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Cotton faces new threat in tarnished plant bug

MISSISSIPPI STATE, Miss. — The Boll Weevil Eradication Program and worm-controlling varieties have allowed tarnished plant bugs to skip to the top spot as cotton’s No. 1 pest.

Once growers removed boll weevils from their lists of pests, they began planting transgenic Bt cotton to control tobacco budworms and cotton bollworms. Producers reduced sprays for those insects, and this allowed tarnished plant bug numbers to grow.

“The tarnished plant bug has filled the vacuum left by the boll weevil since eradication began,” said Jim Robbins, a research entomologist with the Mississippi Agricultural and Forestry Experiment Station in Stoneville, Miss. “Boll weevil eradication programs cost millions of dollars and were very successful, but now other insects have become key pests.”

Robbins said key pests are those that require several applications of chemicals for control. Tarnished plant bugs fall into that category, although stink bugs are serious as well. The tarnished plant bug has five stages before it becomes an adult, and some chemicals that control it at one stage do not work on other stages of development.

Tarnished plant bugs can live in many plants other than cotton, and this makes them more difficult to control.

“Wild host plants are a key player in the life cycle of the tarnished plant bug,” Robbins said. “They can live on about 170 species of wild hosts across the Cotton Belt, and they can reproduce on more than 100 of these.”

Robbins said scouting is a key to controlling the tarnished plant bug.

“Growers really need to pay attention to tarnished plant bugs coming off the wild hosts and invading cotton at pinhead square time,” Robbins said.

The number of insect pests in a field at a certain point in the plant’s maturity determine the type of treatment needed for control. Good scouting lets a producer know when those thresholds are met.

“The thresholds we use in Mississippi prior to bloom are eight adults per 100 sweeps in the cotton,” Robbins said. “After bloom, the threshold moves to 20 per 100 sweeps.”

When it’s time to spray, Robbins said using the right chemical at the right time is important. To delay the onset of resistance, producers should rotate chemicals and modes of action, which means using pesticides that affect the insect in different ways.

Angus Catchot, cotton entomologist with the Mississippi State University Extension Service, said scientists are seeing some signs of pesticide resistance in the tarnished plant bug.

“Pyrethroid resistance has been documented for some time, but now we are seeing the first signs of resistance to organophosphates,” Catchot said.

In recent years, producers have relied heavily on older chemistry to provide plant bug control, particularly after bloom.

“There have been very few chemistries introduced in recent years that provide sufficient control against the tarnished plant bug. This only increases the importance of protecting the chemistry we have by treating only when needed and rotating modes of action to delay the onset of resistance,” Catchot said.

He said a group is monitoring resistance across the Mid-South, trying to improve sampling techniques and studying methods of insecticide application, and much research also is focused on the insect and its control.

“The tarnished plant bug is a serious problem across the state, especially in the Delta region,” Catchot said. “Because of the seriousness of this pest, it has become the No. 1 research focus among applied entomologists in the Mid-South.”

Bonnie Coblentz writes for Mississippi State University Ag Communications.

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