We have received reports of threshold populations of plant bugs from the Gulf to the Tennessee line and from Mississippi to Georgia. If cotton has squares, it should be scouted for adult plant bugs. In Alabama, our threshold is 8 adults per 100 sweeps -OR- to maintain 80% square retention. Some states recommend only treating adult plant bugs if square retention falls below 80%, regardless of the number of adult plant bugs in the field. We have a couple of reasons for treating 8/100, regardless of square retention.
1.
A square will likely not fall off the plant the
day it is fed on by a plant bug. In other words, you can find a field with high
plant bug numbers and high square retention because plant bugs have moved in
recently and fed on squares, but they have not had enough time to fall.
2.
Consider the damage potential of plant bugs. A
threshold level would result in unacceptable square loss in the following week.
The literature says that adult plant bugs feed on 0.6 to 2.1 squares per day. A
threshold population of adult plant bugs is ≈1,160 bugs per acre. Assuming
a plant population of 36,000 plants per acre and an average of 2 pinhead
squares added each week, then a weeklong infestation of a threshold of adult
plant bugs will result in up to of 17,052 squares lost, or ≈24%
of the pinhead squares in the field. If we know that damage potential is
possible, don’t wait for it to happen.
3.
In addition to feeding on pinhead squares, adult
plant bugs also lay eggs. These eggs will hatch out over the following 7-10
days, resulting in an embedded population of immature (nymphs) causing problems
later in the season.
Speaking of egg lay and plant bug nymphs hatching out, this
brings up the timing of the insect growth regulator, Diamond. This chemical is
only active on nymphs and kills them as they molt (shed skin) and grow from one
instar (growth stage) to the next. Immature plant bugs develop through 5
instars over ≈15 days. As mentioned above, the egg stage lasts 7-10
days. This makes the timing of Diamond critical to maximize its effectiveness.
We feel that we get ≈2 weeks (TPB nymphal life length) of residual suppression
from a 6oz rate of Diamond. Timing this application as close to the initial
hatch out as possible helps to get the most “bang for your buck” out of this.
Typically, we say that around 1st bloom is the
best time to apply Diamond. This assumes that a threshold population of adults
in June was present in the field and killed with a well-timed spray when found.
In a “normal” year, adults move into fields when 3 or 4 pinhead squares are
present (≈2
weeks into squaring). Given time for eggs to deposit and nymphs to hatch out,
we typically expect to start finding nymphs around 1st bloom. This
year in many fields, adult plant bugs were found in fields with 1 or 2 pinhead
squares present (≈1st week of square). Additionally, there are a
lot of fields that had a threshold (or higher) level of plant bugs for 7 or
more days before an application could be made due to rain. In these cases,
Diamond should be applied earlier than the 1st week of bloom. The point
of this application is not to time up with crop stage, but with plant bug nymph
emergence.
Bottom line: In many of the phone conversations we have had
with folks this week, we are telling them that if high plant bug populations are
in fields and no adulticide has been sprayed, then Diamond should be applied (with
a knockdown insecticide) 10-14 days after the initial threshold population was observed.
Making a Diamond application does not mean plant bug numbers will not reach
threshold in the following 2 weeks, but it will help to keep numbers suppressed,
especially if rains or other factors will not allow for timely follow-up
applications.
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.
