Bollworm Update Cotton/Soybean

Angus Catchot, Extension Entomologist
By Angus Catchot, Extension Entomologist, Jeff Gore, Research and Extension Entomologist and Don Cook, Research Entomologist July 21, 2012 10:19

This year started out with most folks expecting higher than usual bollworm numbers due to the mild winter. Couple that with increased corn acres and the perfect storm was thought to be brewing. This is why entomologists get “shifty” when asked to predict problems by the media based on winter conditions; it didn’t materialize for various reasons. However, in the last 10 days we have finally started to see a bollworm flight.

Soybeans: Most of the early planted soybeans will not have to deal with bollworms this year.  They will “outrun” them and many will have no insect spray at all. We do not necessarily plant soybeans in MS early just to avoid late season insect pressure but it is a huge benefit. The later planted beans and wheat beans need to be watched closely from this point forward. We are beginning to make bollworm applications in several fields across the state now and moths are still be flushed in some as well.

Bollworm Management Considerations Soybeans: The MS threshold in soybeans is 9 worms ½ inch long per 25 sweeps or 3 worms ½ long per foot of row on a drop cloth. Because sprays have been limited in soybeans to date, you may be very surprised at how well beneficial insects keep numbers down in some fields.  Another thing that needs to be considered when sampling is size of the beans. Last year I had numerous calls about beans that were 50-80% damaged when you parted the canopy but they never caught more than 2 or 3 in sweep net so never sprayed waiting on threshold.  The first question I always ask is “How big are the beans?” Usually the response is chest high or lodging in this situation. Bollworms feed on fruit down in the canopy making it difficult to get an accurate sample on tall rank soybeans.  If you are not getting a good sample with a sweep net, supplement it with a visual sample.  We do not necessarily have a visual sample threshold but you can certainly make some assumptions based on the drop cloth threshold of 3 per ft. of row. Bottom line is if what you are doing is not working, let’s do something different. We will be glad to help just give us a call. This would only apply to limited acres.

What to use:  I think everyone remembers the trouble we had with pyrethroids not controlling bollworms in soybeans last year. I would not recommend a pyrethroid. Products of choice would be Belt or Steward.

Cotton: Many have reported decent egg lays over the last week or so and now I am hearing more reports of 5-20% live worms in bloom tags. There are some targeted applications going out in BGII and WideStrike now.

Bollworm Management Considerations Cotton: The larval threshold in MS is 4% infested plants until cutout, and then it goes to 8% infested plants.  We use the same threshold for Bt cotton and non-Bt cotton. We do not have an egg threshold in MS, however, 25-30% eggs on bloom tags would likely result in a problem in BGII cotton and 10-20% eggs on bloom tags in WideStrike cotton. For now I still consider the pyrethroids my first choice on bollworms in cotton. We get numerous questions about this recommendation.  Many want to know why we recommend pyrethroids for bollworms in cotton and not in soybeans.  They are the same worms, right? Yes, they are the same worms but remember we are often targeting eggs and hatch outs in cotton and ½ inch larvae in soybeans.  Also in cotton, they are somewhat easier to control because they are compromised by the Bt.

Bollworm Management Considerations Cotton: I am going to make an attempt to shed some light on the hardest question I ever get asked when it comes to worm control in Bt cotton. I do not have plant bugs but I do have 30% eggs on bloom tags. Should I make a special trip just for worms? This is an easy call when you have plant bugs in the field.  No question, adding a pyrethroid to your OP increases plant bug control and it is a “free shot” for the worms.  It is much more difficult when you do not have plant bugs. A couple of years ago, a consultant called me and had numerous fields, all planted same variety same planting date. All had uniform egg lay (about 30% on bloom tags) but only half of them had plant bugs.  Where the plant bugs were he was adding a pyrethroid with his OP but needed to know if he should make a special trip across the rest of the fields with a pyrethroid to take care of the worm eggs where he did not intend to spray plant bugs.  I told him just wait, let’s give the Bt a chance to work and we will spray next week if the worms don’t die.  Next week the worms did not die but made it under the bloom tags and 2 well timed sprays 4 days apart still left us with 8% boll damage compared to less than 1% boll damage where we targeted eggs in BGII cotton. Once a worm is allowed to get under a bloom tag they are difficult to control. Research that Jeff Gore conducted years ago showed the very best control he ever got treating worms in bloom tags was 47%. So here is where we are at.  Blooms are the weak point; if the eggs are laid on the bloom (low expressing plant part) chances of survival are very high even in dual gene cotton (greater than 90% from artificial infestations). Anywhere else on the plant, chances are those worms will be controlled just fine by the Bt. Also, it is difficult to find eggs on bloom tags.  So if you are finding 25-30% it will likely be much higher.

Summary: Whether in Soybeans are Cotton, bollworms tend to be down in the canopy so coverage will be critical.  I would recommend that you use 10 GPA by ground and 5 GPA by air to improve coverage to the lower canopy.

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Angus Catchot, Extension Entomologist
By Angus Catchot, Extension Entomologist, Jeff Gore, Research and Extension Entomologist and Don Cook, Research Entomologist July 21, 2012 10:19
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