B. Rogers Leonard, Fangneng Huang and Jack Baldwin
In Louisiana grain sorghum fields, the corn earworm, fall armyworm and sorghum webworm comprise a major insect pest complex that attacks immature grain. Grain yield losses from these pests can be reduced by planting the crop early within the optimum seeding window and by using all available cultural practices to ensure the majority of plants in a field flower within a short time period. After panicle (seed head) emergence, grain sorghum should be scouted once or twice weekly until grain maturity. Action thresholds to initiate insecticide applications for these pests should be based on insect numbers, grain damage and crop maturity.
These three pests comprise the sorghum “headworm” complex and in many instances occur simultaneously across fields. The sorghum webworm is the smallest member of this complex in its larval stages of development. The newly hatched, pale green caterpillar averages less than 0.08 inch in length. Later stages of caterpillars are green-to-tan in color and have a maximum length of about 0.5 inch. They are covered with spines and hairs and have red-to-brown longitudinal stripes on their dorsal (back) surface. The caterpillars move little when disturbed. The adult stage or moth is whitish in color and has a wingspan of 0.5-0.6 inch. Sorghum webworm eggs are oval and slightly flattened. They initially are greenish-white, becoming tannish-brown as they mature. The pupal stage is bi-colored and generally yellow-to-brown on the lower side, but reddish-brown on the top.
Recently, the sorghum webworm has become more difficult to control with the with the pyrethroid insecticides – Karate-Z, Baythroid, Asana XL, Mustang Max and Prolex. These insecticides currently provide satisfactory control of corn earworm and fall armyworm. Historically, the sorghum webworm was more of a problem in South Louisiana, but infestations now occur statewide. Control with an insecticide is justified when five or more larvae are found per seed head. These caterpillars feed on ripening grain and consume the starchy contents of individual seed but leave much of the outside hull intact. The larvae do not spin webs over grain in the seed head, but spin a single thread to suspend themselves and travel throughout the seed head. Occasionally in fields infested with heavy populations, fine webbing can be seen in damaged seed heads. Crop injury in nonmanaged fields may be as high as 50 percent to 80 percent. Yield losses will be minimal, however, after the grain matures to the hard dough stage.
Insecticide screening studies during 2004 documented unsatisfactory control with pyrethroid insecticides and confirmed the effectiveness of Lannate and Tracer as alternative insecticide treatments to manage this pest (Figure 1). These products, in addition to Sevin and Lorsban, are the only products recommended by the LSU AgCenter for control of sorghum webworm in Louisiana grain sorghum fields. These insecticides are also effective against corn earworm and fall armyworm, which are likely to infest grain sorghum plants at the same time as sorghum webworm.(This article was published in the fall 2006 issue of Louisiana Agriculture.)