Volume
4 Issue 9
July 17, 2007
Soybean Aphid Speed Scouting: An Overview
It has been several years now since this field
scouting method for making soybean aphid treatment decisions was
introduced. The procedure was called “Speed Scouting”
by the researchers who developed it. The method speeds up making
decisions on whether to treat or not by using information learned
about the aphid’s population distribution in the field when the
treatment threshold of 250 aphids per plant and 80% of the plants
with these levels is reached.
When discussing aphid per plant thresholds, the response from
people who do the field scouting was less than enthusiastic. No one
wanted to count all those aphids. A result is people tend to
streamline the process, “training” themselves to recognize what a
field infestation of 250 soybean aphids per plant might look like.
The Speed Scouting method formalizes this learning process and
provides a statistical level of reliability you don’t get with your
own personal experience. The method is formally referred to as a
binomial sequential sampling plan. The binomial refers to
two outcomes or classifications. The sequential sampling aspect
refers to the process of keeping track of what is and isn’t infested
and eventually being able to conclude, often times quickly, that the
number of plants needed to make a decision with high levels of
reliability have been sampled.
The protocol for a binomial sequential sampling plan for soybean
aphid was developed by researchers at the University of Minnesota.
Further discussion about the procedure can be found at:
http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sampling.htm
Here is a quick overview:
After collecting data from commercial soybean in southern and
central Minnesota, entomologists at the University of Minnesota
developed a binomial sampling plan, called Speed Scouting for
Soybean Aphid. The recommendation is to use this sampling plan
through the pod set stage (R4).
A ‘binomial’ plan refers to two choices; sometimes
it means a presence/absence count or, as in this case, a pre-set
cut-off number where counting can be stopped. For this procedure,
the binomial sampling cut-off point is 40 aphids per plant. If a
plant has less than 40 aphids, consider it
non-infested; however, if the plant has 40 or more
aphids (remember, counting additional aphids is not
necessary after 40), consider the plant infested.
The field scout keeps track of the plants and how they are
classified. A worksheet really is a must in order to keep track.
Eventually, enough plants (the minimum number of plants is 11) will
have been checked to start making some decisions. The statistical
models these decisions are based on will allow you to make one of
three decisions with a high degree of reliability (the model has
been set at 75% reliability). Those decisions are:
- Do not treat the field,
- Treat the field, or
- Resample the field in 3 to 4 days
The binomial sampling plan can improve the cost (especially in
time commitment) of sampling because every insect no longer needs to
be counted. Though not perfect, the model has attempted to balance
reliability with cost of scouting. When fields are close to
threshold levels, more plant samples are typically required to make
a decision. However, when fields are clearly not at threshold or
easily over threshold, decisions are made quickly and reliably.
You can obtain an
electronic copy of the worksheet
on the web page previously
mentioned.
Modifications for 2007:
After field validation during 2005, results revealed that the
procedure can prematurely reach treatment decisions, typically at
about 160 aphids per plant rather than the threshold of 250 aphids
per plant. To ensure the population is actually increasing, we
recommend that the field be re-checked in 3 to 4 days.
worksheet for soybean aphid speed scouting
pocket-sized field cards for soybean aphid
speed scouting
Reference
Hodgson, E. W., E. C. Burkness, W. D. Hutchison, and D. W.
Ragsdale. 2004. Enumerative and binomial sequential sampling plans
for soybean aphid (Homoptera: Aphididae) in soybean. J of Econ.
Entomol. 97(6): 2127-2136.
Hodgson, E. W., McCornack, B. P., Koch, K. A., Ragsdale, D.
W., Johnson, K. D., O’Neal, M. E., Cullen, E. M., Kraiss, H. J.,
DiFonzo, C. D., and Behnken, L. M. 2007.
Field validation of Speed Scouting for soybean aphid. Online. Crop
Management doi:10.1094/CM-2007-0511-01-RS.
Phillip Glogoza
Regional Extension Educator - Crops
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