Changes in production could usher in new-era pests A low spray environment, changes in tillage practices and even roadside beautification projects could usher in a new era in pest control. Call it, if you will, the era of the "true bug."
Scientists point to three things - boll weevil eradication, Bt cotton and the development of new lepidopterous-specific chemistry - as being largely responsible for the emergence of these true bugs as pests in many cotton-producing regions.
So what are true bugs and how will cotton producers control them in coming years? During Cotton Incorporated's Crop Management Seminar in Memphis, recently, entomologists from across the South provided their observations on the increased importance of these true bugs, defined primarily as plant bugs and stink bugs.
Stink bug populations were higher than they've ever been in the Mid-South in 2000, although this time around the main causal agent was a mild winter which allowed higher numbers to survive, noted Roger Leonard, entomologist with the Louisiana State University AgCenter in Winnsboro.
"But in the future, a low spray environment (when boll weevil eradication is complete) combined with the mild winter conditions could result in even greater problems with stink bugs."
In Louisiana, the stink bug problem was not confined to a single crop host, noted Leonard. "They were a serious issue in field corn, sorghum, soybeans and cotton in 2000. Every time you turned around, they were there."
Organophosphates such as Bidrin and methyl parathion provide excellent control of stink bugs in cotton, noted Jeremy Greene, University of Georgia entomologist. Pyrethroids will also control them (although it differs between species ) and are useful when populations of lepidopterous pests and stink bugs are present simultaneously.
The Southern green stink bug and brown stinkbug are the most common types in the Mid-South. "The Southern green stink bug is fairly easily controlled," Leonard said, "but the brown can be much more difficult. It's tolerant to many insecticides that we have registered."
Other insect pests causing problems in the Mid-South cotton are plant bugs. Cotton fleahopper, tarnished plant bug (lygus) and clouded plant bug are the most common species.
The tarnished plant bug is of particular concern to producers in Arkansas and Louisiana, according to Leonard. The cotton fleahopper is more troublesome to Western cotton producers, although it can be an economic problem in the Mid-South during certain times of the season. The clouded plant bug has been an occasional late-season problem in Mississippi.
Leonard noted that in Louisiana, economic loss to tarnished plant bugs as a combination of yield loss and control costs rose steadily from 1992 to 1996. Economic loss has since declined at a slow rate, but it has not returned to anywhere near 1992 levels.
In Mississippi, economic loss to tarnished plant bugs increased steadily from 1991 to 1994, dropped significantly in 1995, spiked upward in 1996 and has since declined to 1992 levels. Arkansas had low levels until 1993, a spike in 1994, low levels in 1995, a significant spike in 1996 and since then has dropped off to a plateau equal to the spike level of 1994.
The Mississippi scenario "is probably associated with the impact of the boll weevil eradication programs reducing additional control costs in populations of lygus (tarnished plant bug) in those areas," Leonard said.
Late-season management of plant bugs could prove to be tricky in a low-spray environment, according to Leonard. For example, black spots on the outside of bolls, which are usually characteristic of bug feeding, "may not always be an indication of damage on the interior of that boll. This injury does not always pass completely through the carpel walls and damage the lint."
There's also new research on the relationship of cotton boll age and its resistance to late-season lygus injury. A two-year study has indicated that peak abscission of bolls from lygus injury occurred at 60 heat units after white flower. "This declines rapidly until we get around 180 to 200 heat units, eight to nine days after white flower, where there is very little abscission," Leonard said.
"We see yield loss of harvested bolls out to around 330 heat units, beyond which point, we see no additional yield loss or boll abscission. This tells us that when you have a 14- to 16-day-old boll that's accumulated at least this many heat units, it usually can no longer be damaged by lygus."
The proximity of crop hosts like field corn and soybeans as well as native hosts can also enhance populations of these secondary pests, as do crop rotation and conservation tillage practices. The latter "allows vegetation to remain in the fields somewhat longer than in conventionally-tilled plots," Leonard said.
Roadside vegetation in beautification projects, although easy on the eyes, has proven to be a significant host for plant bugs, according to Leonard. "We've found thousands of tarnished plant bugs per acre in samples of roadside vegetation in Louisiana. These hosts can be present throughout the season and are natural havens for the production of these insects."
In addition, the last three years of mild temperatures and low precipitation levels have enhanced overwintering survival of a number of pests.
Entomologists at the seminar also expressed concern over the growing percentage of Western flower thrips in Mid-South seedling cotton.
Recently, entomologists in Arkansas, Alabama, Georgia, Louisiana, Mississippi and Tennessee participated in a survey to determine the composition of thrips populations in those areas. In many of the states, tobacco thrips and flower thrips continue to be the predominant species. However, Western flower thrips, which are tolerant to most standard insecticides, were found consistently in all states except Tennessee. Soybean thrips were common to all the survey regions.
So who is going to figure out how thrips, plant bugs, stink bugs and interact in cotton fields and help the grower chart a course to controlling them?
"New technology coming along is changing the way that we consult," said consultant and grower Ray Young, from Winnsboro, La. , who recalled making a third to a fourth of a bale per acre when he started producing cotton in the 1930.
"We're now working with remote sensing to determine where to spray for plant bugs. Weevil eradication is changing the way we think about raising a cotton crop and many other insects are coming into play. It puts an extra load on the consultants to go out and find out what the problems are."