The plant bug situation has continued to increase over the past week. Going back at least 51 seasons, this appears to be the most widespread and highest pressure we have seen in Alabama. There are fields with numbers as high as 5x threshold in some places. Many fields have received 2 pre-bloom applications, and some have had three.
The question is “what do we do?”
In a lot of these fields, plant bugs are reinfesting so
fast, 7–10-day spray windows are not holding up. Remember that no product provides
residual control of migrating adult plant bugs. We may expect 48-72 hours of
control, but past that, things get dicey. In other cases, the number of adults
in the field is hard to get under control with our “normal” plant bug regime.
In most years, the highest labeled rate of imidacloprid is “good
enough” to get populations under threshold, persevere square retention and reduce
egg lay. In many fields this year, the ≈60% control we get leaves numbers out
there that we can’t live with. In other words, 60% of 8 bugs/100 sweeps is 4.8 bugs
in the field (below threshold), while 60% of 12 bugs/100 sweeps leaves 7.2 bugs
in the field (right at threshold). Again, in high pressure fields, the highest
rate of imidacloprid alone will likely not keep populations in check.
What about pyrethroids? We have a graduate student, TJ
Douglas, who has been screen adult plant bug populations for resistance to
several classes of insecticides this spring. In his preliminary analysis, he
has found HIGH levels of resistance to bifenthrin in the TN Valley (Limestone
County, we have known this for years), SW Alabama (Monroe County), and East Central
Alabama (Elmore County). A population from the SE Alabama (Henry County) came
back with elevated levels of resistance (but ≈62% lower than SW Alabama).
Populations in NW (Cherokee County) and West Central (Dallas County) came back
much easier to control.
What does this mean for 2023 in regions with resistance?
We have known about pyrethroid resistance in the TN Valley;
thus, they have not been recommended for control for years. Over the
past 2 seasons, we began receiving scattered field complaints in SW Alabama. Due
to these experiences and results from lab tests, we are no longer recommending
pyrethroids in SW Alabama for plant bug control. We have not received field
complaints from the Wiregrass or East Central Alabama following pyrethroids, so
this resistance could be “pocketed” and not widespread across the region. It is
hard to say what we will get out of pyrethroids in these regions this year. Choosing
a pyrethroid for plant bug controls may come down to a grower-by-grower
decision based on their experiences. I would be very cautious using a pyrethroid
in the Wiregrass and EC Alabama and scout closely behind to monitor
performance.
What does this mean for 2023 in regions with no
resistance?
In NW Alabama and the Black Belt, we are still comfortable
recommending pyrethroids for control of adult and immature plant bugs.
So, if imidacloprid isn’t good enough and pyrethroids are
resistant, what are we going to do with adult plant bugs?
Centric (thiamethoxam) is still an option. Centric is
a neonicotinoid (like imidacloprid), just a little more expensive. Centric at 2
oz/A has always been better on adult plant bugs than imidacloprid and
resistance monitoring suggests populations are still susceptible across the
state. **Note: Centric often does not provide adequate control of immature
plant bugs.
Acephate is another good option for control of adult plant
bugs. The ½ lb rate of acephate should provide adequate control. In some
fields, acephate plus imidacloprid has been used with positive results. Resistant
monitoring shows that plant bugs are still susceptible statewide. **Note:
Acephate is often associated with spider mite outbreaks in days or weeks
following application.
Transform is probably the best plant bug material currently
on the market. We typically like to hold the “big gun” until bloom, when
immature plant bugs begin to emerge. The 1.5 oz rate of Transform provides good
control of all stages of plant bugs, but at a price. This product is important
for controlling immature plant bugs.
What about tank-mixing imidacloprid and bifenthrin?
We have some data on this from the TN Valley on immature plant bugs. In 2021,
it provided pretty good control. In 2022, it did not provide any measurable
level of control. This mix may work on migrating adults, but it may be a total
miss.
**Bidrin is restricted by label until bloom and cannot be
used in the pre-bloom window**
How do I know if my insecticide worked?
While still important, monitoring adult populations in the
field following an application is not the best way to determine if the most
recent application work. Adults may reinfest fields and it is not uncommon to
find as many or more adults in a field 5-7 days after a spray than before. The best
way to determine the efficacy of an insecticide application for adult plant bugs
is to monitor pinhead square retention 4-5 days later. If retention falls 10%
or more 4 days later, then it is likely that either the application did not work
or could not perform under the pressure. Use this information to guide follow
up applications. If retention is holding (and greater than 85%) but threshold
levels of adults are in the field, then a follow-up application is still
warranted.
What about cotton with immatures present?
In a lot of the older cotton in Alabama, immature (nymph)
plant bugs are beginning to or have been emerging over the past week. In this
situation, where adults are still migrating in and nymphs are showing up, Diamond
is critical for control. We wrote a lot about using Diamond last week (link),
but here are a few reminders: Diamond is not a “knockdown” insecticide and
should be tank-mixed with one. Diamond provides residual suppression and does
not eliminate plant bugs or the need for follow up applications. Think of
Diamond like ThryvOn: populations can still build to damaging levels but are
lower than they would be if not used.
After bloom, acephate, Transform and Bidrin are options to
consider tank-mixing with Diamond.
Getting off to a good start in this cotton with nymphs
emerging will be critical to “set the tone” for the rest of the season.
As always, if we can ever be of any help, or if you would
like to provide input on the situation in your area, please don’t hesitate to
reach out (Scott Graham: 662-809-3368; Ron Smith: 334-332-9501).
For more information on thresholds and insecticide
recommendations, visit the Alabama Cotton IPM Guide (IPM-0415). To
stay up-to-date on the Alabama cotton insect situation, subscribe to the
Alabama Cotton Shorts Newsletter,
Alabama Crops Report Newsletter,
and the Syngenta Pest Patrol Hotline.
