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Integrated Pest Management Newsletters and Information
IPM Stuff
All the pestilence that's fit to print
Issue 6
August 12, 2009
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Table of Contents:
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Upcoming Events
Upcoming events
at the U of M SW Research and Outreach Center (SWROC)
Yield
day- Southwest Research and Center - August 20.
Crop
Weather
Keep up to date on all the
crop weather details during the growing season at the SWROC's weather
page.
While areas of SW, WC and
southern MN received helpful rainfall, other areas remain very dry.
In some fields yield reductions have already occurred. I have some corn
on corn experiments that could be terminal without rain in a couple
of days. Parts of WC MN are also very dry. In addition to crop stress,
the dry conditions are driving populations of several arthropods described
later. We make some guesses on late season problems based on the weather
conditions to this point. Areas of Southwest Minnesota which had heavy
rainfall early in the season might expect to see sudden death syndrome
(SDS) and Brown stem rot issues. Areas which have struggled for moisture
are set up for stalk rot issues this fall and perhaps continued problems
with spider mites and insects associated with dry weather.
 Corn
Corn blotch leaf miner
damage was visible at low levels in plots at Farmfest and we also see
minor injury at the Southwest Research and Outreach Center. Oviposition
injury from the adult fly is visible in the photo the right. This insect
is much more abundant throughout SW Minnesota than usual.
 Aphids
It is gratifying to see that
growers and their advisors are scouting corn. Unfortunately, what some
are finding is an insect problem with little precedent. Corn aphid questions
and concern have increased rapidly the past week. Based on the infestation
in picture to the left, grower concern in some of these fields is understandable
and. A field with a widespread infestation at this level would arouse
my curiosity but not panic.
This is Déjà vu for some corn
growers as high aphid populations occurred in 2008 as well. Come to
think about it, it is not all that unusual to see high aphid numbers
in August corn during dry years. The difference may be more people spending
more time out there lately.
What aphid is it?
Proper identification of the
insects involved should be a priority. Unlike soybean, many aphid species
can be found on corn. In a 2001 paper, Stoetzel and Miller described
13 species of aphids that feed on corn foliage and roots in the United
States.
One of the most common leaf
feeding aphids in corn is the corn leaf aphid (Rhopalosiphum maidis).
Corn leaf aphids are small, pear shaped and blue -green to gray with
dark cornicles. This aphid tends to colonize the upper part of the plant
and migrates from the south each spring. While abundant in 2008, I have
not yet observed this species in any of the corn I have been in this
season.
Most of the aphids in corn
this year (and 2008) are bird cherry-oat aphid (Rhopalosiphum padi).
The small aphid is dark olive and mature specimens have an orange blotch
around the cornicles. This aphid feeds on a range of plants including
corn and small grains (e.g wheat, oats). In these grass crops, it seems
to prefer the leaf sheath area and base of the plant. Overwintering
behavior of these aphids is less clear.
Another relatively common
aphid in corn the past two years is the English grain aphid (Sitobion
avenae). This is a moderate size, bright green aphid with all black
cornicles. It tends to form colonies on upper leaves and ear husks.
I have also seen small colonies
of greenbug in SW MN corn this year.
There are two keys for corn
aphid id included in the references. Both are available on the web.
 The
problem
Here is what we know about
the current problem: Most of the concerns are from infestations of bird
cherry- oat aphid. Aphid suction trap captures at Lamberton and some
other areas are full of bird-cherry oat aphid. In corn, infestations
seem to progress to the ear area and are less abundant on the upper
leaves.
Some hybrids appear to be
more heavily colonized and aphid populations seem to be tracking moisture
stress. Drier fields or areas within fields are expressing higher aphid
populations. Corn on corn is often more heavily infested do to greater
moisture stress. However, extreme moisture stress does not favor aphids.
Corn at wilting point has few aphids left. I suspect that many fields
that are showing high aphid populations are beginning to show stalk
rot at the crowns. Lower leaf loss is visible in many of these fields
but sorting out any aphid contribution to this from drought and spider
mites is not possible.
Some foliar diseases have
been documented to increase stalk rots. We do not know if aphid feeding
or loss of photosynthetic area from honeydew/sooty mold can cause a
similar interaction. We do know that aphid species can transmit virus
to corn but preventing virus transmission by treating aphids is likely
to be futile. Last year, mixed species were present on plants at the
Southwest Research and Outreach Center (bird cherry oat, Corn leaf and
English grain aphid).
Why is the problem so bad
this year?
