Planting dates for grain sorghum in Texas range from January in the Lower Rio Grande Valley to July in the High Plains. And as soon as the seed are in the ground, warfare begins against insect pests.
Hostilities may be limited to short skirmishes or a season-long battle.
“Grain sorghum has at least 21 different pests that feed on it at development stages ranging from planted seed to mature plants,” says Greg Cronholm, Texas Extension agent-pest management. “For example, wireworms eat seed; greenbugs are active from the seedling stage to plant maturity, and midge appear during the flowering stage.
“Most grain sorghum insect pests are ‘occasional’ in that they cause economic damage only in localized areas or only during certain years,” Cronholm said. “Usually only one or two key insect pests appear in any grain sorghum growing area in Texas. These insects, including greenbugs and sorghum midge, occur most years and dominate control measures.
“The good news is several beneficial insects associated with grain sorghum often reduce the severity of damage by the pests. These include spiders, ladybird beetles, lacewing larvae, syrphid fly larvae, minute pirate bugs, big-eyed bugs, and parasitic wasps.”
Because broad-spectrum insecticides will destroy natural enemies as well as the insect pests, producers should not apply pesticides unless necessary. “Killing the natural enemies can result in a resurgence of the treated pest or the increase of a secondary pest such as sorghum headworm (also known as corn earworm) or spider mites.”
Several cultural practices will minimize potential for insect damage.
“Rotating grain sorghum with broad-leafed or tap-rooted crops such as cotton or soybeans will greatly reduce potential problems from insect pests,” Cronholm said.
Destroying grain sorghum plants immediately after harvest and destroying volunteer and alternate host plants significantly reduces the likelihood of damaging levels of insect pests the following year. Such practices are particularly important for reducing populations of southern corn rootworm, cutworms, sorghum webworm, sorghum midge, and stalk-boring insects.
“Johnson grass is a weed-host of many grain sorghum pests, including greenbug, yellow sugarcane aphid, and sorghum midge. Therefore, it should be destroyed as completely as economically feasible,” Cronholm said.
Choice of a hybrid for planting is also important in managing insect pests. Planting hybrids early that have uniformity, disease resistance, standability, earliness, and open heads will avoid infestations by sorghum midge, sorghum headworm, fall armyworm, sorghum webworm, stalk borers, and head-infesting bugs. These hybrids also will be less likely to encounter late-season weather problems.
“Good seedbed preparation coupled with optimum temperatures will ensure rapid seed germination and seedling growth and minimize seed and seedling damage by wireworms, red imported fire ant, and yellow sugarcane aphid,” Cronholm said.
“Treating seed with fungicides and insecticides protects them against some diseases and seed-feeding pests. Unfortunately some systemic insecticides will suppress corn leaf aphids that attract beneficial insects needed for natural control of greenbug and other insect pests.”
The major key to effective, economical pest control is sampling. And because pest numbers can change rapidly, plants should be inspected at least once a week. Daily inspection is recommended when midge are present.
Sampling for soil-dwelling insects, such as white grubs and cutworms, should be done prior to planting, because control measures for these insects must be applied preplant.
“During the growing season randomly-selected plants from all sections of the field (not just the border areas) should be carefully inspected for both harmful and beneficial insects, and for plant damage. Estimates of the number of beneficials and insect pests per plant, and the degree of plant damage, should be carefully recorded as they are essential when making decisions about the possible need for insecticide applications,” Cronholm said.
In addition to visual inspection of plants for pests and beneficials, the “beat-bucket” method should be used to estimate the number of sorghum headworm, fall armyworm, sorghum webworm, and bugs in sorghum heads. To use this technique, sorghum heads are vigorously shaken into a 2.5- to 5-gallon plastic bucket; the number and kinds of insects are recorded, and the information subsequently used when making insect management decisions.
“Once the levels of the various insects and the physical damage to the plants are determined, the producer can use economic threshold tables for guidelines on insecticide applications,” Cronholm said.
These tables and other pertinent information are available from bulletin B-1220, “Managing insect and mite pests of Texas sorghum” on the Web at http://lubbock.tamu.edu/.