Starve or move on

USDA/ARS entomologists at Stoneville, Miss., aren't giving preseason populations of tarnished plant bug much of a choice in an area-wide management study. The pests can either starve or move.

The study, with the help of $1.3 million in-house funds, focuses on four large areas near the research facility at Stoneville, each 9 square miles, where cooperating growers raise cotton, soybeans and corn.

In the spring, ARS spray teams climb into tractor-mounted sprayers and apply broadleaf herbicides on weeds along marginal areas around fields, roads and ditches. Two of the four areas are treated, the remaining two areas are checks. The checks and treated areas are alternated each year.

The purpose of the project is to reduce the numbers of tarnished plant bugs that build up on the plants before they go to cotton. Then, during the growing season, three crews sample cotton, corn and soybean fields once a week for plant bugs.

It looks promising. “What we've found the last two years, is that there are fewer plant bugs to treat in-season in the sprayed areas,” said USDA/ARS entomologist Gordon Snodgrass.

The tarnished plant bug overwinters in the Mid-South as an adult and starts to enter diapause toward the end of August. It breaks diapause in December, when it will mate and start laying eggs. “In January, in mild winters, you've actually got young plant bugs out there on host plants,” Snodgrass said.

“By the middle of March during most years, you'll have the first generation of plant bugs. That generation and the one that follows it are in these broadleaf plants. What we want to do is select a wide area, knock out the broadleaves and see what happens.”

The entomologist didn't include a grass herbicide in the treatment because grasses aren't preferred hosts for the plant bug (plant bugs prefer to feed on plant blooms) and he wanted to leave a refuge for predatory insects. “We're not trying to make bare ground,” Snodgrass said.

Entomologists have known for years that it was possible to reduce tarnished plant bug numbers by destroying alternate hosts, according to Dick Hardee, a USDA/ARS entomologist based in Stoneville. “We've just not been able to figure out how to do it economically and safely.”

As part of the study, ARS will monitor any buildup of herbicide residue in streams and standing water and see what effect the project has on wildlife. “We know that ecologically, we're changing the area. We don't want to have a (negative) influence on wildlife,” Hardee said.

“The chemicals we use are 2,4 — D-type chemicals that break down fairly quickly,” Snodgrass added. “We also want to know if we're leaving predators out there and we're finding quite a few out in the grasses. But we're still collecting data on that.”

This year, the study will be expanded with the planting of nectariless cotton both inside and outside the area where the host plants are being treated “to see if we can reduce the in-season plant bug numbers even more,” Hardee said.

“Normally, you get 30 to 40 percent reduction in nectariless cotton, even higher in some cases,” Snodgrass said.

A similar but smaller program is being conducted by entomologists at the Northeast Research Station in St. Joseph, La. That program, centered on the Panola Company farm, in Newellton, is now in its second year.

“The first year of the study, our reduction in plant bug numbers was as high as 70 percent where the weed hosts were sprayed,” said Gene Burris, an entomologist at the station. “We have not been able to measure the impact in the field during the season because of the boll weevil eradication program. Hopefully, this year, we'll get better results.

“We're also looking at some fall treatments where we can work with lighter rates of products. You never really totally eliminate the plant bugs and sometimes, you may require both a spring and fall application.”

Expanding a plant bug management project to include the entire Mid-South “is very doable,” said Burris. “The farmers are doing a lot of burndown work these days anyway. It would be just a matter of picking up an additional 8 to 10 percent in field margins that are really rough and need some extra control.”

Plant bug populations are definitely benefiting from a low-spray environment created by the advent of Bt cotton and boll weevil eradication, Snodgrass noted. But the growth of corn acreage in some areas and Group IV soybeans are also part of the plant bug equation and a good reason to reduce plant bug numbers early in the season.

“The Group IV soybeans bloom fairly quickly in May and June and when they bloom, they attract the plant bugs. So you get a generation of plant bugs made in Group IV soybeans. They can also reproduce in corn.

“In June and July, the Group IV soybeans quit blooming and corn is starting to mature and is not attractive to the plant bugs. So you have two crops putting out adult plant bugs at around the end of June or the first or second week of July. A lot of your weed hosts mature then, too.”

The result is that the plant bug is becoming more of a midseason pest on cotton, Snodgrass said.

Plant bugs have also shown resistance to insecticides, noted Snodgrass. “When they get a high level of pyrethroid resistance, the enzymes that confer it also give them multiple resistance to some of the carbamates, organophosphates. They're pretty tough.”

The tarnished plant bug, also known as lygus, was the third most damaging cotton pest in Louisiana in 2000, the fourth most damaging cotton pest in Arkansas and Mississippi and the fifth most damaging cotton pest in Missouri and Tennessee. And it's climbing up the ladder of importance.

“We think that with boll weevil eradication success right around the corner along with wide-scale use of Bt cotton, that tarnished plant bugs and nematodes are the things that growers are going to need the most research on,” Hardee said.

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