Many aphid species thrive
under dry cool conditions. Work with several species of aphids and plants
show increased reproduction on moisture stressed plants. Fungi that
control aphids are most effective under cool, wet conditions. It remains
to be seen whether this is a problem which will continue to be an issue
in corn production or just a result of the past two season's weather.
Economic injury
Most, if not all, of the economic
threshold work is based on corn leaf aphid and corn prior to pollination.
There is no reason to suspect that pre-pollination thresholds would
apply to corn at this time of year, nor is there a reason that damage
from other species would be similar. Remember corn leaf aphids prefer
the upper part of the plant. Post tassel corn leaf aphids are generally
believed to not impact yield even with high populations. Bottom line
is - No documentation for late season yield effects from aphids.
We know more than 700 aphids
or so can cause yield loss to early reproductive soybean (before full
pod) under stress conditions. However, based on the difference in biomass
alone, one could infer that corn could tolerate many times the number
of aphids without losing yield. I do, however, acknowledge that inferences
and assumptions are dangerous. Nevertheless, corn and soybeans do accumulate
yield in very different fashion and different aphid species feed differently.
It would be a big stretch to directly translate between aphid problems
in corn and soybean.
Making a decision
Aphid populations often collapse
dramatically through weather and natural enemies. We recommend ignoring
aphids in post-pollination corn unless: 1) populations are very heavy
above the ear and photosynthesis may be impacted from sooty mold and
2) predators and disease are rare in the field. Even then, the yield
benefits of treating are uncertain. I would be inclined to leave the
aphid problems alone.
If you decide you must treat,
consider the following: Coverage and control in 8-10 foot tall corn
will be difficult. Spraying by ground is not possible in most fields,
particularly narrow rows. Use as much water as practical (4-5 gallons
by air).
Insecticide applications will
remove beneficial and poor coverage can lead to rapid population rebounds.
Aphid problem fields are in drier areas. Flaring of spider mites present
in the lower canopy is also a concern. It may be wise to use an insecticide
that provides spider mite control when spraying aphids in corn. Fungicide
applications to corn have the potential to reduce the efficacy of aphid
killing fungi in the field.
Please leave adequately wide
check strips if you do treat. I will collate as much of this data as
I can this fall.
Selected references
Steffey, K.L, M.E. Rice, J.
All, D.A. Andow, M. E. Gray, J.W. Van Duyn (eds.). Handbook of corn
insects. 1999. The Entomological Society of America.
Stoetzel, M.B and G.L. Miller.
2001. Aerial feeding aphids of corn in the United States with reference
to the root-feeding Aphis maidiradicis. Florida Entomologist84(1). 83-98.
Hein, G.L, J.A. Kalish, J.
Thomas. 2005. Cereal aphids. Identification
and general discussion of the cereal aphids specied most commonly found
in Nebraska small grains corn, sorghum and millet. NebGuide G184.
Soybeans
Soybean aphid
Areas of Minnesota where soybean
fields have reached economic threshold have expanded; in part due to
the summer dispersal that occurred in late July through last week. Fields
treated before August 1 should be re-evaluated to determine if re-colonization
has occurred. On the up side, we are starting to see aphid mortality
from fungal disease in experiments at Lamberton.
Downy mildew is unusually
abundant in some fields even in low rainfall areas. Fungicide applications
are not likely to control this disease.
Spider mites
While areas receiving heavy
rainfall last week may have had the spider mite problem solved, areas
with a couple inches or less may be still at risk. Spider mites have
reached near damaging levels in some soybean fields that have received
spider mite treatments two weeks or more ago. Remember to use a different
active ingredient when re-treating fields.
 I
need a vacation
While dissecting corn roots
for a corn rootworm experiment I happened to glance at the vegetation
to my right. A large number of large bright red aphids were present
on plants with tri-lobed leaves. Trying to sound impressive I pointed
out to my student workers that those look like brown ambrosia aphids
on giant ragweed. After pontificating on the high level of host preference
by most aphid species I happened to look at the top of the plant. I
was probably right on the aphid but the plants were green coneflower
and not ragweed. Regardless that it works in many cases and rating corn
roots for rootworm damage can cause one's mind to wander, plant id based
on the aphid species indicates a padded room may not be far off.
Until next time,
Bruce Potter
IPM Specialist SW Minnesota
University of Minnesota
Extension
Department of Entomology
University
of Minnesota Southwest Research
and Outreach Center
23669 130th Street
Lamberton,
MN 56152
Ph:
507.752.5066
Fax: 507.752.5097
E-mail: bpotter@umn.edu
http://swroc.cfans.umn.edu/SWMNPEST/swmnpest.htm
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