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IPM specialist warns: Beware of evil that lurks in grain sorghum

Its picture may not be on any U.S. Post Office wall, but grain sorghum producers need to know how to identify and stop public enemy number one, the dreaded chinch bug.

“This pest causes economic damage three out of five years,” says Jim Swart, Texas Extension integrated pest management specialist at Commerce.

“Chinch bugs come out of grasses into grain sorghum and then into the center of fields,” Swart told a group of Denton County farmers at a milo seminar recently.

Swart gave farmers, some of whom are novice sorghum producers, a primer on insect pest management in milo.

“Just two or three chinch bugs per plant, identified on 20 percent of a sample can cause economic damage if not treated,” he says.

Swart says Gaucho, used at planting or as a seed treatment has provided the best control in test plots. “It is the only treatment that provides an economic return on investment,” he says, “even when no significant pest population exists. We're probably seeing a response to managing low level pest infestations.”

Swart said greenbugs may not be the John Dillingers of milo fields, but at high enough numbers they can steal profits.

“Greenbugs suck juices from the plant and also inject toxins that cause further damage. Gaucho, again, is the recommended treatment.

Swart says a Gaucho seed treatment will keep greenbugs at bay for up to 100 days after planting. “We also see hastened maturity with a Gaucho treatment. We're not sure why because we see that even without insect pressure.”

Swart says greenbugs feed on the underside of leaves. “Like other aphids, they secrete honeydew. And they are prolific. A female can increase a population 20-fold in one week.”

Infestation symptoms include leaf reddening. “But if growers see that reddening and no insects, beneficials probably have taken them out.”

Spray decisions, he says, should depend on the numbers of greenbugs per plant and the number of mummies (shells left after beneficials parasitized the pests).

“Moisture conditions also play a role. “Producers are less likely to treat during a drought. Also, smaller plants will withstand less damage than larger ones. And consider overall field conditions before committing to a treatment.”

Swart says rain and predators will suppress greenbug populations. “Infestations are worse during dry periods.”

He recommends growers examine 40 plants, selected randomly from across the field. “On smaller plants, less then 6 inches tall, examine the whole plant and if 20 percent show viable populations, consider treatment.”

Swart said another pest, the corn leaf aphid, “is not nearly as damaging and may attract beneficial insects. It rarely causes economic damage.”

A yellow sugarcane aphid is less common but can be devastating. It also injects a toxin into the plant. “It only takes a few to cause severe damage, including delayed maturity, lodging, and plant death.”

Swart recommends a seed treatment or an at-planting insecticide.

The sorghum midge also may cause serious damage to milo. “The midge is a small, mosquito-sized, orange-colored insect that blends in with sorghum flower parts. Early infestations typically are below damage levels,” he says.

“The first two or three generations often develop in johnsongrass. As they move into grain sorghum, numbers increase during flowering. Damage occurs when larvae feed on newly fertilized ovaries, preventing normal kernel development.”

“Scout at mid-morning,” Swart says, “to give the midge a chance to emerge in the heads. By early afternoon, the adults begin to die so try to take your sample between 10 a.m. and 2 p.m., if possible. Use close-range inspection of the head and start at the edges first.”

Swart says early planting may head off midge infestations. “Planting by the first of April will allow sorghum to flower and set seed before high midge populations develop. Also, plant hybrids with uniform maturity dates to avoid late flowering.”

He says crop uniformity sometimes depends on weather conditions.

He says eliminating johnsongrass also helps manage midge. “They overwinter as larvae in johnsongrass spikelets.”

Swart said growers may need to make multiple insecticide applications to kill adult midge populations before they lay eggs. The crop is most vulnerable during a seven to ten-day flowering stage.

Economic threshold is two adult midge per head. “As the value of grain sorghum goes up, the threshold may drop to one midge per head,” Swart says.

A few sorghum hybrids may show some resistance to midge infestations.

Swart also discussed caterpillar pests — corn earworm and fall armyworm. As whorl worms, they feed in the whorled sorghum leaves.

“Pull whorls apart,” he advises. “If worms are there, excrement will be present. Also look for shot holes on the leaves.”

The plant can tolerate some leaf damage, Swart says.

Worms may be more troublesome as head feeders. “When larvae hatch from eggs laid in sorghum heads they pose more threat than leaf feeders. Use a beat bucket to collect samples.

“Treatment is justified if feeding reduces leaf area by 30 percent. Or if feeding damage is observed on developing grain.”

Swart says insecticide options for sorghum insects should begin with seed treatments (Gaucho, Cruiser).

“Those will provide good suppression of chinch bugs, and all the early season aphids (greenbug, yellow sugarcane aphid, corn leaf aphid). The granular insecticides provide limited protection from chinch bugs and aphids but have not provided the economic returns of seed treatments in our research trials.”

For sorghum midge, Swart says many labeled pyrethroid products “will do an excellent job. Some of the organophosphates are also pretty good, but do not provide as much residual control as the pyrethroids (two to three days versus four to five days).

“Most growers are using pyrethroids to control headworms, too,” he says. “These are cheap and effective. Some organophosphates are fair, but generally more costly.”

Swart said other insects inhabit grain sorghum fields and not all are villains. Aphids may succumb to lady beetles or lacewings. “Be aware of potential for biological controls. Parasites and predators work for free.”

He says farmers should learn to recognize the good guys. “Some beneficial insects resemble pests,” he says. “A big-eyed bug and a minute pinch bug resemble chinch bugs. It's important to learn to identify beneficials and let them work for you.”

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