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Aphis glycines feeding on a soybean
leaf (click on photo to enlarge).
Photo John Wedberg |
The literature and our experience in 2000 suggests
the time for rapid population increase of the soybean aphid, Aphis glycines,
occurs during V3-R5 growth stage.
In 2000 I stated that the aphids we found during peak
abundance in mid-July probably got their start in mid-June. In other words they were in
the fields at low levels for about a month before numbers were large enough to capture
attention. However, small colonies on even 100% of the plants in June will not reduce
yields through direct feeding damage. Spraying such fields will not protect you for the
rest of the season.
In China, three periods of damage to soybean have been
identified:
1. From seedling stage to blooming stage of soybean, the aphid population reaches its
highest peak. Colonies concentrate on tender leaves and branches (e.g. new trifoliolate
leaves).
2. In late July, the top growing point of soybean stops
growing, the aphid colonies alter their positions from the top to the middle or the lower
part of the plant and feed on the underside of soybean leaves.
3. From late August to early September the aphid colony
begins to multiply rapidly again. Afterwards it migrates back to the overwintering host,
where eggs are laid in preparation for winter.
Scouting
Look for aphids on new, upper leaves, stems
and petioles. Fewer aphids are found on older, lower leaves. A hand lens or magnifying
glass would help spot these tiny insects.
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| Fig. 2. Symptoms of aphid feeding on soybean. (Click
on photo to enlarge). Photo John Wedberg |
Symptoms of direct damage from aphid
feeding
Symptoms of aphid feeding are leaves with rolled edges and puckering (Figure 2),
easily confused with virus symptoms, yellow leaves, stunted plants with shortened
internodes and petioles, early maturity, sooty mold growth, defoliation. The long term
effect of aphid feeding on the plant is fewer pods per plant, reduced seed weight,and less
overall yield.
Aphids suck plant sap, filter it through their system and
pass a liquid excrement high in sugar. It is called "honey dew" and you will
probably find a sooty mold growing on the layer of honey dew on soybean leaves later this
summer. The plants will appear grey to black.
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| Aphis glycines |
Identification
Aphids that look identical to most of us can be separated into different species because
of subtle differences in setae (hair) patterns and length, length of segments of
appendages, minute differences in coloration etc. It is critical to make sure that you
know the species with which you are dealing because such aphids can have different host
ranges and biology.
The large infestation of the soybean aphid (Figure 3) in
2000 suggests it has been present in Wisconsin prior to 2000, but at levels too low to
detect. If you find aphids colonizing soybean, the odds are very good they are soybean
aphid.
Other aphids in Wisconsin
The soybean aphid is not the only aphid found in Wisconsin. The corn leaf aphid, pea
aphid, the cotton/melon aphid, Aphis gosspyii, and other species of aphids live
in Wisconsin. The "buckthorn aphid", Aphis nasturtii is another aphid
that feeds on buckthorn, the alternate host of the soybean aphid. The buckthorn aphid
doesn’t colonize soybean, however, but moves to plants in the Solanaceae (e.g.
tomato, potato) upon leaving the buckthorn host. Most aphids do not colonize soybean.
Soybean viruses can be transmitted by
aphids
A separate issue in damage to soybean from the soybean aphid is the potential of
transmitting viruses. Based on a 1999 survey of soybean viruses in Wisconsin, Soybean
Mosaic Virus (SMV) and Alfalfa Mosaic Virus (AMV) are the most prevalent aphid-transmitted
viruses in Wisconsin. Symptoms of SMV are cupping down of the
leaves, a mosaic of light and dark green areas on the leaves, and leaf yellowing. SMV is
able to infect soybeans, snap beans, and many other legumes. The source of SMV is infected
soybean seed that germinates into infected seedlings. Long term effects of SMV are reduced
yield, stunting, delayed maturity, reduced seed quality and bleeding hilums on seed.
AMV produces rugose leaves, and large chlorotic patches. AMV
has a broad host range including soybean, alfalfa, clover, snap bean, and pea. Bright
yellow mosaic leaves and vein yellowing are common symptoms of AMV on soybeans. Several
species of aphids transmit AMV. Seed transmission of AMV has not been reported in
soybeans.
Although most aphids do not colonize soybean, winged aphids
that move into soybean, make a few test probes, and then move on, cannot be ignored as
potential vectors of soybean viruses.
Aphid infestation
and damage tends to be worse on fields planted late
Most of the heavily infested fields that had signs of damage in 2000 were planted after
June 15. Low soil moisture exacerbates the situation. All of the fields planted by
early May that I examined in 2000 held their own, although I realize there was
probably exceptions.
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| Fig. 4. The common buckhorn (large woody schrub) is the alternate host
of the soybean aphid. Photo Craig Grau |
Aphid biology
To overwinter in the upper Midwest, our aphids must have a very cold hardy stage. For
aphids, the egg is the most cold-resistant stage, and these are laid in late-summer to
early-fall and on a non-crop host. For the soybean aphid this is the buckthorn shrub
(Rhamnus spp.- Fig. 4). They will complete a generation on this overwintering host
(called the primary host) in the spring before moving to cultivated crops (the alternate
host). Bug people call these spring aphids fundatrices, and they are all winged females
that give birth to living young (in most cases) without fertilization. There are no males.
