• Late in the growing season, corn earworm larvae can be found feeding on the seeds and flower stems of sorghum.
• Plants are most vulnerable to injury during the bloom to milk stages, and corn earworm larvae can heavily damage seed heads.
GRAIN SORGHUM acreage in North Carolina and Virginia is up this year and new growers may face late season insect problems.
Grain sorghum acreage has increased significantly in North Carolina and southeast Virginia this year, thanks primarily to a new incentive program by Murphy-Brown.
Many growers may not be familiar with late season insect control.
In Virginia, Ames Herbert, says, “We have been sampling sorghum heads for worms this week in four different locations and all but one have worm levels that exceed the 1 worm per grain head threshold. Some samples reached 30-55 per 10 heads, way over threshold.
Late in the growing season, corn earworm larvae can be found feeding on the seeds and flower stems of sorghum. Plants are most vulnerable to injury during the bloom to milk stages, and corn earworm larvae can heavily damage seed heads.
An average of one larva per seed head can reduce yields by 5 percent and two larvae per seed head can cause a 9-10 percent yield loss.
While there are a number of insecticides that will control corn earworms, finding one that’s labeled for use on grain sorghum is little more of a challenge.
Herbert says there may be additional insecticides that are approved for use on sorghum, but the only ones he could find are Baythroid, Karate/Warrior, Lannate, Mustang Max or Blackhawk (the new Tracer).
Of those, Lannate will give the quickestkill but has essentially no residual activity.
Pyrethroid-based insecticides should also do well on corn earworms in grain sorghum. However, researchers don’t know whether there will be resistance problems in sorghum. If so, using pyrethroids would not work.
In pyrethroid vial tests, Virginia Tech researcher Sean Malone continues to find high, up to 45 percent survivorship of corn earworm. Malone works at the Tidewater Agriculture Research and Extension Center, along with Herbert, who is the State IPM leader.
Herbert points out that residual activity should not be a problem on grain sorghum at this time of year because worms are only a problem in the grain heads of sorghum and earworms will not have time to produce another population before grain sorghum heads mature.
He says the damaging action of corn earworm in grain sorghum is in the head, not the foliage.
If spraying sorghum, only the heads need to be treated, not the foliage, so use a sprayer configuration that gives good access to the head, get the water content right and sprayer pressure right to direct as much spray as possible to the head, he adds.