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Economic Threshold of
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Economic Threshold of
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New!
Continuing Education: Impact of Aphids on Soybean Yield Components
New!
Commentary
Performance
of Insecticides
| 2005:
Yield
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Aphids
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2008:
Yield
New!
Stage specific economic injury levels for V5, R2, R4, and R5
Economic thresholds are not picked out of thin air. Nor are
they set by consensus. Inaccurate economic thresholds can
be very costly if we consider the many acres of soybeans
involved every growing season since the discovery of the
soybean aphid.
Economic injury levels and action levels (economic thresholds)
are actually quantified through slow, tedious, and unglamorous
research.
Setting economic threshold requires a yield loss function.
That is,
a basic quantification of yield loss incurred relative to
insect numbers. At the moment, for example, we want to know
how many aphids cause how much yield loss in soybean.
Yield here pertains to tonnage, oil content, and protein content.
Although Stern et al. (1959) defined what economic injury and
economic thresholds should be, they never actually advanced
the mathematical procedure needed to set the economic
thresholds. Perhaps they did not even know how. But they
started the ball rolling.
Stern, V. M., R. F. Smith, R. van den Bosch, and
K. S. Hagen. 1959.
The integrated control concept. Hilgardia 29:81101.
In my book, the landmark publication that actually cemented
the whole economic-injury concept was made in 1972 when
Jay Stone and Dr. Pedigo, for the first time, advanced the
necessary procedures for calculating the economic-injury
level of the green cloverworm on soybean. This should be a
required reading for anyone interested in how economic
thresholds are set.
Stone, J. D., and L. P. Pedigo. 1972.
Development and economic-injury
level of the green cloverworm on soybean in Iowa. J. Econ.
Entomol.
65: 197-201.
Our
SD soybean aphid economic threshold was made
possible by the successful establishment of a yield loss
function through controlled field research in 2003-2004.
Graduate
student Eric Beckendorf conducted the field research
to
generate data for his MS thesis. Eric graduated in December
and is now biological technician for the USDA-ARS-NCARL
here in Brookings, SD.
The images below show the basic procedures and
results
from our recently completed research:
New!
Commentary
New!
Continuing Education: Impact of Aphids on Soybean Yield Components
We have thresholds available for V5, R2,
R4, and R5. Growers
who want to tank mix an insecticide and herbicide can use the
V5 thresholds; but V5 may still be too early and aphid numbers
may not increase if natural enemies were present and
did their
job. Also, most insecticides may only give about a month
of
residual activity thus leaving your soybeans unprotected later
in the season.
On the other hand, there is a substantial yield penalty if soybean
aphids are not controlled immediately. Those who have a lot of
confidence in natural enemies
may elect to
give natural enemies
a chance to work, and spray later
at R4 or R5,
if still necessary.
However, all spraying must be finished
at R5 as aphid numbers
will start to decline dramatically after R5.
Also, the
soybean plant
itself will be senescing soon.
Once again, in 2005, we
lost 5
bushels per acre just by waiting to
spray from R2 (July 28) to R4 (August 11). Five bushels times $6.00
per bushel is $30.00 per acre lost by sitting on ones hands. This, as
Buford would say, is called "opportunity cost." "Not to mention the fact
that R4 appears to be the end of the lag phase and the beginning of
exponential growth of them aphid population as predicted by the
logistic curve", Buford further opined. "Personally, I prefer to drive my
ground
rig between them rows when them beans are not inextricably
intertwined yet."
Soybean aphids can also reduce seed oil content by up to a certain
percentage point per 1000 unit increase in peak aphid numbers.
This is in addition to the reduction in yield components (pods per
plant, seeds per pod, seed size, and biomass).
These new information must be included in the economic threshold
calculation. Surely, you did not forget about the oil, did you?
What
about protein? You have that data, right?
Still 250?
From your basic agronomy class, do you remember when oil and
protein are deposited in the seed? If you want to prevent reduction
in oil content, when should you manage soybean aphids?
Still 250?
Future
research beyond thresholds would be the
role of ants in
influencing the success or failure of biocontrol agents; and quantifying
the contribution of biocontrol agents in terms of bushels per acre or
dollars and cents. For if we do not or cannot quantify the monetary
worth of natural enemies, biocontrol will forever be in the fringes of
integrated pest management.
Biocontrol researchers will be better served if they can show exactly
how many bushels
of soybeans were they able to increase by working
on
lady beetles, minute pirate bugs, and parasitic wasps. Simple
question: exactly how many bushels of soybeans? Or, maybe, this is
not a simple question after all.
Lastly, what's this thing I heard that the Asian lady beetle is actually
suspected of driving some of our native lady beetles to near extinction?
© Copyright 2008 by South Dakota State University. All
Rights Reserved.
| SD Soybean Aphid Biology | Insecticide Performances: | 2005: Yield | Aphids | 2004-I | 2004-II | 2002 | New! Commentary
Prepared and posted by Mike Catangui on December 23, 2002. Last updated on Monday, October 6, 2008.
