Urbana, Illinois
July 30, 2004
A lot of
people continue to monitor soybean fields for the presence of
soybean aphids, and scrutiny for this pest is a good thing. We
wish sometimes that scouting for other pests would be as
diligent. Fortunately, scouting efforts thus far in 2004 have
revealed few soybean aphids in fields throughout the state. In
last week's issue of
the
Bulletin (issue
no. 18, July 23, 2004), we noted that some people have
observed "hot spots" of high densities of soybean aphids in some
fields, and we encouraged everyone to keep their eyes on the
development of these localized infestations. The recent weather
conditions (cooler than usual) have been beneficial for
development of soybean aphids, so a full-blown, fieldwide
infestation could evolve from a hot spot. We learned firsthand
last year how quickly aphid populations could become
threatening.
We have read
one unconfirmed report that soybean aphid densities have reached
or exceeded the economic threshold established for the
Midwest--250 aphids per plant during the R1 through R4
stages--in northern Illinois. If this report is accurate, people
in northern counties should ratchet up their scouting efforts.
Obviously, soybean aphid densities have not been as high in 2004
as they were at this same time in 2003, so it is unlikely that
economic infestations will develop in most fields in Illinois
this year.
Most people
will agree that one of the most challenging aspects of managing
soybean aphids is scouting for and counting the pests. Counting
aphids and determining an average density for a field are
particularly difficult when numbers of aphids are low. To
address this concern, entomologists at the University of
Minnesota have developed a more efficient scouting process,
which they refer to as "speed scouting."
The scouting
plan is a binomial sampling plan, or a modified sequential
sampling plan. The objective is to enable scouts to sample for
aphids more efficiently and to make decisions about control more
quickly. The plan is explained thoroughly on the
University of Minnesota "Soybean Aphid in Minnesota" Web site.
Most helpful are the example forms that are completed for a "do
not treat decision," a "treat decision," and a "resample the
field decision." In these examples, mock numbers of soybean
aphids are used to explain how decisions to treat or not to
treat can be made quickly, and how continued sampling is
required under certain circumstances. Marlin Rice, extension
entomologist at Iowa State University, also recently wrote an
article about speed scouting for soybean aphids for
Integrated Crop Management, which includes yet another
magnificent photograph.
Because the
key to the speed scouting procedure is efficiency, the binomial
sampling cutoff point is 40 aphids per plant. In other words,
you need not count more than 40 aphids on any given plant. If
you quickly determine that there are more than 40 aphids on a
given plant, you record your observation ("+") and move on to
the next plant, per instructions. The fewest plants you will
sample with this procedure is 11, and the most you will sample
is 31.
Because the
speed sampling procedure is explained so thoroughly and so well
in the aforementioned articles, we will not repeat the articles
in their entirety in this issue of the Bulletin. However,
it is important to note that this procedure does not create a
new threshold of 40 aphids per plant. This sampling plan uses
the percentage of infested plants (at least 40 aphids on a plant
is considered infested) as an indicator of damaging soybean
aphid populations. There is a statistical relationship between
the economic threshold (250 aphids per plant) and the proportion
of plants that are infested (i.e., 40 aphids per plant).
As we have
frequently stated, new information will derive from research
conducted on soybean aphids and from our annual experiences with
this invasive species. As this new information accumulates, we
will make you aware of it in as timely a fashion as possible. We
will try to stay one or more steps ahead of the soybean aphid
and other insect pests so that you can be prepared with the most
current information.
Kevin Steffey,
The
Bulletin |