Several generations can be produced in a short time frame because of parthenogenesis. If
populations on cultivated crops become excessive and/or if plant quality deteriorates, the
next generation will have winged females. They will leave the field and migrate to new
fields. There will be no males until late summer and after the winged females have
migrated back to the overwintering sites. Mating will occur there followed by the laying
of eggs for next year’s problems. Both winged and wingless aphids can be found
in soybean and we probably have overlapping generations; the wingless aphids will be
yellow-green as will the winged forms. However, the winged forms will be slightly darker,
primarily a result of the wings.
Role of soybean variety
We have not found any evidence of host plant resistance being expressed.
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Aphids on the back of a soybean
leaf. The white flecks are the remains of the shed skins of immature aphids as they grow.
The yellow/green aphids are soybean aphids; the chocolate colored structures that are the
size of aphids are the remains of aphids killed by a fungal pathogen. Photo courtesy of
Union Grove Co-op. |
Biological control
Our best help will come from a fungal pathogen that usually catches up with aphids by
mid-summer. As diseased aphids dry up they may look to you like a small rust fleck on the
soybean leaf. Take a small pointed object and probe gently. You’ll be able to move
the aphid remains. Or take a hand-held magnifying glass and you’ll be able to tell
them from rust pustules or other organisms.
Adult lady beetles are usually plentiful. The black and
bristly alligator- shaped insects are larvae of lady beetles. They will also have a few
orange patches on their body. The orange spheres with black spots (about the size of a
dried pea) that are stuck to the plants are pupae of lady beetles. You may also find small
brown/green insect larvae shaped like the lady beetle larvae. They have a set of fangs for
mouthparts with which they suck the body fluids from aphids. These are aphid lions. You
will also find 1/4" long maggots on the leaf. These are syrphid fly larvae and they
also prey on the aphids. You will also find aphid mummies-brown/tan aphids that appear to
be inflated like a balloon. These have been parasitized by parasitic wasps. If you find a
pin-sized hole in the back of the mummy, it indicates that the wasp grub has emerged as an
adult wasp to search for aphid prey.
| Table 1. Insecticides
labeled for soybean that have a label for aphids in other crops. We have no
data at this time to suggest if they will be effective on aphids in soybean. |
| Product |
Rate |
| * Asana XL |
5.8-9.60 fl. oz./A |
| *Warrior T |
3.2-3.84 fl. oz./A |
| *Penncap-M |
2.0-3.00 pints/A |
| *Ambush 2E |
6.4-12.8 fl. oz./A |
| *Pounce 3.2EC |
4.0-8.0 fl. oz./A |
| Lorsban 4E |
1-2 pints/A |
| Dimethoate |
(see product label) |
* Restricted use material
When a range of rates are given, use no less than the middle part of the rate range. |
Insecticides
Although there are insecticides labeled for soybean that are labeled for aphids
on other crops, no insecticide is labeled for the control of aphids on soybean. However,
it is legal to apply the material to the labeled site for an insect not listed on the
label. There should, however, be strong evidence either from the manufacturer, university
research or other field research showing that the insecticide will control or reduce
populations when used against the non-labeled pest (the aphid in this case). This
situation is analogous to using a pesticide at less than labeled rates. It is legal to do
so, but the manufacturer does not have to stand behind the performance of the product.
There are no efficacy data for the soybean aphid. Asana XL and Warrior T are labeled for
cotton aphid in cotton but only for "suppression" and not control, which
suggests this aphid is tough to control.
In April, 2001 Cerexagri (formerly Atochem) received a
supplemental label for Penncap-M 2FM (Restricted Use) to control soybean aphid in
soybeans. The application range is 1-3 pints per acre with a restriction of no more than 2
applications per season and a pre-harvest interval of 20 days. Table 1 lists Insecticides
labeled for soybean that have a label for aphids in other crops. We have no
data at this time to suggest if they will be effective on aphids in soybean.
There is no way to assure farmers that the cost will be worth it.
Coverage is critical because aphids
are present in all parts of the canopy, and canopy penetration is tough at this time of
year in soybean. By air I would use no less than 3-5GPA and by ground no less than
30-40GPA and I would use flat fan or cone nozzles on ground-driven equipment. Ground
application will have the advantage of more thorough canopy penetration and coverage but
it will also run down drilled soybean, and this is a cost to factor in when you are
deciding if insecticide use if justified.
Every field has to be examined on a case-by-case basis.
See the Wisconsin Crop
Manager- Insects for the latest information on insecticide trials and
thresholds.
See also NCSRP Plant
Health Initiative for regional information about soybean aphids and SMV:
Soybean Aphid,
Soybean Viruses
and Soybean Mosaic
Virus
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