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29
Aug
2013
Bollworms invading some soybean fields, passing others by
Author: Gus Lorenz, Extension Entomologist

Bollworm numbers are very high in soybeans in some locations right now, but overall, populations are extremely spotty.  It’s hard to figure out why they show up in some fields and not others.  We observed a field today that was running 65 bollworm larvae per 25 sweeps, along with an assortment of saltmarsh caterpillars, loopers, and green cloverworms just outside Lonoke (Fig. 1).  The threshold for bollworm is 9 per 25 sweeps.  In some areas, fields may be running right around threshold.  It’s really a strange year for this pest.

Fig. 1 - A mixed catch of insects from a soybean field near Lonoke.

Fig. 1 – A mixed catch of insects from a soybean field near Lonoke.

Our trap counts are low right now and have been for a couple of weeks so we are surprised to see numbers this high around Lonoke.  It just goes to show that the bollworm traps don’t always tell you what’s going on.  Usually fields at the most risk are fields that are blooming and setting pods and have not achieved canopy closure.  That’s not always been the case this year.  In fact, in many cases, fields that are lapped are experiencing bollworm problems and fields nearby that aren’t lapped are well below threshold.

Bollworm/ corn ear worm

Bollworm/ corn ear worm

With the high numbers of larvae in these spotty scattered fields and high heat, we may see some slippage in control.  Just remember, the treatment is only as good as the application.  Based on the current situation, we make the following suggestions to obtain adequate bollworm control in soybeans:

  1. Based on our studies this year and observations the past two years, we recommend you don’t try to control these bollworms with pyrethroids.  In our trial last week we got about 50% control with pyrethroids.  Consider the field mentioned above.  At 65 worms per 25 sweeps, 50% control would leave about 32 worms per acre, about 3.5X threshold.  I know a lot of folks want to use pyrethroids−they are much cheaper, but they simply aren’t working for us in most cases.
  2. Consider using Belt, Besiege, Prevathon, or Steward.  Early observations are that a new product Intrepid Edge is also working very well.  We have observed excellent control with Belt at 2 oz/ acre, Besiege at 8 oz/ acre, Prevathon at 14 oz/ acre, and Steward at 1 gal/ 20 acres.  The rate on Intrepid Edge is 5 oz/ acre.  The advantages of these products are that they provide very good control of caterpillars in soybeans and give outstanding residual control.  With the exception of Besiege, they have little impact on beneficial insects.  However, for the same reason they don’t impact beneficial insects they also provide no control of stink bugs and other non-caterpillar pests of soybean.  Besiege is a premix of Chlorantraniliprole, the same active ingredient as Prevathon and Karate, so will provide some control of stink bugs but also hit beneficial insects.
  3. To insure adequate application, we recommend a minimum of 5 gallons per acre (GPA) by air and 10 GPA by ground.  Also, consider the addition of an adjuvant, such as crop oil, with your insecticide application.
  4. Scout fields closely following application to make sure adequate control was achieved and for subsequent problems with bollworms and other pests.

Bean Leaf Beetles This insect continues to be a problem for many growers.  We have received several reports of BLB at extremely high levels.  Remember that we generally consider BLB to be a defoliator pest, but they do feed on pods and can cause damage.  Pod injury caused by BLB enables infection of the developing bean seeds by diseases and can result in incomplete development of the seed/ pod, cause harvest loss due to the beans sticking to the pod and result in poor seed quality and dockage.  To determine if you are looking at old or new damage, remember that fresh BLB pod injury initially appears green.  After a couple days the scars appear white and old damage will look dry and brown.  New damage and old damage should be an indication to you how much damage has occurred and is currently occurring.  Also, remember that most of the damage is usually in the upper portion of the plant so damage may look worse than it is if you do not inspect the whole plant.  While we don’t have a threshold for pod injury, logic, BLB numbers, crop value, etc. will have to play a part in whether or not treatment is necessary. We put a test out last week and pyrethroids weren’t very effective.  Bifenthrin was best.  While many treatments reduced numbers fairly well at 3 days after treatment, by 7 days most treatments were back to the same levels before treatment was made. Consider the stage of the soybeans;  if the field is late-R5 or R6 you may be able to avoid treating if the defoliation and/or pod damage is not that bad.  As mentioned earlier, in most of the fields we are seeing, the damage is limited to the top of the canopy.

Bean leaf beetle

Bean leaf beetle

Bean leaf beetle damage on soybean

Bean leaf beetle damage on soybean


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