Sebe Brown, Dr. David Kerns, Dr. Jeff Davis, LSU AgCenter Entomologists, Dr. Ronnie Levy LSU AgCenter Soybean Specialist
With agronomic production rapidly approaching a close, soybean loopers are still a concern across the soybean producing regions of Louisiana. Soybean loopers are late-season defoliators that typically migrate to the Mid-South in August. Soybean loopers have the ability to build large populations quickly and are exaggerated by the use of broad-spectrum insecticides for three-cornered alfalfa hoppers and stink bugs. The threshold for soybean loopers in Louisiana is 150 worms in 100 sweeps or 8 worms ½ inch or longer per row foot. Soybean looper control is primarily limited to lepidopteran specific insecticides. Insecticides recommended for use against soybean loopers in Louisiana include: Lannate, Tracer, Steward, Intrepid and Belt.
Because soybean loopers are foliage feeders, adequate insecticide coverage is essential to limiting defoliation and reducing population numbers. Soybean loopers often initiate feeding in the lower portion of the canopy defoliating soybean plants from the inside out. This cryptic behavior allows soybean loopers to stay protected from some predators and insecticide applications in the dense canopy of soybean plants. Thus, good insecticide coverage is essential for optimal control of soybean loopers.
In recent years, Belt has consistently provided satisfactory control of soybean loopers. However, this year Dr. David Kerns and I have received numerous phone calls throughout the Northeast region alerting us to instances of apparent reduced efficacy of Belt applied at 2 oz/acre for soybean loopers 14 days after application. However, data generated by Dr. Kerns at the Macon Ridge Research Station confirms the activity of Belt, applied at 2 oz/acre delivered at 10 gpa by ground, against soybean loopers 15 days after application. Results of these trials can be found in the graph below.
Similarly, Dr. Davis and the Soybean Entomology Laboratory has seen 100% soybean looper field control 14 days after treatment when applied at 15 gpa. His residual efficacy after 21 days is holding at 75% control, similar to previous years. In addition, his lab has been testing 2012 soybean looper field collections this year with diet incorporated Belt. Concentrations for LC50 and LC90 are not different from lab colonies or previously tested field colonies from 2009, indicating there is no resistance to this product.
So why the apparent reduction in field efficacy of Belt? One unifying theme related to all of these calls was the application of Belt by air. Aerial application typically utilizes 2 to 5 gpa and application speeds in excess of 100 miles per hour. Small variations in wind speed, direction and ambient air temperature can have profound effects on where an application is intended to land and where it actually settles. Thus, aerial application with products intended to cover leaf area may experience reduced initial efficacy or shorter residuals due to inadequate coverage. Although Belt is translaminar and will absorb into the leaf tissue, it will not translocate throughout the plant; therefore, good coverage is essential in thick canopied crops. However, as the loopers move from the inner canopy to the outer leaves, mortality should increase.
Regardless of the insecticide used, there are several approaches one might take to counter coverage issues: 1) apply by ground when possible, 2) utilize a minimum of 5 gpa when applying by air, 3) initiate the insecticide application at a slightly lower action threshold, and 4) increase the insecticide rate.