As we observed and stated last week, soybeans in Alabama are experiencing the most widespread outbreak of foliage feeding insects of the past 41 seasons. This outbreak has been going on for about three weeks or more. However, some growers are just now discovering the situation. This has led to the most calls and questions that Dr. Tim Reed and I have ever received during one week. I will attempt to summarize the questions and what our responses have been.
Most calls begin with – “I have worms in my soybeans, when
do I need to treat and what insecticide do I need to use?”
First of all, in most all fields we are dealing with a mix
of soybean loopers (SL), velvetbean caterpillars (VBC), and green cloverworms
(GCW). Loopers are statewide, VBC are a significant part of the mix in south
and central Alabama while GCW are a large part of the mix in northern areas of
the state. Loopers are resistant to pyrethroids, Lannate and all of the older
chemistry. Since they are part of the mix statewide, we must go to the newer
chemistry (in alphabetical order): Belt 3 oz.; Blackhawk (Tracer) 2 oz.;
Intrepid 4-8 oz.; or Steward 7 oz. Prices for Belt, Blackhawk and Steward will
be somewhere between $15 and $20 per acre. Intrepid will range from about $7 to
$14, depending on the rate used. The 4 oz. rate has been what most growers have
chosen due to economics. However, where significant foliage loss has already occurred
and worm numbers are high, this 4 oz. rate is not adequate. At least 5 or 6 oz.
of Intrepid will be needed.
In addition to worms, most all soybean fields have some
level of stink bugs present. None of the worm materials will control stink
bugs. Therefore, a pyrethroid or acephate (Orthene or generic) at 0.75 lb./ai
must be added. This rate of acephate makes pyrethroids the most economic
choice. The majority of our stink bugs are the green or southern green species,
which pyrethroids control very well.
Now back to what a threshold is on foliage loss. Depending
on the stage of beans, our threshold is 20-30% foliage loss. That does not mean
that we wait for that level before treating. We treat in advance when we find
worm numbers that will give that level of loss. Also, we must allow 4-5 days
for products like Intrepid to control the worms. However, foliage consumption will
normally end much sooner. Big factors, which are seldom mentioned in discussing
foliage loss thresholds in soybeans, are the amount of moisture available to
the crop and the reproductive stage of the beans. Soybeans can compensate at a
much higher level when soil moisture is available and we have that statewide at
present. Also, soybeans will replace lost foliage when they are still in the
How many worms does it take to cause 20-30% foliage loss? In
traditional row beans, where a drop cloth can be used, somewhere between 4 and
8 larvae per row foot will give 20-30% foliage loss. The size of the beans is
also a variable here. Due to the good moisture in August, many of our beans are
waist high and have lots of foliage. Many of our beans in 2012 are drilled.
Therefore, a sweep net is needed to get a measure of worm numbers. I suggest
taking 10 sweep samples. Some states recommend treatment when about one larva
per sweep is found. I have found in 2012 that it takes about 3 or 4 worms per
sweep to cause 20-30% foliage loss. Another factor to consider is how long the
worm infestation has been ongoing. In fields I have monitored, the answer is
three or more weeks. New worms just keep hatching, so where a subthreshold has
been present for 15-20 days, much of the lower canopy has already been consumed
by the worms. In this situation a 10% foliage loss can turn into a 30-40% loss
in 7 days.
In summary, I feel that our Extension educational program on
soybean insects has been very inadequate and we promise to do better beginning
with winter meetings. However, growers must also do their part in managing
insects in soybeans, especially when they are worth $17-18 per bushel. We do
not know what level of insect pressure 2013 will present, but if we plan to
stay on top of the insect situation, soybeans will need to be monitored by a
trained or experienced person at least weekly during the season, just like we
have done in cotton for the last 60 years.