Stage of sorghum crop mandates need to scout for headworms
Friday, August 17, 2018
Oklahoma producers should still be scouting for headworms if their sorghum crop is between the milk and soft dough stage.
“Late-planted sorghum in vegetative stages should be fine, but plants just coming into susceptible reproductive stages need to be scouted weekly,” said Jessica Lindenmayer, graduate researcher with Oklahoma State University’s department of entomology and plant pathology. “Headworms eat developing seeds.”
Sorghum headworms are a complex of two species, the fall armyworm and the corn earworm. The fall armyworm has a prominent white or yellow inverted “Y” on its head, which allows it to be distinguished from the corn earworm. Corn earworms do not have the inverted “Y” and vary greatly in color, ranging from light green or pink to nearly black.
Producers should examine sorghum heads to determine the need for control of panicle-feeding bugs and headworms.
“The good news is that one technique works for both pests,” Lindenmayer said. “The shake bucket-baggie method of scouting works well for sampling fields.”
Use a 2.5-to-5-gallon bucket or white garbage can. Carefully move to a plant without disturbing it, quickly place the head into the bucket and shake it vigorously. Examine five sorghum heads at six locations. Count all headworms that fall into the bucket or garbage bag.
“Inspect at least one sorghum head per acre of field, a minimum 30 samples, and continue scouting sorghum until the grain has reached the hard dough stage,” Lindenmayer said.
Typically, the rule of thumb for headworm thresholds is based on a plant population of 25,000 plants per acre, a grain price of $4 per bushel and a treatment cost of $12 per acre, said Tom Royer, OSU Cooperative Extension Integrated Pest Management coordinator.
“The threshold makes a difference as to whether the headworms are large, greater than one-half inch or mostly medium sized at a quarter to one-half inch,” he said. “If they are large, the rule of thumb is 0.5 worms per panicle. If they are medium size, the rule of thumb changes to three worms per panicle. If they are a 50:50 mix of medium and large size, the threshold is 1.5 worms per panicle.”
If insecticide costs are different simply calculate the new price divided by 12 and multiply the answer by the threshold.
“We would advise selecting the insecticide that works best on a particular pest if we had only one; however, producers now have to consider the presence of sugarcane aphids in all of our sorghum pest management decisions,” Royer said.
If sugarcane aphids are already present and building, a producer must consider using either Transform or Sivanto. “That narrows the choice options for combining another product to control headworms because pyrethroids, while effective, will flare the aphids,” Lindenmayer said.
After reviewing data from multiple years of insecticide trials throughout the southeastern United States, data suggest that products containing chlorpyrifos provide spotty control of headworms. Lindenmayer said Prevathon and Blackhawk provide excellent control of headworms. Diamond while effective on headworms is a growth regulator and will take some time to work.
“We have not seen any data to suggest Diamond will flair sugarcane aphids,” Royer said. “If a producer planted a sugarcane aphid resistant variety and his or her scouting has not revealed any sugarcane aphid presence, a product such as Besiege, Cobalt, Concero or Stallion might be an option. They all contain a pyrethroid but if sugarcane aphid is not an issue, they should be effective.”
The Oklahoma Cooperative Extension Service is one of two state agencies administered by OSU’s Division of Agricultural Sciences and Natural Resources, and is a key part of the university’s state and federally mandated teaching, research and Extension land-grant mission. The OSU department of entomology and plant pathology also is part of DASNR.