Volume
2 Issue
11
August 2,
2005
Soybean Aphid: The Minnesota Aphid
Express Has Left on Track 9
 Soybean
aphid populations have been rising in a number of
locations in NW Minnesota. The MN Department of Ag pest
survey from July 20 to 26 illustrates the wide spread
nature of the aphid population (figure
1). This week’s survey should be posted by
Friday, if you wish to keep up with the most recent survey
observations.
http://www.mda.state.mn.us/pestsurvey/PestReports/PestReport.html
Otter Tail
County is not well represented in the state survey. This
is the area where reports of soybean aphid actually have
the largest populations. Populations have exceeded
treatment thresholds and wide spread applications are
occurring this week around Fergus Falls and Underwood
areas with applicators working from early morning to dusk
to accomplish their workload.
Fields to the north and west are more variable, yet
there have been limited applications against aphids.
Smaller fields with wooded, protected areas have had the
greatest numbers of aphids and fields have been treated.
Larger fields have high percent of the plants with aphids
present (80 to 90% of the plants), but populations are
generally averaging below the 250 aphids/plant treatment
threshold. Aphid per plant numbers range from low-teens to
hundreds, but the average per plant has been in the 40 to
50 aphid/plant range.
 The
chance of precipitation is low for the next week and
forecasted temperatures (high 70’s to mid-80’s) will drive
aphid reproduction. We can anticipate increasing
populations over the next 2 weeks. Add to the equation the
high numbers of winged aphids (figure
2) reported from infested fields in the SE and
the significant southerly winds (The Minnesota Aphid
Express is on the way) and we potentially have lots of
incoming aphids that can also increase populations
rapidly.
At least two chemical trials currently being conducted
in the Fergus Falls area have had very high levels of
immigration as evidenced by lots of winged aphids and the
establishment of 1st instar nymphs (basically new-born
aphids; very small and white). At least one field on the
NWROC has wide-spread populations that are increasing and
it is estimated will require treatment within 2 weeks.
Bottom line – scout those fields, key
into what 250 aphids per plant looks like. Because our
soybeans are into reproductive stages, aphids have moved
off of the top leaves and now can be found anywhere on the
plant, including stems. Look for small white aphids, these
are the 1st instars and are probably no more than 2 days
old.
One last word of caution. Trials this
summer are indicating that the Synthetic Pyrethroids (e.g.
Warrior and Asana) are not providing the same level of
long term aphid suppression as witnessed in previous
years. In trials at Underwood, soybean aphids are
re-infesting pyrethroid treated plots in approximately 11
days. This may be due to the high temperatures we have
been getting, the efficacy of pyrethroids decreases
significantly when temperatures get into the 90’s.
Consequently, don’t assume fields are safe that have
already been treated with pyrethroids to prevent aphid
populations from developing (i.e. ‘just in case’
spraying). Because the pyrethroids don’t seem to be
holding as long in this warmer summer, keep an eye on
these fields as well.
Additional Comments on Insecticide treatments.
When treating, consider the insecticide choices for your
situation. Aphid kill, residual, and yield gains are not
consistent among individual insecticides.
- Pyrethroids (Warrior, Mustang, Asana, Baythroid)
have a long residual, and work best at temperatures
below 90°F. Organophosphate products have a fuming
action, and may work well in heavy canopies or at
higher temperatures under all circumstances.
- Tank mixes of fungicides, herbicides or foliar
feedants, with an insecticide targeted for the soybean
aphid, are generally compatible, but a jar test is
always recommended. Crop damage has been reported
where organophosphate insecticides have been added to
herbicides, so caution should be used. Consider the
optimal timing and method (GPA, pressure, nozzle type)
for all the components in the tank mix.
- Good coverage is important, particularly with
aerial application. High spray volumes and high
pressure help to move the insecticide down into the
canopy.
- It is suggested to leave an unsprayed check strip
for comparing against sprayed areas to determine the
performance of the insecticide and the value of the
treatment.
- Spraying at early reproductive stages poses a
threat to bees. Communicate treatment plans to
beekeepers and follow precautions to minimize bee
kills. In areas with concern about honey bees,
pyrethroids are a better choice for application than
other formulations.
These recommendations are the
cooperative effort of
researchers throughout the North
Central states, funded with
soybean checkoff dollars through
the North Central Soybean
Research Program.
Soybean
Aphid Insecticides
Natural Management for Soybean
Aphid
Intense rainfall may kill many aphids by dislodging them
from the plant. High humidity that follows rains should
increase the potential for aphid fungal infections.
 Numerous
predators and parasites attack aphids. Important predators
of the soybean aphid are lady beetle larvae, lacewing
larvae and predatory bugs. Evidence of parasitic wasp
activity is the presence of aphid mummies on leaves
scattered through the aphid colonies.
Aphid populations may decline rapidly in response to
other factors. Crowding and declining quality of food
resources often stimulate the development of winged adult
aphids. There were numerous cases reported since August
2001 where aphid populations declined rapidly as the
aphids developed wings and migrated from those fields.
For a reference on
management, access
Soybean Aphid Management,
NDSU Extension Service Circular, E-1232. November 2004
Ian
MacRae, Entomologist
U of MN NWROC, Crookston
and
Phillip Glogoza
Regional Extension Educators - Crops, Moorhead
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