We are quickly entering the spring season, when planting is on the mind, and we start thinking about weeds and pests. Our anxiety rises when considering treatment options for the critters out there who want to eat or crowd out what we are trying to grow for our livelihood. I thought it was a good time to review just why it is a good idea to have a professional out there looking at your fields, and why, sometimes, they don't recommend doing anything, even when you think you see something that worries you. Read on!
Thresholds in scouting
Why we don’t always treat when we see a problem
When I scout a field, I look for anything out of the
ordinary during the growing season. Each
part of the season brings a new set of challenges. Early on, there are weeds to consider,
emergence problems, and seedling diseases.
Later in the season, I look for nutrient deficiencies, insect feeding,
and root and foliar diseases. I look for
damage from wind and drought.
When I do see something that is out
of the ordinary, I don’t always recommend acting on it. I’m not just looking at insects for the
opportunity to spray insecticides, and I’m not going to tell the producer to
find a fungicide whenever I see a disease.
Why is
that?
There are several reasons.
First, there isn’t
always something we can do about the problem. For example, if there’s a water shortage or a
drought in a dryland or rainfed field, I can note the leaves curling and plants
wilting, but there’s no pivot to turn on.
All we can do is hope for rain. In another case, there’s a relatively new pest haunting
soybeans in my area called the soybean gall midge. We have a couple years of research on this the little
fly-like creature, but the adults spend their
time in the ditches and woods surrounding the fields and only come into the
fields to lay their eggs. Once they
hatch, the larvae, which are barely visible to the naked eye, quickly find
cracks in the stems to hide away, burrowing into the interior of the
plant. Neither the adults nor the larvae
would be affected by an insecticide treatment in the field—the adults aren’t in
the field for very long, and the larvae are too well protected by their
burrowing habits. So alternate and
preventative treatment considerations are being discussed. For now, we observe and report.
Second, it isn’t
always the right time of the year to treat the problem. I recently saw some corn plants with yellow
whorls and interveinal chlorosis. This
definitely could be the sulfur we had yet to apply to the field, but more
likely it was due to a pH problem. When
the plants are in the field, it isn’t the right time to be applying lime to
correct the pH. So, again, we can apply
the sulfur, but have to schedule the lime for after harvest in the fall or next
spring before planting. Alternately, the
problem could be larvae that had eaten their way through much of the
vegetation, but most of them were already in pupae—they won’t be eating
anything as adults, so it doesn’t warrant treating pupae who won’t respond to
the insecticide. Also, just because you
see evidence of feeding on plants, doesn’t mean you see the critter that has
been feeding on them—you may be treating a critter that isn’t even there
anymore.
Third, the problem
isn’t always at the economic threshold to be able to warrant treating the
problem. No field is going to be
perfectly pest-free, and no plant is going to be perfectly free of some sort of
feeding or disease. Universities come
out with what they call “economic thresholds”, or the point at which a pest is
going to create enough harm to the plant that yield will be adversely affected,
and the extra expense of treating the field is worth it. Not only are you paying for the chemical, but
there’s the application fee if someone else does the application, and equipment
cost, fuel, and time spent driving in the field if you do the application
yourself. The potential income from
yield lost has to outweigh the cost to treat the pest. In the case of insects, it may be number of
insects per plant or amount of total feeding on the plant (percent of
defoliation), in which every leaf on the plant is considered for total
loss. In the case of diseases, it may be
percent of the field affected by the disease or number of lesions on each
plant.
Sometimes, scouting a field is more
of an art form than a cut and dry numbers game. Just because you see the critter out there—or
even the evidence of it, doesn’t automatically mean there is something that can
or should be done about it. But that
also doesn’t mean you shouldn’t be out there watching things grow, and keeping
an eye on things. Because the moment
that threshold is crossed, and the season is right, timing is of the
essence. It helps to have someone that
knows what they are looking for, and knows what they are looking at.
It helps to have someone outstanding
in the field. [Yup. I had to throw that pun in there.]
Who’s looking out for your crops?
Thanks for reading.
Julie S. Paschold
originally written June 16, 2020
repost April 17, 2023