Nikolas
Clarkson is a returning intern for us at the Hale, Swisher, and Floyd IPM Unit
with Texas A&M AgriLife. Nik is a
senior at Texas Tech nearing the end of his collegiate career. Part of Nik’s duties are to write and help
educate in an applied way on some of the subjects he is learning about with us
this summer. Here is Nik’s write up on
Bt resistance in the bollworm population and what we might be expecting from
the worms as they move into more cotton fields this August.
Nikolas Clarkson’s Bollworm
Bt Resistance and Status August 2018
It has been an
interesting summer this year in West Texas with our cotton growing season. We
are at that point of the growing season where we have fought through drought,
plant growth, weeds, and pest issues and are inching our way toward the finish
line. However, we still have a couple more hurdles to climb before we reach
that sweet harvest date. One of those hurdles is the cotton bollworm and now is
the time to prepare for their full on arrival.
As people in the
cotton industry in West Texas we need to understand the importance of the
bollworm and their relationship with cotton. The bollworm has been known since
around 1796 and has been a pest in cotton ever since. The states that see the
bollworm the most are Texas, Louisiana, Mississippi, Oklahoma, and Arkansas. The
bollworm is a pest that doesn’t just attack cotton either most people can find
them in corn known as corn earworms and you can find them in sorghum, soybeans,
and peanuts as well but our focus is on cotton. The bollworm has 4 stages in
its life cycle which are the egg, larva, pupa, and the adult moth. The cotton
bollworm hatches out of its egg about 3 days after being laid in the hot West
Texas weather, so this gives us a short period of time before they turn into
larva and start destroying bolls. The cotton bollworm can be detrimental to a
cotton field in a hurry if we do not catch them in time. They can also be very
hard to control once they have entered the bolls so trying to catch them before
that threshold point is important.
We’ve been lucky
through the years with Bt, but we are
seeing resistance of the cotton bollworm in these technologies. The cotton
bollworm resistance is getting worse with last year being the worst year so we
need to be knowledgeable about cotton bollworm resistance in Bt varieties moving into the bollworm
season this year. In an article by Steve Byrns of Texas A&M Agrilife
Communications he caught up with Dr. David Kerns who is the Texas A&M
Agrilife Extension entomologist and IPM coordinator for the state of Texas. Dr.
Kerns and his colleagues worked on a project and collected data on 2016-2017
and was quoted stating that bollworm populations were showing “high resistance to the Cry1Ac technology
and about 70-75 percent of these populations showing resistance to the Cry2
toxins.”
There was also
some resistance seen in Cry1F. This can be worrisome for us because this means
our second generation Bt technologies
are seeing quite a bit of resistance. This means for producers with Bollgard
II, WideStrike, and TwinLink just need to be aware for any bollworm resistance
as these are made up of the Cry1 and Cry2 toxins. For now it seems that if we
have third generation technologies planted we should be okay, but we should
still be cautious. WideStrike 3, Bollgard 3, and TwinLink Plus have Vip3A toxin
which Dr. Kerns says is still “pretty new, so it has not been grown
extensively.”
Burns also
mentions during the interview that Dr. Kerns says “that they haven’t found
resistance to Vip technology yet but that it is expressed better in corn than
cotton. When cotton is stressed it may not express enough of the Vip3A to
provide desired control.” So as consultants, researchers, and producers we just
need to be aware that we can possibly still suffer damage and need to be able
to find these worms knowing they are out there.
Now that cotton
bollworms are vying to reestablish themselves as the number one cotton pest and
are becoming increasingly resistant to Bt
varieties we need to make sure that we are properly trained on how to scout for
them in the field. We are coming up on the date of August 20th which
every year is our most important week of scouting because that is when we tend
to see a huge wave of bollworms coming in.
For us working in
the Hale, Swisher, & Floyd IPM Unit, Blayne is very particular about our
field scouting methods. First, we get out across the field. For each data set, we start with whole plant
inspections, scouting for the eggs and larvae on 2 plants. We usually get about 5 separate data sets for
10 whole plants inspected total, but this varies on field size. Next we make
use of a drop cloth that allows us to either check 4.5 or 6.4 row feet
depending on row spacing and literally pop off all insects from the dozen or so
plants in that area under the cloth. This allows us not to check 100 plants and
get an average for a field with 4 or 5 drop cloths. We bang the plants on the
drop cloth hard enough and see what falls out. Sometimes you will find the worm
straight up on your drop cloth, other times a small boll will fall off and the
worm will be burrowed in the hole it has created, or you can find the small
worms in bloom tags that have fell on the drop cloth as well. It is also
important to pick up dropped squares and bloom tags in the area to see if you
find bollworm damage or the worm itself. For the plant inspections, the
bollworm can be found anywhere but usually lower on the plant and around bolls
and squares. When inspecting it is easy to either see the worm boring into or
out of the boll or finding the hole and feeling that the boll is not strong and
firm but soft and squishy. Don’t forget
to check under the plants for worms that have fallen with damaged fruit. These worms will crawl back onto plants to
attack more fruit quickly.
A bollworm egg
will be colored white as a pearl, perfectly round and dome-shaped like a ball,
and are ribbed all the way around. They are very small and can be easy to miss
because they are just a little bit smaller than a pin head. Usually you are
supposed to find and eggs laying on one of the top of the uppermost leaves of
the plant, but they can be found anywhere from bolls, squares, bloom tags that
haven’t fell off yet, or even on the stem of the plant. When we scout for the
worms themselves we need to know how to identify them by their anatomy, size,
and color. The cotton bollworm has 5 pairs of prolegs, 3 pairs of slender legs
behind the head, and under a magnifier the skin looks rough and spiky. Their
color can range from a light-dark green to a pinkish and brownish nearly black
color. We classify the bollworm into 3 different stages which are small medium
and large. The first 3 days the worm is considered small and goes from 1/16” to
¼” in that time. The worm right out of
the egg is easy to miss because it’s so small so it is important to have a
magnifier. The worm is then classified as medium in days 5-8 at ½” to 7/8”
tall. Then days 10-12 the worm is 1 1/8” to full grown at 1 3/8” tall.
If
we come to a point where we unfortunately have to spray for bollworms it is
critical to know what we can spray and how much it costs. Chemistry and market
depending, it can cost between $6.80 and $38.00 per acre for bollworm control
products.
Some currently work better
than others, while some protect beneficial arthropods and others are broad
spectrum.
There can be many decisions
going into which product you might need. Here is a short list of chemicals and
rates for the cotton bollworm provided by the Texas A&M Agrilife Extension
Cotton Insect Management Guide that can help you decide:
https://cottonbugs.tamu.edu/fruit-feeding-pests/bollworm-and-tobacco-budworm/
Sources
Agriculture Yearbook Committee. “Insects.” The Yearbook of Agriculture 1952, United
States Department of Agriculture, 1952, pg. 511-512
“Bollworm and Tobacco Budworm.” Cotton Insect Management
Guide,
cottonbugs.tamu.edu/fruit-feeding-pests/bollworm-and-tobacco-budworm/. Accessed
10 August 2018.
Metcalf, C. L., Flint W. P. “Destructive and Useful Insects.”
Theirs Habits and Control, R. L.
Metcalf, McGraw Hill Book Company, 1962, pg 578
Williams, Rob. “Rob Williams.” Texas IPM Program, 30
Apr. 2018, ipm.tamu.edu/2018/04/experts-cotton-farmers-may-face-increased-bollworm-pressure-this-season/.
Thanks Nik!
Blayne
