THE SOYBEAN
APHID IN NEBRASKA
Soybean
Aphid
Soybean Aphid Picture
Soybean Aphid
Significant
soybean aphid populations have surfaced in the past few days in Nebraska.
Interestingly, the first fields located with populations high
enough to cause concern were in Adams and Dawson counties, quite a bit
west of where we are. However,
the beans are ahead of us in development and may have been more favorable
for population buildup.
We
are entering the period of time when we spotted soybean aphids in northeast
Nebraska last year. This, along
with the possibility that the delayed growth stage and cooler weather
is more favorable for aphid development, means that we should be on
the lookout for soybean aphids. The
following article was in newsletter #9, but since people have a tendency
to lose things, we thought we would provide it for you again.
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.
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). Asana, Furadan 4F, Lorsban 4E, Mustang, Penncap-M,
Pounce, and Warrior are all labeled for aphid control. Check the label
for rates and restrictions.
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
.
Grasshopper
control with ATV-RAATs
Recent grasshopper
activity has increased concern about serious damage occurring when maturing
grasshoppers move into cropland in the next few weeks. Treating immature
grasshoppers now in the areas adjoining cropland is a good strategy
to managing later grasshopper movement.
These adjoining areas can be treated
using the reduced agent and area treatment (RAAT) strategy of control.
The RAAT method was developed to allow economical application of insecticides
to reduce populations below a level that will cause economic damage
in rangeland. This method can be modified for use in range, pasture,
and non-crop areas adjoining cropland to prevent movement of grasshoppers
from these areas when their food sources dry up.
Typical RAAT treatments are applied via
air with lower chemical rates. Treatment is applied only to about 50%
of the acreage by treating alternate swaths. This strategy also has
been shown to be effective with an ATV sprayer applying narrow treatment
swaths. Any sprayer could be used to treat the swaths, but an ATV will
be able to cover rougher terrain. An advantage of this method is the
use of narrow swaths that reduce the distance the grasshoppers need
to move to enter a treated area.
The RAATs strategy takes advantage of
natural grasshopper behavior -- over time they will move from feeding
zones into treated zones and be controlled. Also, natural enemies will
survive in the untreated zones and continue to exert a controlling influence
on the grasshopper population. The effectiveness of the RAATs strategy
relies on several factors. First, and most importantly, the grasshoppers
must be immature to enable better control. Third and fourth instars
are the ideal stage to target. Next, the residual of the product used
must be maximized to enable grasshoppers to move from the untreated
zones into the treated zones to contact the insecticide. In addition,
application techniques must be ideal to enable uniform application of
the product in the treated zones. Control in a RAATs treated area will
likely be slightly lower (5-15%) than a comparable blanket treatment
area, but the much- reduced cost makes the method a great benefit.
Three products are labeled for grasshopper
control in range, pasture and non-cropland areas (carbaryl, malathion
and diflubenzuron) and would be appropriate for treating these areas
around cropland. The residual for Malathion will be short, perhaps only
a few days. Carbaryl (Sevin XLR+) will last longer and at higher application
rates may last two to three weeks.
Diflubenzuron (Dimilin 2L) has a very
long residual and will last 28 days or more at higher rates. The longer
residual products have a better chance of success in the ATV-RAATs program.
Techniques for ATV-RAATs application are quite different from those
used in aerial RAATs.
Whereas application rates for aerial
RAATs are reduced, recommendations for ATV-RAATs are to use the maximum
labeled rate (1 oz/A Dimilin 2L, 32 oz/A Sevin XLR+). The RAATs strategy
works by taking advantage of the movement behavior of the grasshoppers
in that over time they will move into the treated zones and be controlled.
Also, by leaving some areas untreated, natural enemies will survive
and continue to exert an influence on the grasshopper population.
The amount of treated area with this
program will vary depending on grasshopper density and grass forage
coverage. The percent of the area treated should be equal to the grasshopper
density with a minimum of 20% coverage. For example, if you have 30
grasshoppers per square yard in the areas to be treated, leave two untreated
swaths for every swath that is sprayed (33% coverage). A second major
factor that will impact optimum swath width is the density of foliage.
Dense, lush foliage will slow grasshopper movement from the untreated
to the treated strips. In these situations consider an increase in treated
area (narrower skip swaths).
Mixing and application of these products
is enhanced with the use of 8
oz/A crop oil to the mixture. Tests also have shown that canola oil
may offer some advantages over crop oil in attracting grasshoppers into
the treated areas. To maintain mixability, use at least 15% crop oil;
for example 1.2 oz crop oil plus 7 oz canola oil.
More information on cropland grasshopper
management and ATV and aerial RAAT treatments can be found on the UNL Department of Entomology
Web site.
Gary
Hein, Extension Entomologist, Panhandle REC
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