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Link Found Between Pigweed And Plant Bugs

• By Amanda Huber, Southeast Editor •

dirty cotton bloom

Blooms with dried, brown anthers, or “dirty blooms,” indicate tarnished plant bug or plant bug nymphs fed on larger squares the plant did not shed — photo by Ronald Smith, Auburn University; bugwood.org

The reasons to aggressively battle glyphosate-resistant Palmer amaranth (pigweed) are well known. This pest will outcompete any crop, and if left uncontrolled, can take over entire fields. But even with 20 years of research into this formidable foe, new evidence suggests yet another reason: a link between Palmer pigweed and tarnished plant bugs.

Research led by University of Georgia graduate student Taylor Randell with UGA Extension weed specialist Stanley Culpepper and Extension entomologist Phillip Roberts showed pigweed attracts tarnished plant bugs into the field. The insects then feed on cotton plants.

In research conducted in 2017-2018, plant bugs damaged 9% to 15% of cotton terminals in plots kept free of Palmer amaranth.

However, when pigweed was allowed to remain in the plot for up to 35 days before removal, damaged cotton terminals ranged from 25% to 39%. Cotton terminal damage from plant bugs influenced by the presence of Palmer amaranth caused yield loss at the Macon County site in one of two years.

Check Fields For Both Pests

Roberts says tarnished plant bug is becoming a more frequent pest but is still considered an occasional pest.

“I have heard more chatter on this pest in the past few years than in the previous 15,” he says. “It is a very serious pest in other parts of the country. Absolutely check your fields for them. If you have plant bugs, they can be bad. But it’s rare that we’re going to spray for them. Maybe 5% to 10% of the acres will need to be treated to control this pest.”

The link between Palmer amaranth and tarnished plant bug adds more science behind UGA’s decade-long recommendations for managing Palmer amaranth. These include:

No Palmer up at planting.

 Two effective at-plant residual herbicides used at rates that will not hurt the cotton crop.

Timely post applications.

 Improving late-season weed coverage and control, while reducing cotton injury by applying conventional chemistry through your layby rig or hooded sprayer.

What To Look For

As for the tarnished plant bug, according to the “North Carolina Cotton Insect Scouting Guide,” early season monitoring for activity is recommended. This is especially important for retention counts of small squares at approximately 1/8 to 3/16-inch long, including bracts. If square retention is greater than or equal to 80%, more comprehensive sampling for live tarnished plant bugs is probably unnecessary.

As recommended in the guide, “If retention rates of small, upper and other first- or second-position squares drop below this level, further sampling for live tarnished plant bugs may be needed. Usually, one terminal square (or its missing position) and one first or second position square (or its missing position) two or three nodes from the top of the plant are inspected per plant.

These inspections are from 25 randomly selected plants in a field for 50 squares total. Sweep net sampling for tarnished plant bug adults and large nymphs typically involves taking two sweeps at six to 10 locations per cotton field.”
But Roberts says producers shouldn’t be too aggressive in treating plant bugs.

“Plant bugs are a problem when we start to square,” he says. “We have good thresholds, and if you have a problem, address it. If not, let Mother Nature and beneficial insects come into cotton and start establishing to help us with pests later in the season.”

As Roberts points out, squaring cotton is attractive to predatory insects like big-eyed bugs that help with other pests.

“Squaring is a critical window in that you don’t want to go in and upset the apple cart, so to speak. Tarnished plant bug is more of a pest in other parts of the country than in Georgia.”