High Plains & West Texas
All Texas sorghum growers at some point likely have concerns about worms feeding down in the whorl in grain sorghum from about the 8-leaf stage until flag leaf emergence. You may not immediately notice the worms feeding themselves, but as grain sorghum grows and larger leaves unfurl, you see the “shotgun” symptoms of the feeding. There can be holes in the leaves all over, and sometimes these holes take on a symmetrical appearance.
Leaf feeding in the whorl in itself is not of great concern. In spite of the trashy appearance of the sorghum crop, in most cases only a minimal amount of the actual leaf area has been lost. This is similar to a peppering of hail that rags up some of the mid-size and larger leaves but results in potential grain yield losses of only a few percent at most. Economic thresholds for spray treatment are usually 30% or more leaf area loss.
The greater concern is that whorl worms—this is often spoken of as a “Complex,” which contains fall army worm and sorghum headworm (same as corn ear worm or cotton bollworm)—could feed low enough in the whorl to damage the developing head though overall this is not normally a significant issue. Historically this is tough to deal with as typical insecticides used to control worms do not penetrate the whorl. Texas A&M AgriLife entomologists continue research on newer insecticides including Prevathon (Dupont), Besiege (Syngenta), and Belt (Bayer) which are larvicides and have some ability to move within the plant. But at this time insufficient data exists to make recommendations regarding the use of these products for grain sorghum for worm feeding in the whorl where head damage is occurring.
For background reading on the grain sorghum whorl worm complex, consult “Managing Insect and Mite Pests of Texas Sorghum,” B-1220 (2007) which you may read/print/download from http://lubbock.tamu.edu/files/2011/11/sorghum_guide_2007.pdf Sorghum growers deal with these worms more commonly in the head as this leads to direct grain loss. For further information on this type of damage again consult the above Texas sorghum insect guide.
The Potential Onslaught of Whorl & Head Worms—High Plains
Summer 2014 fall army worm moth flight in the Texas High Plains has been extremely high with pheromone trap counts as high as 1,000 per week. Currently (early August) moth flight numbers have declined from their peak as moths are between the second and third generation. Dr. Pat Porter, AgriLife Extension entomologist, Lubbock, cautions growers to especially be on the lookout beginning by the end of the week of August 10th for the anticipated third generation moths, which could even higher. This could translate to extreme pressure on grain sorghum either a) feeding on any remaining sorghum still in the whorl stage, or b) head feeding from emergence to flowering to grain development and maturation. Dr. Porter believes that scouting is crucial as we finish the 2014 growing season in the High Plains. For further information from Dr. Porter’s comments and Lubbock fall army worm trap counts consult the July 2014 editions of “Focus on South Plains Agriculture” newsletter at http://lubbock.tamu.edu/focus-newsletter.
