It has been a number of days since we last reported on
Alabama insects. So what has been happening? First of all, August has been
dominated early-on with little insect activity other than the bug complex in
cotton. Second has been the weather, we had sun yesterday, August 19, for the
first time in about 10 days. During this period, rainfall has been measured in
inches varying from low single figures to double digits, depending on the
location. Up to 4 inches in a 24 hour period has been common. This has resulted
in bridges and roadways being washed out and public schools closing during the
beginning of the fall term. Agriculturally wise, sprayers have been unable to
get into fields and aerial applicators are over booked. Fortunately, insects
have overall been low with just a few exceptions. The bug complex has been over
threshold in many but not all cotton fields. In the past few days the
caterpillar complex has built to damaging levels in soybeans. Some fields have
been sprayed by plane for the lep complex, primarily loopers and velvetbean
caterpillars. The heaviest infested fields are those where growers just preventatively
added a pyrethroid when they were applying a fungicide.
It has been my experience in research plots that an
application of a pyrethroid just prior to an infestation of foliage feeding
caterpillars is like throwing gasoline on a fire. We usually see much higher
numbers of soybean loopers where a pyrethroid application has been made within
the previous 10-14 days. My advice to growers is that if they feel they must
add something to their fungicide, just add 2 or 3 ounces of a product like
Dimilin which does not disrupt the beneficial insects.
Once we have a damaging level of foliage feeders the first
thing we need to do is to quantify the species present and the numbers. Five to
seven caterpillars per foot of row will usually result in 30% or more foliage
loss and therefore require controls. Treatment thresholds for sweep nets on
drill beans are not quite as well defined. However, 2 to 5 per sweep would
likely require controls. As to species, velvetbean, green cloverworms, corn
earworms (podworms) and stink bugs can all be controlled with a pyrethroid.
However, if many soybean loopers are in the mix, then one of the newer lep
materials will be required. These are, in chemical alphabetical order: Belt, at
2-3 oz.; Steward at 7 oz.; Intrepid at 4-6 oz.; Prevathon at 18-20 oz.; tracer
or Blackhawk at 1.5-2 oz. per acre.
One additional thing should be added about soybean looper
control, especially if application is by air. Looper eggs are deposited on the
lower leaves of the plant, therefore they infest the plant deep within the
canopy. As the larvae mature they feed upward through the canopy. In order for
any of the newer chemistry to be effective, the leaf containing the droplet has
to be eaten. Therefore, it may take several days to fully suppress a looper
population. It has been my experience that the surviving larvae behind application will be early
instar larvae found lower in the canopy. Most of these looper insecticides are
very rainfast and have long residual on the leaves, so don’t panic if you see a
few small loopers behind an application.
I will be on research farms for the next several days and will
get back soon if we see or hear of damaging levels of insects developing.
