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Soybeans: Watch for late-season defoliators

Soybeans: Watch for late-season defoliators

As Louisiana soybeans progress into late summer, producers and consultants should be wary of late-season defoliators such as soybean loopers, velvet bean caterpillars and lingering populations of corn earworms.

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 0.5 inch or longer per row foot.

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 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.

Insecticides currently recommended for use against soybean loopers in Louisiana include Belt, Prevathon, Besiege, Intrepid Edge, Steward and Intrepid.

Once soybeans reach R6.5, yield is set and protection from soybean looper defoliation is no longer critical. 

Numbers of corn earworm moths are declining and instances of corn earmworms infesting soybean fields around Louisiana are becoming fewer. Of the previously mentioned caterpillar pests, corn earmworms are the only insects that feed on fruiting structures of the soybean plant.

The Louisiana threshold for corn earmworms is 3 worms per row foot or 38 in 100 sweeps, after bloom.

Corn earworms in Louisiana have the highest levels of pyrethroid resistance in the United States. Therefore, control of corn earmworms with the use of a pyrethroid alone is strongly discouraged.

Products recommended for control of corn earmworms in Louisiana are Belt, Prevathon, Besiege, Intrepid Edge and acephate plus pyrethroid.

Finally, when making insecticide application decisions for caterpillar pests in soybeans: insect species present, insect numbers present and defoliation percentage should be taken into consideration.

After bloom, soybeans can tolerate 20 percent defoliation and not experience a significant yield loss. However, with insects such as corn earmworms, pod and seed injury can occur without excessive defoliation.

Stink bug issues in much of the soybean acreage around Louisiana have been minor this season. Brown stink bugs have been the predominate species found in north Louisiana with increasing reports of red-banded stink bugs occurring as the season progresses.

Brown and red-banded stink bugs can be difficult to control with pyrethroids, and acephate + pyrethroid would a better control option in fields where populations of these insects occur.

The Louisiana threshold for brown stink bugs is 36 insects in 100 sweeps and the red-banded threshold is 16 insect in 100 sweeps.

TAGS: Management
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