigation/g690.htm).
To get a copy of the original USDA booklet using these photos, call
Pat McGrane at the NRCS office in Lincoln, 402-437-5328, or email him
at pat.mcgrane@ne.usda.gov”
“Before you
irrigate check soil moisture in the active root zone. Below are the
average root depths of selected corn and soybean stages.”
|
Root
Depth (feet)
|
Corn
Stage
|
Soybean
Stage
|
|
1.0
|
4 leaf
|
Vegetative
|
|
2.0
|
8 leaf
|
Early
bloom
|
|
2.5
|
12
leaf
|
Full
bloom
|
|
3.0
|
16
leaf
|
Pod
development
|
|
3.5
|
Silking
|
Seed
development
|
|
4.0
|
Blister
|
Full
seed fill
|
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