The cotton insect situation is as it has been all year, all over the board. Reports from South Alabama are that plant bugs are finally beginning to build in many fields but continue to be the heaviest near corn. Overall, bug pressure has been light in Central Alabama, but the bug complex (stink bugs and plant bugs) are beginning to be more common in some fields. Pressure is also picking up in some areas of North Alabama.
Bug Complex
In some fields, plant
bugs are easily found above threshold (3 per 5 row feet) and dirty blooms are
apparent. This makes for an easy treatment decision. In other fields, numbers
have been just below threshold for a couple weeks and the call is more
difficult. In many cases, we have recommended “cleaning up” these fields.
Conducting research on subthreshold populations is difficult, but I believe
multiple weeks of numbers just under threshold will result in similar losses to
one week of populations at threshold. Depending on planting date (or really age
of cotton) insecticide options vary. In fields in early bloom, Transform is
still a good option, however, fields at the 3rd week of bloom or
later need a material that also cleans up stink bugs. That means a pyrethroid
or organophosphate. A few things to consider for decisions are rain patters,
variety and efficacy. We observed ≈10%
damage from bollworms in our Non-Bt cotton at the Prattville research station
this week. While overall pressure was low in our Bollgard 2 (DP 1646) cotton,
consider a pyrethroid for bug control to get the additional bump on bollworms.
Overall, stink bug damage has been light across the state,
but our observations on research farms and in grower fields in central Alabama
is that stink bugs are just now beginning to infest older fields. Threshold during
peak bloom is 10% internal damage of 10-12 day old bolls (1 inch diameter, about
the size of a quarter). Sample 20-25 bolls across a field and look for signs of
feeding.
Aphids
The aphid fungus has caused populations to crash in many
areas of the state. Before treating for aphids, look for the grey, fuzzy
looking cadavers. The fungus is highly effective and will crash populations
across a field in a matter of a few days.
Rainfastness Considerations
While some fields are still pretty dry, many are receiving rainfall
pretty consistent. Rainfastness of insecticides has been a common question over
the past couple days. Several factors impact the effectiveness of an
insecticide application after a rain event.
In general, most insecticides are “safe” after about 6
hours. If a rain event happens 3-4 hours after an application, I would consider
a reapplication. If rainfall happens 4-8 hours after, I would scout the field
3-4 days later to determine efficacy and treat accordingly. Two insecticides that
are “exceptions” to the rules are Diamond and Acephate. Diamond has better
rainfastness (≈2 hours) than
most insecticides (Miss
State Blog). Many of the insect growth regulators (like Diamond) have good
rainfastness. On the other side, Acephate is among the least rainfast materials
we have. At least 8 hours of no rain is needed, and up to 12 hours is better.
Amount and Intensity of Rainfall: More rain will intuitively
have more impact than less rain. A hard, driving rain will probably be more detrimental
than a slower, steady rain. In other words, a half inch in 30 minutes will
probably be worse than a half inch in 3 hours.
Activity of Insecticide: Systemic insecticides (move
inside the plant) are less likely to be severely impacted than contact
insecticides. Most systemic insecticides require 2-4 hours to move into the
plant. While some contact insecticides aren’t very rainfast, insects may come into
contact quickly and have enough exposure immediately following the application
to die.
The question remains, should you go ahead and “beat the rain”
or hold off until it passes. That can be a difficult one to answer. Action will
depend on how bad the infestation is, how likely the rain is, and how quickly
you can get back into the field. If plant bug populations are well above
threshold (3-4x threshold), I would consider taking the risk of wash-off and
partial control vs allowing them another week to feed if I could not get into
the field.
Soybeans
Things have been relatively quiet on the soybean front,
outside of kudzu bug. This has been the worst year of my career with kudzu bugs.
Many fields reached threshold for adults and needed treatment. In general, we
do not recommend treating adult kudzu bugs, however populations were so high
they could not be left unchecked. Kudzu bug feeding causes similar impacts as
drought stress in soybeans. Many dryland fields across the state could not take
the additional stress of kudzu bugs during a droughty period. We have observed
reproduction in south and central Alabama and have even seen some naturally
occurring fungus taking out kudzu bugs in central Alabama. We typically wait to
trigger on nymphs and use a threshold of 1 nymph per sweep (=15 in 15 sweeps). Most
pyrethroids provide good control of kudzu bug.
Defoliating caterpillar and stink bug populations have been
low thus far in soybeans across the state. We will continue to monitor the
situation and provide updates as pressure builds.
If we can be of any
help, please don’t hesitate to reach out and let us know (Scott Graham:
662-809-3368; Ron Smith: 334-332-9501). To stay up-to-date on the Alabama
insect situation, subscribe to the Alabama Cotton Shorts Newsletter,
Alabama Crops Report Newsletter,
and the Syngenta Pest Patrol Hotline.
