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Wisconsin Agriculturist

Looper alert for Louisiana soybeans

Many fields in north and central Louisiana are under heavy pressure with soybean looper.

I have received several calls from farmers, consultants, and dealers during the last week on the performance of Intrepid. At this time, I have not see any field control problems that I would call an outright failure when I am comfortable that the correct rate and timing of Intrepid was used. Based upon conservations with several folks working in soybean IPM, it appears that Intrepid is working okay in Louisiana and surrounding states. There may be some other application problems that I am not aware of which also may have affected efficacy.

Dr. Jeff Davis’s laboratory (LSU AgCenter Department of Entomology) has confirmed considerable variability in Louisiana soybean looper responses to Intrepid, but fortunately the 2010 field control issues have not been correlated to insecticide-resistant populations. This may change as the season continues.

I have not heard of any significant problems with Belt, Steward and Larvin (except for one each with a wash-off possibility) or Tracer.

All of the products other than Intrepid kill soybean loopers relatively quickly and you can see significant efficacy within two days. More insects will continue to die for about a week after application. Intrepid requires much more time before it can be evaluated in the field.

Most of the confusion is associated with that fact that this compound is an insect growth regulator and requires at least four to five days to really show a reduction in field numbers. Treating small larvae (<1/2 inch) with Intrepid does not increase the speed of kill as it does for other products. Larvae have to ingest (eat) the product and begin the moulting process for toxicity to occur. This appears to occur most readily at about five to six days after application.

If you watch a field closely, you can usually tell that Intrepid was used when a sweep net sample shows a high proportion of the larvae are in the same growth stage. This is the point where many larvae have attempted to moult and cannot. They starve or desiccate and usually within 24 to 48 hours the population is nearly destroyed.

Performance in field trials for the past several years has provided confidence in this product because of its rain fastness and residual. The other insecticides work very well, and certainly are options, but have not always demonstrated the same rain fastness and residual efficacy. Belt is a new product recently registered by the EPA in soybean and LSU AgCenter scientists are trying to characterize its activity.

How long do we need leaves? Dr. Jim Griffin’s (of the LSU AgCenter) data show that harvest aid application between R6.5 and R7 is sufficient to prevent any significant yield loss. A student of Dr. Angus Catchot (Mississippi State University entomologist) has also shown that some leaves are required until soybean reach the mid-R6 stages. In the MSU trials, only 100 percent leaf removal loss on soybean plants beyond the R6 stages caused a significant yield loss. That loss was in the range of two to three bushels. They are repeating those studies this year.

These results suggest that as long as soybean plants retain some active leave tissue and are protected until the mid-R6 stages, or until the plants are safe for harvest aids, final yield should not be compromised. Dr. Catchot has provided a graph of the 2009 results illustrating yield loss at several levels (17 to 100 percent) of manual defoliation in the upper, lower, and total canopy of R6 stage soybean plants.

The second issue to be aware of is that tobacco budworms (TBW) have been found in at least one group of soybean fields. Some of the suspected control problems using a pyrethroid (alone or in a mix) that I have been informed of may be related to this pest. It is uncommon, but tobacco budworm moth catches have been much higher than normal this year. If you suspect TBW in a soybean field then insecticides of choice would be Belt, Steward, or Tracer.

TAGS: Soybean
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